List of Ph.D. Theses

 
 

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Tripurari Srivastava
Prof. Joydeep Chakrabortty
14109883
Phenomenological and Theoretical aspects of BSM scalar in the light of low energy constraint
20.11.2018 (Thursday)
12:00 PM
FB382 (Prof. A. K. Raychaudhuri seminar hall)
In 2012 the last piece of the most successful theory of elementary particles, the "Standard Model", was discovered in the form of a neutral massive scalar, the "Higgs", at the Large Hadron Collider (LHC). Though this completes the story of the SM, many observations hint that there must be a theory Beyond the SM (BSM). Most of the proposed BSM frameworks contain an extended scalar sector to break extended gauge symmetries and(or) generate masses for the exotic fermions. Thus, it is very important to analyse the impact of these "BSM" scalars. We have performed a model independent analysis of a doubly charged scalar coupled with charged leptons and gauge bosons. Restricting ourselves to the regime of conserved charged-parity (CP), we assume only a few nonzero Yukawa couplings between the doubly charged scalar and the charged leptons. Our choices allow the doubly charged scalar to impinge on low-energy processes like anomalous magnetic moment of muon and a few possible charged lepton flavour violating (CLFV) processes. These same Yukawa couplings are instrumental in producing same-sign dilepton signatures at the LHC. Our findings would be useful to test the phenomenological significance of a doubly charged scalar by using complementary information from muon (g − 2), CLFV and collider experiments.
Having discussed that, I will move on to a specific scenario of BSM. The left-right symmetric model is considered to be an excellent candidate for BSM physics. This model contains doubly charged scalars accompanied by many other exotic particles. I will discuss the theoretical constraints on the extended scalar sector, namely perturbativity, unitarity and vacuum stability.
As these exotic particles are heavy, there is a possibility that they appear as off-shell propagators in different low energy processes. In the presence of off-shell propagators, the decay width calculation is a little involved. I will talk about the proposal which can ease this computation. Though this prescription was proposed initially for off-shell scalars, it has been later modified such that the effect of off-shell particles having any quantum numbers can be encompassed.


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Ashutosh Kumar Singh
Prof. Amit Agarwal
13109879
Nonlinear and anisotropic optical conductivity in Dirac materials
15.11.2018 (Thursday)
10:00 AM(Refreshment @ 09:45 AM)
FB382 (Prof. A. K. Raychaudhuri seminar hall)
Optical response, in general, can be quantified in terms of optical susceptibility or optical conductivity which reveal the system’s behavior to an external field in terms of Polarization or Current density respectively. Such description of response function is best provided in terms of density matrix, which is used to describe the carrier dynamics via Equation of Motion (EOM) approach. The EOM yields a set of coupled differential equations popularly known as Optical Bloch Equations (OBEs). Most of the studies in Nonlinear optics have been carried out either by direct integration of OBEs or they involve perturbative treatments in powers of the electric field. These kinds of study are very useful for ultrafast spectroscopy when the timescale over which a pulse lasts is approximate of the order of few femtoseconds. However, if a Continuous Wave (CW) is used instead of ultrafast pulses and different kind of decay channels are replaced by phenomenological damping in OBEs, the time evolution of the system attains quasi-equilibrium steady state owing to the balance between continuous excitation of carriers and relaxation due to damping. This approach leads to rather elegant and intuitive results which can easily be manifested in various kinds of optical transport measurements.
The present thesis explores the nonlinear optical response in Dirac materials in the presence of a CW light. We develop an analytical framework to obtain the steady state density matrix of a two band system in general. Using the steady state solution of the photo-excited density matrix, we study in detail about the nonlinear optical conductivity, polarization rotation, anisotropic photo-conductivity and wave-mixing effects in graphene. Interestingly the well known Kubo formula for optical conductivity, appears as a limiting case (linear response and Infinite coherence time) of our more general non-linear formulation.


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Himanshu Gupta
Prof. Sagar Chakraborty
13209861
Fluid Instabilities in Inhomogeneous Intracluster Medium
02.11.2018 (Friday)
11:30 AM(Refreshment @ 11.15 AM)
FB382 (Prof. A. K. Raychaudhuri seminar hall)
Galaxy clusters are the largest known gravitationally bound objects in the Universe. The convective instabilities in the intracluster medium (ICM), a weakly collisional dilute plasma permeating the galaxy cluster, is of main interest in the thesis. It consists mostly of fully ionized hydrogen ions, helium ions, and the free electrons. The magnetic field in the ICM is too weak to be of any dynamical significance. However, it is capable of making the heat conduction as well as the particle diffusion strongly anisotropic. This anisotropy helps the system in becoming convectively unstable, arguably leading to a turbulent state and thus to a more "conducting" state. The ICM facilitates the heat-flux buoyancy driven instability (HBI) & the magnetothermal instability (MTI) depending on the sign of the temperature gradient along with specific configurations of the gravity and the very weak magnetic field. The HBI dominates in the core of a galaxy cluster while the MTI in the outskirts.
We have applied the stability analysis techniques, usually employed for studying the Rayleigh–Bénard convection, to analyze the MTI and the HBI. Our formalism has made it easier to incorporate the explicit boundary conditions used in the numerical experiments of the ICM. We find how the marginal state depends on the plasma-beta factor, which mode becomes unstable first, etc.
The process of diffusion may lead to helium sedimentation in the cluster core. Hence, the composition of the ICM can vary radially from the cluster center. In order to account for such inhomogeneity and its possible effect on the MTI and the HBI, we have introduced a concentration gradient and magnetic field-mediated anisotropic diffusion in our model. Our studies show that a suitable concentration profile of helium ions is capable of stabilizing the aforementioned instabilities. Subsequently, we have also found that many cool-core galaxy clusters may be in the supercritical regime and hence there is a possibility of the existence of helium fingers (defined in line with the salt fingers found in the oceans) in the core of the ICM conducive to the HBI.
A part of the thesis work is focused on the outskirts of the cluster where the MTI is supposedly in action. We show that the outcomes of the stability analysis as done on the plasma in the sheared state due to, say, the ubiquitous differential rotation. We have analyzed how the stability of such a sheared state depends on the instabilities generated due to the concentration gradient of the plasma. We also show that a suitable concentration gradient can counteract both the Kelvin-Helmholtz instability and the magnetorotational instability as found in the sheared ICM.
References: [1] S. A. Balbus, Astrophys. J., 562 (2):909, 2001. [2] E. Quataert, Astrophys. J., 673:758, 2008. [3] M. E. Pessah and S. Chakraborty, Astrophys. J., 764:13, 2013. [4] H. Gupta, S. K. Rathor, M. E. Pessah, and S. Chakraborty, Phys. Lett. A, 380 (31-32):2407, 2016. [5] S. Sadhukhan, H. Gupta, and S. Chakraborty, Mon. Notices Royal Astron. Soc., 469 (3):2595, 2017. [6] H. Gupta, A. Chaudhuri, S. Sadhukhan, and S. Chakraborty, Mon. Notices Royal Astron. Soc., 474 (1):636, 2018.


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Shamik Ghosh
Prof. Pankaj Jain
12109875
Cosmology beyond the Cosmological Principle
25.10.2018 (Thursday)
11:00 AM(Refreshment @ 11:45 AM)
FB382 (Prof. A. K. Raychaudhuri seminar hall)
The Cosmological Principle implies that the distribution of matter and radiation in the Universe is spatially isotropic and homogeneous. The Cosmological Principle is an assumption and is not a manifestation of symmetries of the underlying theory. For our (3+1) dimensional spacetime, this assumption allows us to treat the 3-space as a maximally symmetric subspace which gives us the Friedmann metric. Thus it is a foundation of the very successful LCDM model of cosmology, which is built on the Friedmann metric. However over the last decade gathering evidence suggest that on the largest length scales there is a departure from statistical isotropy.
One key observation has been the hemispherical power asymmetry (HPA) in the Cosmic Microwave Background (CMB) temperature. The HPA implies a difference in power of the CMB in the two hemispheres about (l=232 deg, b=–14 deg). I will discuss some theoretical expectations for an identical HPA which might be observable in the CMB polarization signal. I will talk of estimators we introduced to test this signal in both the harmonic space and the pixel space. I will also discuss in brief the effects of sky masking and mitigating estimator bias. I will present our current limits on the HPA in CMB polarization from PLANCK 2015 polarization data. I will also present forecasts for the detection of this signal from future CMB experiments like CORE.
The dipole in the NVSS radio galaxy catalog is another observation that violates the assumptions of statistical isotropy. Due to the local motion of our frame of observation, we do expect to observe a dipole in both the CMB and the radio sky. But the velocity from the radio continuum survey is found to be much larger than the measurements from CMB. This can arise if there is an intrinsic dipole in the large scale structures. I will discuss the theoretical possibility of such an intrinsic dipole arising from a large wavelength mode perturbation. I will also discuss the measurement of variation of the log N - log S slope. Finally I will cover some updates to these work and future course of action.


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Sanghamitro Chatterjee
Prof. Sudeep Bhattacharjee
13109884
Studies on surface wettability of atomically heterogeneous systems created by microwave plasma
generated low energy inert gaseous ion beams
21.08.2018 (Wednesday)
12:00 AM(Refreshment @ 11:45 AM)
FB382 (Prof. A. K. Raychaudhuri seminar hall)
Ion beams are increasingly becoming one of the most important tools, with the rapid development of many basic, applied and inter-disciplinary research in physics and material science. Researchers use ion beams having energies ranging from a few eV to some MeV for studying the basic and applied aspects of ion-matter interactions. Low energy (0 - 5 keV) ion beams (LEIB) have attracted special attention due to their nature of interactions with matter, where, a few sub-surface atomic layers can be tailored. Thus, materials irradiated by LEIB exhibit interesting surface properties that were lacking in the unirradiated condition.
Surface wettability, which is the spreading and gathering of a liquid drop on a solid surface, is an important surface property that is determined by the interplay between the cohesive and adhesive intermolecular interaction between the solid and liquid surfaces in contact. Thus, wetting is related to the surface free energy of a solid and indirectly dictates its adhesiveness. Thereby, it contains useful information about the surface topography, heterogeneity, and composition. The phenomena of wetting is ubiquitous in nature; starting from non-wetting lotus leaf to the lubricating fluid present in the cornea of human eyes, and thereby has drawn significant attraction of scientists. It is also important in many industrial processes such as lubrication, self-cleaning, and microfluidics. Hence, controlling wettability of a solid surface is an important requirement. Conventionally, wettability is controlled by engineering of surface topography, chemical texture or chemical coating, which widely falls under the so-called Wenzel and Cassie-Baxter regimes. Textured surfaces are expected to produce significant hysteresis brought about by the defect induced pinning.
The present research demonstrates a distinct way of controlling and studying wettability. Here, metallic (Au, Cu, Al) thin films (~ 200 nm) are irradiated by low energy (0.5 keV) noble gas (Ar, Kr, Ne) ion beams, obtained from a microwave induced plasma. It has been found that the irradiated metallic films become hydrophobic and the large scale tunability in the wetting behaviour cannot be explained by the nanometer scale roughness generated due to irradiation. However, the implanted noble gas ions stay within the near surface atomic layers and are responsible for an appreciable modification in the dispersive intermolecular interaction in the near surface region, leading to a reduced surface free energy which explains the observed transition. A few degrees of hysteresis is also observed, which is believed to have stemmed from retention of a thin liquid layer behind the receding three-phase contact line, and that the nanoroughness or the atomic scale heterogeneity do not induce appreciable pinning.
Since, the present investigation demonstrates utilization of a microwave plasma ion source for studies on surface wetting, it represents a cross-disciplinary area of research- where the physics of plasmas, ion beams and surface wettability have interplaying roles. Hence, the systematic study follows the following sequences: first, a computational work has been performed on a simple 1-D wave induced plasma where it has been found that the effect of varying length and neutral gas pressure have self-similar effects on various discharge parameters, namely, the electron energy probability function, electron temperature and plasma density, which was further validated by experiments on the existing microwave plasma setup. Thereafter, successful extraction of beams have been demonstrated experimentally with the available set up and different regimes of the current-voltage characteristic curve was analysed, followed by measurements of beam flux at different plasma conditions. It has been observed that the low voltage (≤ -50 V) regime of the beam characteristic is due to space charge limited flow, which deviates from the well-known Child-Langmuir law. The high voltage (-50V to -3kV) regime is Schottky-like, which stems from penetration of extraction electric field through the plasma electrode apertures. These are original researches carried out and also aimed at optimization of the system. These studies were followed by the investigations of wetting phenomena as described above.


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Ms. Suchita
Prof. R.Vijaya
12109876
Continuous wave, broadband, erbium-doped fiber laser and study of its temporal coherence characteristics
08.08.2018 (Wednesday)
08:30 AM
FB382 (Prof. A. K. Raychaudhuri seminar hall)
A spectrally broadened light output can be generated from an erbium doped fiber ring laser (EDFRL) by introducing a specialty fiber inside the cavity, and thus enabling the third order nonlinear optical process of four wave mixing (FWM). The intra-cavity FWM will generate new sidebands which will undergo further gain and generate more mixing products during the multiple roundtrips, leading to spectral broadening. During this process, the gain and the new sidebands play an important role in modifying the phase correlations in the spectral output of the laser. The temporal coherence length of broadband lasers is important in deciding their suitability for applications ranging from coherent communications to incoherent imaging, and has to be quantitatively estimated through reliable experiments.
In this thesis, a broadband EDFRL is constructed based on multi-pump-induced FWM in the wavelength range of 1560-1600 nm. The required multiple pumps for the FWM process are generated within the ring cavity itself by a suitable length of erbium-doped fiber and specialty fiber. The prevalent FWM theory is improved to interpret the two-pump and four-pump induced side band generation in the first and second orders, after including the effect of phase mismatch at the wavelengths used in the experiments. The temporal coherence of this broadband EDFRL is studied and compared with the results from different optical sources by analyzing the interference features from a fiber-based Mach-Zehnder interferometer (MZI). The fixed path length MZI is able to extract the temporal coherence characteristics of the source from the wavelength-dependent visibility results. A standard narrow line-width commercial laser is used as a reference source in these studies. The broadband source is filtered down to a multi-spectral source with several narrow spectral lines, and made comparable in linewidth to that of the standard laser source for clarity in interpretation of results.
The work also involves simulations by including all the likely phase contributions. The combined results from experiments and simulations clearly imply that the process of generation of the light in the broadband source is influencing its temporal coherence. The FWM process and the ring cavity have a combined effect on the temporal coherence property of the nonlinearity-assisted broadband EDFRL. The FWM sidebands are also individually tested for their temporal coherence property with and without the cavity effect, which helps to explain the loss in visibility due to the intra-cavity noise arising from the amplified spontaneous emission which is not entirely annulled in broadband lasers. In addition, the phase correlation within the spectrum of the broadband source is also demonstrated by providing two optical inputs to the MZI.


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Mr. Ummer K.V
Prof. R.Vijaya
10709070
Role of Band-edge modes and in-plane modes in two- and three-dimensional photonic crystals
02.08.2018 (Thursday)
11:00 AM (Tea will be served at 10.45 AM)
FB382 (Prof. A. K. Raychaudhuri seminar hall)
Periodic dielectric structures have been studied in recent years for many interesting optical properties such as negative refraction, superprism, and super-collimation effect. The periodicity in their dielectric constant will discretize the electromagnetic modes within the structure into allowed propagating modes and the modes with no propagating component – constituting the photonic stopband. Depending on the extent of overlap of the photonic stopband with the emission spectrum of an embedded emitter in a photonic crystal (PhC), the suppression in spontaneous emission within the stopband and enhancement in emission at the band-edges can be expected. The reduced group velocity at the band-edges provides increased time duration for light-matter interaction.
In this thesis, the quantitative enhancement in emission and absorption of Rhodamine-B dye due to the band-edge effect in 3-D ordered pseudo gap polymer-based photonic crystals has been studied. The motivation is to identify the ideal parameters for designing stand-alone miniature low-threshold lasers through a combined effect of absorption and emission enhancement from band-edges. The combined effect has resulted in a 70% enhancement in emission intensity over the intrinsic emission of the dye within a PhC.
Silicon-based PhC platforms are desirable since they help in large-scale on-chip integration of optical components for miniaturization of optical devices. In this work, the 2-D photonic crystal slab with a hexagonal lattice of air holes on SOI wafer has been fabricated by electron beam lithography (EBL), and its Fano resonances are characterized by a simple out-of-plane experimental set-up. The quality factors of these Fano resonances are associated with the lifetime of the leaky modes of the slab which are present in the band structure above the light line for the cladding. A higher quality factor at any wavelength will be helpful in building applications such as Si-based micro-devices and narrowband optical filters.
In addition to the applications based on the photonic stopband, the allowed Bloch modes of photonic crystals possess unusual dispersion characteristics which result in various optical phenomena such as negative refraction, self-collimation, and super-prism effects. In our work, an all-angle (±90º) self-collimation effect with a bandwidth of 15% has been obtained from a two-dimensional rectangular PhC. All-angle (±90º) negative refraction from a two-dimensional honeycomb lattice of air holes in Si shows a wavelength resolution of 0.2λ when imaged with a point source.


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Mr. Alekha Chandra Nayak
Prof. Pankaj Jain
11209062
Phenomenological study of very special relativity
July 3, 2018 (Tuesday)
4 PM (Tea @ 3.45 p.m.)
FB382 (Prof. A. K. Raychaudhuri seminar hall)
The special theory of relativity has been experimentally tested to unprecedented degree of accuracy. The Standard model of particle physics respects Lorentz invariance and successfully describes all particle interactions. However, it breaks several discrete symmetries such as P, T, CP or CT, which were once thought to be preserved in nature. With the violation of these discrete symmetries, it is possible that only a subgroup of the Lorentz group may be preserved in nature. The resulting framework along with translational symmetry is known as very special relativity (VSR). It turns out that this is both necessary and sufficient to explain the null result of Michelson-Morley experiment and its consequences. VSR has a preferred direction which manifestly breaks Lorentz invariance. Remarkably it provides an alternative approach for neutrino masses which does not require addition of a new fields in the Standard Model and it also conserves lepton number. We study collider, astrophysical and cosmological implications of models based on VSR.


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Tanmay Maji
Prof. Dipankar Chakrabarti
12209874
Transverse structure of proton and azimuthal spin asymmetries in the SIDIS processes
June 5, 2018 (Tuesday)
4:00pm(tea @ 3:45pm)
FB382 (Prof. A. K. Raychaudhuri seminar hall)
In last two decades, there have been a lot of investigations on the three dimensional structure of nucleons. Understanding of the spin and the orbital angular momentum (OAM) structure in partonic level is a challenging task in this field. Three dimensional structures are encoded into the distribution functions e.g., generalized parton distribution functions(GPDs), transverse momentum dependent parton distribution(TMDs), generalized TMDs (GTMDs) etc. Wigner distributions are higher dimensional distributions which are not accessible in experiments but at different limits they reduce to different parton distributions and thus play important role in understanding the 3D structure of nucleons. Since these distributions are non-perturbative in nature and difficult to be calculated in full QCD, people study these objects in QCD inspired models. We present a model prediction to the non-perturbative transverse structures of proton. To investigate these distribution functions of nucleons, we construct the Light-front quark-diquark model (LFQDM) where the wave functions are adopted from soft-wall AdS/QCD. The model is consistent with the quark counting rules and the parameters of the model are determined by fitting the experimental data of Dirac form factor and Pauli form factor. The TMDs provide a three-dimensional structure with the probability of finding a quark with longitudinal momentum fraction $x$ and transverse momentum $\bf{p}_\perp$ inside a proton. We also calculate the five-dimensional Wigner distributions and present the quark orbital angular momentum(OAM), spin-OAM correlations inside a proton.In this model, We also present the single spin asymmetries (SSAs) for SIDIS e.g., Collins asymmetries, as well as the double spin asymmetries (DSAs) which involve the T-even TMDs at the leading twist. The model results of asymmetries are compared with the COMPASS and HERMES data for $\pi^+$ and $\pi^-$ channels. Fragmentation functions are taken as a phenomenological input. We propose a parameter evolution approach for TMD evolution and compare with the QCD evolution of unpolarised TMDs. The parameter evolution approach for TMD evolution gives quite satisfactory result for azimuthal asymmetries as the asymmetries are defined by the ratio of TMDs. Our model has predicted sizeable Collins asymmetry for the upcoming EIC data. We then extend our investigation for T-odd TMDs and azimuthal spin asymmetries generated by them in SIDIS process. The spin asymmetries associated with T-odd TMDs namely, Sivers and Boer-Mulders asymmetries require gluon exchange in the final state. To get the nonzero T-odd TMDs, the wave functions are modified by including a spin dependent complex phase which reproduces the final state interaction (FSI) effect. We also predict the Sivers asymmetry and Boer-Mulders asymmetries in the SIDIS process for $\pi^+$ and $\pi^-$ channels and compared with the HERMES, COMPASS measurements.


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Anmol Thakur
Prof. Amit Agarwal
12109063
Density and Current Responses in Dirac Materials
18th May, 2018
11:00 am (Tea: 10.45 am onwards)
FB382 (Prof. A. K. Raychaudhuri seminar hall)


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Sunil Kumar
Prof. Y. N. Mohapatra
Y9109079
Organic Semiconductor Homojunction Diodes: Mechanisms Controlling Current and Capacitance Characteristics
15th April 2018
03:30 PM
SCDT (Seminar Room)
Electronic devices based on organic materials, particularly, diodes and transistors have significant technological potential due to their advantages in processibility, flexibility and cost-effectiveness. The role of organic/organic & metal/organic interfaces is crucial in controlling the device characteristics. However the underlying physical mechanisms taking place at these interfaces are not well understood, and need special attention. A detailed understanding of mechanisms at the interface will help design structures and applications in the field of organic electronics. My thesis aims to study in detail the undoped homojunction diode, which is a sandwich structure consisting of doped and undoped layer of the same organic semiconductor. Our principal focus is to demonstrate the effect of the thickness of the undoped layer on the device characteristics. The configuration of the specifically designed device under test is ITO|p-MTDATA|i-MTDATA|Al. Intrinsic layer thickness is varied from 10 nm to 100 nm so as to demonstrate the control over both forward and reverse current density-voltage (J-V) and capacitance-voltage (C-V) characteristics. The specially designed device structures for this purpose have been fabricated using a state-of-the-art automated multi-chamber Cluster tool. The prototype materials m-MTDATA and F4-TCNQ are used as matrix and dopant, respectively and the doping is achieved by co-evaporation. The devices with thin intrinsic layer showed Zener type behavior exhibiting high current density beyond a certain reverse threshold voltage. In order to delineate underlying mechanisms, the J-V characteristics were studied by varying the temperature from 200K to 300K. The forward bias characteristics are controlled by the high-low junction to result in nearly temperature independent ideal exponential J-V regime prior to the space charge limited regime even for samples with intrinsic layer thickness as low as 10 nm. However, the reverse characteristics are controlled by Fowler-Nordheim (F-N) tunneling at the cathode/intrinsic layer interface. The barrier controlling F-N tunneling is found to be temperature independent but sensitive to the intrinsic layer thickness. This is interpreted in the framework that the thickness of the intrinsic layer modulates the Fermi level responsible for barrier height. The capacitance spectroscopy of m-MTDATA based homojunction and intrinsic diodes has been analyzed to investigate the physical mechanisms controlling the charge processes. The capacitance exhibits voltage dependence in reverse bias which is a signature of the defect states acting as traps. In order to investigate the defect states, temperature dependent capacitance-frequency (C-f) characteristics have been analyzed. The defect states are estimated to have Gaussian distribution and the associated parameters have been calculated. In the low thickness homojunction we are able to probe the parameters associated with localized level in the HOMO states, demonstrating consistency with Gaussian disorder model of energetic in organic semiconductors. In homojunction diode, having low intrinsic layer thickness (10nm), the capacitance in deep reverse bias starts decreasing nearly exponentially and goes below the geometrical capacitance (Cg) after a critical electric field inside the device. This decrease in the capacitance is interpreted on the basis of F-N tunneling of carriers. The small signal capacitance in such cases will have negative contribution which is directly related to delay time introduced by charge transport. Using F-N tunneling, the characteristics have been modeled in deep reverse bias, and the estimated barrier height matches with the values calculated from J-V characteristics. The technique also allows the determination of mobility, which in turn yields the disorder parameters through its temperature and field dependence. The study reported in the thesis thus gives a coherent account of mechanisms underlying current and capacitance characteristics of homojunction organic diodes. We thus demonstrate determination of useful intrinsic transport and disorder parameters from such characteristics.


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Pratim Roy
Prof. Tapobrata Sarkar
11109067
Information Geometry: An AdS/CFT Perspective
26th April 2018
3:00 PM
FB382 (Prof. A. K. Raychaudhuri seminar hall)
In the context of statistical physics, information geometry addresses the question - how do we distinguish two infintesimally separated states in a physical system? This is done by defining a Fisher information metric on the parameter space of the system (classical or quantum). This metric in turn allows the introduction of the notion of a line element which quantifies the distance between two infinitesimally separated states. Recently, there has also been a proposal to calculate a counterpart of this information metric from the celebrated AdS/CFT correspondence, which relates classical gravity to strongly coupled field theories. In the first part of the present thesis, to illustrate the utility of the information geometric approach to physical systems, we investigate phase transitions in magnetic systems, as well as a toy model of a liquid system. With this serving as the motivation, we undertake to investigate information geometry in the context of AdS/CFT. As the first investigation in an AdS/CFT context, we first investigate the phase transition of rotating and non-rotating black holes in the extended phase space paradigm. Briefly, this proposal modifies the well-known first law of black hole thermodynamics by considering the cosmological constant as a variable parameter. Next, we introduce the notion of holographic complexity, which is a quantity related to the Fisher information. We explicitly calculate complexity for field theories dual to Schwarzschild and Reissner-Nordstrom black holes in a near horizon expansion. We have also numerically computed the complexity for a Reissner-Nordstrom black hole with a spherical horizon. Lastly, we investigate how the complexity behaves along the renormalization group flow by holographic methods. In systems where there is a phase transition, we track the behaviour of the complexity across it and comment on how the complexity behaves near the phase transition.


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Alestin Mawrie
Prof. Tarun Kanti Ghosh
12109061
Electrical, optical and thermoelectric transport properties of spin-orbit-coupled fermionic systems
Feb 06, 2018
4:30 PM
FB382 (Prof. A. K. Raychaudhuri seminar hall)
The fundamental role of spin-orbit interaction (SOI) in condensed matter systems is one of the interesting topics in modern days research. The strong SOI in group III-V semiconductors makes them a suitable candidate for the spintronic application. The SOI associated with holes in p-doped III-V semiconductors is comparatively stronger than that of the electrons in n-doped III-V semiconductors. The hole in bulk p-doped III-V semiconductors is fully described by the 6 X 6 Luttinger Hamiltonian which gives a quadratic momentum dependence of SOI. On the other hand, two-dimensional heavy-hole gas (2DHG) formed at the p-type GaAs/AlGaAs heterostructure is associated with two types of SOIs, namely Rashba SOI (RSOI) and Dresselhaus SOI (DSOI), which are both cubic in momentum. Using the Kubo formula and the Green's function technique, we present a theoretical study to understand the influence of higher-order SOIs on electrical and thermoelectric transport coefficients as well as on optical properties. We also discuss magneto-transport properties of 2DHG with k-cubic RSOI in presence of a quantizing magnetic field. References: [1] R. Winkler, Spin-orbit Coupling Effects in Two-Dimensional Electron and Hole Systems (Springer, Berlin 2003) [2] A. Mawrie, T. Biswas and T. K. Ghosh, Journal of Physics: Condensed Matter 26, 405301 (2014). [arXiv: 1405.4533] [3] A. Mawrie and T. K. Ghosh, Journal of Applied Physics 119, 044303 (2016). [arXiv: 1511:04917] [4] A. Mawrie and T. K. Ghosh, Journal of Physics: Condensed Matter 28, 425302 (2016). [arXiv: 1601:00591] [5] A. Mawrie, P. Halder, B. Ghosh and T. K. Ghosh, Journal of Applied Physics 120, 124309 (2016). [arXiv: 1605:05162] [6] A. Mawrie, S. Verma and T. K. Ghosh, Journal of Physics: Condensed Matter 29, 465303 (2017). [arXiv:1705.02483]


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Jishnu Goswami
Prof. Dipankar Chakrabarti
11109065
Numerical Studies of Borici Creutz fermions on lattice
21st December, 2017
3:30pm (tea@ 3:15pm)
FB382
Lattice field theory is a well known technique to study any field theory non perturbatively from the first principle. However the discretization of the fermion action suffers from the notorious fermion doubling problem. There are many prescriptions to overcome this issue but in the price of some basic symmetries of the lattice action. There is a no-go theorem which tells that fermionic lattice action with locality, hermiticity and chiral symmetry must have doublers. In general, chiral symmetry is sacrificed to get rid of the doublers. Nevertheless the search for chiral symmetric lattice action which is computationally not very demanding is still going on. Recently motivated by the fact that electrons on a graphene lattice are described by a massless Dirac-like equation, Creutz and Borici proposed a four dimensional euclidean lattice action describing two flavors of fermions on an orthogonal lattice. This action minimize the doubling problem but in this case hypercubic symmetry is reduced to cubic symmetry. There is not much evidence in the literature to prove whether this fermion is suitable or not for numerical studies. So, we initiated a systematic numerical investigation of the Borici-Creutz action. We first studied the chiral symmetry breaking and mass spectrum in Gross Neveu model with this fermion formulation using HMC(hybrid monte carlo method) in two dimensions. Encouraged by the two dimensional results we extend our work to four dimensions, where we use mixed action approach to study this fermion. We use Borici Creutz valence quarks and asqtad sea quarks. As Borici Creutz action breaks hypercubic symmetry, we need to add counter terms which can be generated through radiative corrections. In our study we fix the counter terms non perturbatively. We show that our results for partial quenching, pion chiral log and delta mix etc. are consistent with the chiral perturbation theory predictions.


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Dipak Rout
Prof. R.Vijaya
11209064
Localized surface plasmon resonance effect on the optical properties of metal-dielectric photonic crystals
06.11.2017 (Monday)
11:30 AM (Tea @ 11.15 AM)
FB382
Many exotic phenomena are observed during electromagnetic wave propagation through periodic dielectric media with feature sizes comparable to the length scale of the wavelength of light. Photonic crystals are one such class of periodic structures which effectively block a set of wavelengths in transmission leading to band gap effect. The optical properties of these dielectric crystals can be modified further by introducing metal nanoparticles into them. Metal-dielectric photonic crystals allow better control of light-matter interaction as they offer a higher refractive index contrast to the incident radiation and support plasmonic resonances. They are useful for applications ranging from integrated photonics and solar energy conversion to chemical and bio-molecular sensing. This work explores the possibilities of tailoring the optical properties of metal-dielectric hybrid structures by a synergistic combination of the photonic stopband and the plasmon resonance of metal nanoparticles. By a proper choice of the size of dye-doped polymeric colloids and the gold nanoparticles, we have studied the effect of plasmonic resonance on the stopband characteristics of photonic crystals. In addition, the fluorescence intensity of the dye molecules in the crystals is measured using a standard laser-induced photoluminescence experiment, and the fluorescence decay time of the excited state is measured using a time-correlated single photon counting system. We have demonstrated an enhanced emission from gold-infiltrated photonic crystals as a result of increased plasmonic absorption and a resonant energy transfer between the dye molecule and the gold nanoparticles. A reduction in the radiative decay time and a complete suppression of the non-radiative decay route is achieved in the metal-dielectric photonic crystal. Suppression of non-radiative decay route is useful to enhance the fluorescence efficiency of the dye and to reduce the threshold for lasing. In addition, a considerable increment in the Raman signal intensity is achieved when the gold nanoparticles are surface-coated over the dielectric photonic crystals due to the combined effect of the stop band and the localized surface plasmon resonance of the gold nanoparticles. The plasmonic-photonic interaction in enhancing the dye emission is further studied in complex designs such as heterostructures made from dye doped 3-D colloidal photonic crystals embedding a planar SiO2 defect doped with gold nanoparticles. The reason is the physical intuition to be gained about spontaneous emission control in non-local designs. Significant spectral narrowing and reduction in the fluorescence lifetime is achieved in such structures. The role of defect mode and localized plasmons in modifying the emission characteristics from the heterostructure is addressed through this work. Thus the thesis establishes the possibilities of localized surface plasmon modes in modifying the optical properties of metal-dielectric photonic crystals towards amplification of emission and low-threshold lasing applications.


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Debarchan Das
Prof. Zakir Hossain
11109866
Heavy fermion behavior and magnetic ordering in Ce(Cr,Ti)Ge3 with hexagonal perovskite structure
16-08-2017 (Wednesday)
4:30 p.m. (Tea at 4:15 p.m.)
FB382
Strongly correlated electron systems have attracted considerable attention of condensed matter physics research community as they exhibit wide variety of fascinating phenomena which include heavy fermion behavior, Kondo effect, valance fluctuation, quantum criticality, superconductivity etc[1,2]. However, among the vast number of existing intermetallic compounds, only a small fraction exhibits orders of magnitude enhancement of the conduction electron effective mass at low temperature, known as heavy fermion system and sometimes they even enter in the superconducting state alongside this enhanced conduction electron effective mass, known as heavy fermion superconductors (HFSC). Unlike the conventional s-wave superconductor where superconducting pairing is mediated by phonons, in HFSC superconducting pairing is believed to be mediated by magnetic interactions. Thus, Heavy fermion and HFSC systems have continued to dominate the field of condensed matter research for the past several decades. In our effort to search for new rare earth based intermetallic compounds we have synthesized polycrystalline samples of CeCrGe3 and performed a comprehensive study of the low temperature properties by means of x-ray absorption spectroscopy (XAS), magnetic susceptibility, isothermal magnetization, electrical resistivity, specific heat and thermoelectric power measurements. The results corroborate that the compound undergoes a ferromagnetic transition below Tc ~ 70K. The Kondo lattice type of the resistivity and the large value of Sommerfeld coefficient (γ = 130mJ/mol/K2) convince us that CeCrGe3 is a moderate heavy fermion system[3]. The observation of HF/ Kondo lattice behavior even in the ferromagnetically ordered state is quite rare. In order to gain further insight into the magnetic ordering in CeCrGe3, we have performed muon spin relaxation (μSR) experiments which reveal the presence of a bulk magnetically ordered state[4]. In order to unveil the magnetic structure of this system, we have carried out powder neutron diffraction study on CeCrGe3. Neutron diffraction investigation reveals that both Ce and Cr orders ferromagnetically at ~70 K [5]. Interestingly, the observation of this high value of ordering temperature for the Ce moments is very unusual. In addition, we have tried to tune the magnetic ordering in CeCrGe3 by Ti doping at Cr site. The results on CeCr1-xTixGe3 suggest a possible existence of a bi-critical point in the system[4]. Superconductivity was not found down to 2K. [1] G. R. Stewart, Rev. Mod. Phys. 56, 755 (1984). [2] G. R. Stewart; Rev. Mod. Phys. 73, 797 (2001). [3] D. Das, T. Gruner, H. Pfau, U.B. Paramanik, U. Burkhardt, C. Geibel and Z. Hossain, J. Phys.: Condens. Matter 26, 106001 (2014). [4] D. Das, A. Bhattacharyya, V. K. Anand, A. D. Hillier, J. W. Taylor, T.Gruner, C. Geibel, D. T. Adroja and Z. Hossain, J. Phys.: Condens. Matter 27, 016004 (2015). [5] D. Das, S. Nandi, I. da Silva, D. T. Adroja and Z. Hossain, Phys. Rev. B 94, 174415 (2016).


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Sayandip Ghosh
Prof. Avinash Singh
11109869
Unified Description of Electronic Structure and Anti Ferromagnetism in a Three-Orbital Model for Iron Pnictides
18/07/2017 (Tuesday)
11:00 AM (Tea @ 10:45 AM)
FB382
Iron pnictides, the new class of high T_c superconductors after cuprates, exhibit a rich temperature-doping phase diagram in which the parent compounds undergo a tetragonal-to-orthorhombic structural transition and a magnetic phase transition to the stripe antiferromagnetic (AFM) state. With doping, both are suppressed and superconductivity arises. The electronic structure calculated using first-principle techniques and probed by angle resolved photoemission spectroscopy experiments reveal multiple bands crossing the Fermi level resulting in two nearly circular hole pockets and two elliptical electron pockets. Magnetic excitations in the magnetic state examined by inelastic neutron scattering measurements yield well defined spin wave excitations with a maximum at the ferromagnetic zone boundary. Moreover, existence of ferro-orbital ordering between the xz and yz Fe 3d orbitals is also observed. Understanding these electronic and magnetic properties within a single theoretical framework has been an outstanding challenge. The single-band Hubbard model is our starting point in which the stripe AFM state is stabilized at intermediate hole doping. More realistic two-orbital models yield correct Fermi surface topology, but I will show that the spin wave dispersion does not agree with experimental findings. Motivated by this, I will present and investigate a three-orbital itinerant-electron model and show that essential features of electronic structure and magnetism can be well understood. Existence of the weakly gapped magnetic state will be shown to arise from a composite effect of orbital hybridization and magnetic order. I will also demonstrate how the effective spin couplings are generated from the particle-hole propagator, which explains the microscopic origin of the ferromagnetic spin coupling along b direction required to understand the observed spin wave dispersion. Finally, the genesis and stability of the newly discovered double-Q spin-charge-ordered state will be discussed.


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Nimisha Raghuvanshi
Prof. Avinash Singh
Y9209042
Magnetic excitations in multi orbital models for iron pnictide.
16/05/2017 (Friday)
11:00 AM (Tea @ 10:45 AM)
FB382
Study of magnetic excitations provides detailed understanding of effective magnetic interactions in correlated electron systems. Spin wave excitations in iron pnictides have been intensively investigated since the discovery of magnetism in these iron based superconductors. Angle resolved photo emission spectroscopy and inelastic neutron scattering experiments have revealed various features of electronic Fermi surface and magnetic excitations in this (pi, 0, pi) ordered antiferromagnet. These observations have highlighted the limitations of various existing theoretical models and approaches. In this talk, I will discuss calculation of magnetic excitations in different multi-orbital models for iron pnictides aimed at understanding the major characteristic features of magnetic excitation spectra within a single theoretical framework. We propose that the characteristic maximum spin wave energy at the ferromagnetic zone boundary is the result of strong ferromagnetic spin couplings generated due to the exchange of the (virtual) particle-hole pair. Conditions on the microscopic Hamiltonian parameters of the multi-orbital model which favor the stabilization of the spin density wave state and correct sign of the orbital order within the constraints on the Fermi surface structure are identified. The role of nesting in the stabilization of the spin density wave ordered state is examined.


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Anup Kumar Singh
Prof. Rajeev Gupta
Y8109062
Anomalous resistivity behaviors of thin films and environmental effects in devices of amorphous Indium Gallium Zinc Oxide.
17/05/2017 (Wednesday)


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Biplab Bag
Prof. Satyajit Banerjee
10109873
A study of interplay between magnetic and superconducting order in BaFe2-xCoxAs2, and driven vortex state properties in 2H-NbS2 superconductor.
17/05/2017 (Wednesday)
3.30 PM
L-6
In iron pnictide superconductors, we investigate the competition and interplay between magnetic fluctuations and superconductivity. Using bulk and local magnetization techniques, we do a detailed doping dependence study to observe the interplay between superconductivity and magnetic correlations with doping in BaFe2-xCoxAs2 single crystals (underdoped, optimally doped and overdoped). Our study reveals an inhomogeneous superconducting state below the superconducting transition temperature (Tc). Above Tc, we observe the presence of positive magnetic response in our samples. Using high sensitivity magneto-optical imaging we find that the positive magnetic response in our samples appears due to presence of a short range magnetic correlation in the sample. On application of magnetic field we find an unusual suppression in the magnetic response along with an enhancement in the diamagnetic shielding (superconducting) response in our samples. We show the presence of superconducting fluctuation response above the bulk superconducting transition temperature. We have found that the strength of the short range magnetic order, and superconducting fluctuations are the strongest for the optimally doped sample (which possess the highest Tc) and weakest for the over doped sample. We believe our results provide some evidence of interplay between magnetic and superconducting order in iron pnictide superconductors and magnetic fluctuations may play a role in mediating superconductivity in these compounds. A part of the thesis work is also devoted to exploring a new jamming transition in the driven vortex matter in 2H-Nbs2 superconductors. Using four probe transport characteristics, we reveal two different depinning threshold regime; (i) conventional depinning from the static pinned vortex state and (ii) unjamming transition from the drive-induced jammed state. We show the unjamming transition has unusual negative velocity fluctuations. We identify the unique characteristics of the unjamming transition which are found to be distinct from conventional depinning. We analyze this transition in terms of IV scaling relations and Cohen-Gallavotti non-equilibrium fluctuation relation (GC-NEFR).


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Anshuman Dey
Prof. Tapobrata Sarkar
11109063
Holographic Renyi entropy, entanglement entropy and thermalization in higher derivative gravity theories.
21st April, 2017
11 a.m.
FB382
The present thesis seeks to understand three important aspects of quantum field theory : thermalization, entanglement entropy and R ́enyi entropy, in the context of the gauge/gravity duality with higher derivative corrections. All these as- pects are of great interest in the condensed matter and statistical physics community, and it is important to understand the predictions of the AdS/CFT correspondence in these contexts. In the first part of the thesis, we consider a gravity model for holographic thermalization, generalizing the Einstein-Maxwell action by including a specific class of four derivative terms which involves a coupling of the Maxwell field to the bulk Weyl tensor. The key interest here is to understand how the coupling constant of the four derivative term affects the thermalization in the boundary field theory. Here we also investigate the effect of the chemical potential in the boundary theory on thermalization, and find subtle interplay between the coupling constant and this chemical potential. In the second part of the thesis, considering a similar gravity model, we set out to explore the rich phase structure of the dual field theory in a fixed charge ensemble, and for this purpose we construct appropriate black hole solutions perturbatively, and analyze black hole thermodynamics and compute the entanglement entropy of the dual CFT. Finally, the third part of the thesis is devoted to the study of another important quan- tity in quantum information theory, namely the R ́enyi entropy, from the holographic viewpoint, in the context of modified gravity. We investigate this by considering four different gravity models, charged and uncharged, and carry out extensive investigations of some aspects of the dual conformal theories.


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Seema Devi
Prof. Asima Pradhan
Y9109081
Application of correction techniques and statistical analysis on polarized fluorescence and synchronous fluorescence spectroscopy and study of polarized fluorescence imaging for characterization of cervical precancerous tissue.
27/03/2017 (Monday)

FB382


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Upkar Kumar Verma
Prof. Y. N. Mohapatra
Y8209064
Trapping and recombination in P3HT:PCBM bulk heterojunction solar cells; influence of device structure using photocapacitance, impedence and photovoltage decay techniques.
28/12/2016 (Wednesday)

SCDT seminar room


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Jitesh Barman
Prof. Krishnacharya
111009868
Electric Field Controlled Interfacial Phenomenon at Liquid-Solid/Liquid Interfaces.
27/12/2016 (Tuesday)
11.00 AM
FB382
Wetting of liquids on solid/liquid surfaces is one of the most important interfacial phenomenon from fundamental as well as application point of view. Wettability of a surface can be controlled by manipulating the surface free energy of solids either passively by coating or actively using external stimulus e.g. temperature, radiation, electric filed etc. Here we report external electric field induced manipulation of liquids on solid and liquid surfaces. Electrowetting on dielectrics (EWOD) has been established as an effective tool to reversibly manipulate the surface wettability. Firstly I will talk about the application of EWOD in open-microfluidics. Surface grooves with triangular cross-section will be used as open-microfluidics. Static and dynamics of liquid transport in the grooves, actuated via electrowetting, will be discussed along with appropriate theoretical model. Subsequently, EWOD on dielectric lubricating fluid coated solid surfaces, to reduce hysteresis, will be presented. Nepenthes pitcher plants have motivated researchers to explore lubricating fluid coated slippery surfaces. I will demonstrate how using external electric field, electrically tunable slippery surfaces can be fabricated with electric field dependent slip velocity. Multiple aqueous droplets on such lubricating fluid coated surfaces show spontaneous coalescence or pseudo non coalescence depending on the lubricating fluid film thickness. External electric field can also be used to control the coalescence or non-coalescence on such surfaces. Towards the end, I will talk about specially engineered superoleophobic surfaces which repel low surface tension liquid (oils and hydrocarbons). Ultraviolet radiation induced reversible wettability change, from superoleophobic to oleophilic, will also be discussed.


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Pankaj Chaturvedi
Prof. Gautam Sengupta
10109874
AdS/CFT correspondence, holographic superconductors and quantum phase transitions.
25/11/2016 (Friday)
12.30 PM
FB382
We will describe three specific examples of the application of the AdS/CFT Correspondence ( Gauge Theory-Gravity Duality) to the construction of holographic superconductors and the study of holographic quantum phase transitions. The first example will deal with a (1+1)-dimensional holographic superconductor from a boundary quantum field theory dual to a bulk (2+1)-dimensional charged rotating BTZ black hole. Subsequently we will describe a (2+1)-dimensional p-wave holographic superconductor from a boundary field theory dual to a (3+1)-dimensional bulk charged AdS-Born-Infeld black hole. Finally we will characterize a holographic quantum phase transition in a (2+1)-dimensional boundary field theory dual to a system of two mutually interacting massive real scalar fields in a (3+1)-dimensional bulk AdS soliton background.


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Tanay Nag
Prof. Amit Dutta
11109870
Quenched and periodically driven closed quantum systems: entanglement, decoherence and dynamical localization.
04/11/2016 (Friday)
10 AM
FB382
Dynamics across a quantum critical point is necessarily non-adiabatic due to a diverging time scale in the vicinity of the quantum critical point (QCP) where the system becomes infinitely sluggish. The defect density in the final state is given by the celebrated Kibble-Zurek (KZ) law where the defect density is expressed in terms of quenching rate and the critical exponent associated with the quantum phase transition (QPT). We probe the temporal behavior of decoherence factor (DF) by considering a central spin model where an external qubit is globally connected to the environmental spin chain. We show that the DF shows power law behavior in the vicinity of a QCP while on the far limit DF exhibits a Gaussian fall for integrable environment. We successfully predict the scaling behavior of the decay constant associated with DF in both the limits using KZ scaling argument. In parallel, we probe the periodically driven system in the light of Floquet theory. We have studied the dynamics of a one-dimensional lattice model of hard core bosons (HCB) which has an initial current carrying ground state in the superfluid phase. The system is subjected to periodic delta-function kicks in the staggered on-site potential. It is found that the current vanishes (and the work done saturates) in the in the limit of an infinite number of kicks and large frequency. This phenomena can be attributed to a dynamic localization of the hard core bosons. We also have shown that the particles propagates in a light-cone-like fashion in real space. This study has been generalized to a situation when the staggered potential varying sinusoidally in time to estimate the behavior of maximum group velocity as a function of frequency and amplitude.


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Gyanendra Kumar
Prof. R. Vijaya
Y9109068
Nonlinear dynamical studies on Erbium doped fibre laser subjected to cavity loss modulation.
26/09/2016 (Monday)
10.00 AM
FB382
Highly-stable erbium-doped fibre amplifiers and lasers are useful in traditional fibre-optic communication due to their wavelength range of operation lying in the low-loss window of glass fibres. Fibre lasers operating in the chaotic domain are equally useful, but for widely disparate applications ranging from secure communication systems to chaos-based LiDARs and random number generators. An understanding of the conditions for switch-over from linear to nonlinear dynamical regime of fibre lasers, their tendency for chaotic operation, and possibilities of controlling their dynamics are essential for these applications. We study the fundamental as well as sub- and super-harmonic resonance characteristics of the erbium-doped fibre laser in the regime of its relaxation oscillation frequency, the evolution of its chaotic dynamics, and the conditions leading to its bi-stable and multi-stable operation using the cavity-loss modulation technique through experiments. At appropriate modulation parameters, the laser exhibits different periodic states (period-1, period-2, period-4, period-8 etc) eventually leading to deterministic chaos through the period-doubling route. This work shows that we can repetitively drive the non-autonomous class-B laser into and out of linear, nonlinear (of different extents), and chaotic dynamics with a very simple approach as required for applications. The nonlinear oscillator model of the laser provides a good agreement between the theory and experiments. Convenient user-defined parameters such as the pumping ratio and the biasing voltage of modulation enable a fine control on the laser dynamics. The alternate technique of pump modulation is also analyzed for the sake of completeness, and the advantage of loss modulation is established.


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Chandan Mandal
Prof. Dipankar Chakrabarti
11109864
Nucleon structure in light front quark models in AdS/QCD.
14/09/2016(wednesday)
3.30 PM
FB382
The investigation of the structure of nucleon has a long history which goes back to early fifties but the structures are still not completely resolved. In parallel to the efforts of theoretical modelling, different experiments are still going on to understand the insights into nucleon structure. In the recent time, light-front AdS/QCD has emerged as one of the most promising theoretical framework to investigate the non-perturbative QCD/hadronic structure. The thesis mainly focuses on the application of AdS/QCD models to investigate several nucleon properties such as electromagnetic and gravitational form-factors, generalized partion distributions (GPDs), transverse charge and magnetization densities in transverse plane etc. In the second part of the work, we investigate various quark-diquark model where the nucleon wavefunctions are constructed from the soft-wall AdS/QCD correspondence and compare with the soft-wall AdS/QCD models. The results obtained are compared with experimental data wherever available or other phenomenological models.


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Anirban Bagui
Prof. S. Sundar Kumar Iyer
Y7109861
Effect of electric-field annealing during solvent drying step of active layer in organic solar cell devices.
05/08/2016 (Wednesday)
12.00 PM
SCDT seminar room
The performance of organic electronic devices can often be improved by improving the nano-morphology of organic active layers in those devices. In this thesis, the role of thermal annealing of polymer layers during solvent drying in the presence of a constant electric-field to improve the the nano-morphology of the films thus formed was studied. Comparative studies of morphological and electro-optical properties of the P3HT:PC61BM based blend film and bulk hetero-structure solar cell devices were carried out to understand the role of 'electric-field annealing'. The external quantum efficiency (EQE) and power conversion efficiency (PCE) of the solar cells made showed significant enhancement due to better charge transport in the case of electric-field annealed devices. In order to study this process step on the hole mobility in the P3HT layer, devices with holes as the primary charge carriers were fabricated. The hole mobility in P3HT was found to increase monotonically for annealing field strengths up to 2000 V/cm. The current density voltage (J V) data corresponding to the space charge limited currents (SCLC) at various low temperatures for P3HT based hole-only devices were fitted with the empirical model and the Gaussian disorder model (GDM) to interpret the data. X-ray diffraction (XRD) measurements confirmed increase in crystallinity and crystallite size of the films for electric-field annealed samples. The observed changes in the charge transport properties due to electric-field annealing of the films at the time of their fabrication have been explained based on the above measurements and analysis. Solar cells in inverted structure have also been made with appropriate modification in device structure to get better performing devices by electric-field annealing. Finally, the effect of electric field assisted treatments was studied on devices fabricated with PTB7 (a new promising polymer for organic solar cells) as the device active layer. The experimental results presented in this thesis confirm that application of electric-field during annealing of organic films during their formation is a useful processing step to fabricate higher mobility polymer films for building improved organic electronic devices.


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Gangadhar Behera
Prof. S. Anantha Ramakrishna
Y9109065
Nanostructured plasmonic thin films for enhanced optical properties.
20/04/2016 (Wednesday)
4.00 PM
FB382
The unique optical properties of nano-structured plasmonic films have attracted great attention due to their potential applications in solar cells, photo-detectors, sensors, nano-imaging devices, thermal emitters and many more. Surface plasmon polaritons (SPPs) are surface electromagnetic waves that exist at the interface between a metal and a dielectric material. In this work, the significant role of the surface plasmons in novel optical phenomena like enhanced transmission or enhanced absorption in nano-structured plasmonic thin films is investigated in detail. The extra-ordinary transmission of light through an array of holes in thin plasmonic films for use as conducting transparent electrodes with enhanced functionality due to large local fields is proposed. By experiments and simulations, these structured metallic electrodes are also shown to enhanced absorption for solar cell applications. Complementary layers of ladder-like plasmonic structures fabricated by laser interference lithography (LIL) are investigated for enhanced optical properties in the visible-IR bands. Possible applications as polarization dependent sensors at IR frequencies is also discussed. Enhanced absorption from a trilayer plasmonic system consisting of structured hole arrays in gold film separated from the bottom gold layer by dielectric spacer is reported. The fabrication by LIL and optical characterization of these samples are presented. A new approach to design dual band perfect absorber in the visible to the NIR region with top metallic patches on a SiO2 coated Si substrate is reported. A physical model for these absorbers is presented. A combined structuring of metallic disc and grating arrays on glass substrates that give rise to triple-band perfect absorption at visible frequencies is also reported. The physics behind these enhanced phenomena are discussed. Further, a system consists of metal-dielectric multilayers grating structures that show polarization dependent and independent unit absorption at visible to near-infrared frequencies are also discussed. The results have good potentialfor realizing low-cost large area nanostructured plasmonic thin film devices for different device applications.


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Reeta Pant
Prof. Krishnacharya
Y9209066
Investigation of Tunable Wetting and Slippery Behaviour on Hydrophilic Surfaces.
19/04/2016 (Tuesday)
11.00 AM
FB382
Interaction of a liquid with a solid surface is one of the most fundamental surface/interfacial phenomenon which primarily governs the wetting behaviour of the liquid-solid system. The surface wettability can be controlled by manipulating the surface free energy of the solid and/or liquid-solid interfacial energy with passive or active methods. Reversible switching of surface wettability using polystyrene/titania nanocomposite based responsive surfaces upon ultraviolet (UV) exposure and annealing will be discussed. As prepared nanocomposite coating shows hydrophilic behavior which become superhydrophoboc upon annealing at 180 degree for 90 mins. This happens due to increase in surface roughness during annealing. Subsequently upon UV exposure, the superhydrophobicity slowly decrease and the samples become superhydropholic with almost 0 degree water contact angle. The UV exposed superhydrophilic samples recover their superhydrophobicity upon annealing again at 180 degree. Detailed investigation of static and dynamic wetting transition will also be presented. Subsequently superhydrophobic and lubricating fluid infused slippery surfaces will be discussed. Based on energy minimization, stability criteria for lubricating fluid infused slippery surfaces will be established. Initially we observed that slipping water droplets sink into lubricating silicone oil layer due to strong polar interaction between hydrophilic substrate and water molecules. This sinking can be prevented by using hydrophobic surfaces. Annealing silicone oil coated substrates makes the surface hydrophobic. Optimization of annealing parameters will be discussed in detail. Characterization of the stable slippery surfaces using contact angle hysteresis and slip velocity measurements will be presented. Subsequently, I will discuss the effect of substrate roughness on the stability of slippery surfaces. It was found that small nanoscale roughness enhances the stability of the lubricating fluid as well total slippery behavior. Fabrication of stable slippery surfaces based on nanostructured formed by nanoparticles enable us to get rid of the hydrophobic surface requirement. Hydrophilic silica and titania nanoparticles were used to cast nanostructured film which after coating with silicone oil show stable slippery behavior. Towards the end, I will discuss about the effect of surface wettability on the lubricating film thickness underneath a test drop. Electrical measurements were used to measure the thickness of the lubricating film underneath a test drop.


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Dheeraj Pratap
Prof. S. Anantha Ramakrishna
Y9109063
Anisotropic and Hyperbolic Metamaterials in the Cylindrical Geometry.
25/03/2016 (Tuesday)
11.00 AM
FB382
Metamaterials are artificially designed composite materials that are structured on subwavelength lengthscales. They are often inherently optically anisotropic, having direction dependent properties, due to their structure. Acid anodization of an aluminium micro wire was used to fabricate a nanoporous alumina microtube with non-branching nanopores aligned along the radial direction. This is essentially a metamaterial fiber with anisotropy in the cylindrical geometry. These nanoporous alumina micro tubes show enhanced optical and thermal properties, of which a study of the optical properties is presented here. Not only are these nanoporous alumina structures anisotropic, but they are radially inhomogeneous as well due to the radius dependent size of the nanopores. These metamaterials were homogenized to obtain the electromagnetic effective medium parameters (dielectric permitivity tensor)using techniques of transformation optics and Maxwell-Garnett homogenisation theory. Nanowires of silver were deposited electrochemically within the nanopores of the cylindrical nanoporous alumina to yield a metamaterial with hyperbolic dispersion. These metamaterial fibers exhibit novel optical modes that are described by Bessel functions of fractional and imaginary orders solely due to the anisotropy. Particularly for the high transverse momentum modes (whispering gallery modes), these modes are confined increasing near the centre of the waveguide in sheer contrast to conventional fibers where they localise to the edge. The inhomogeneous nature of the metamaterial fibers also cause extremely strong confinement of the modes near the center. The nanoporous alumina microtubes coated with active dye molecules of Rhodamine-6G show highly enhanced fluorescence, which is highly quenched in the presence of silver nanowires within the nanopores. Other studies, such as the possibility of manipulating the structure of the nanopores and a study of fluorescence from molecules deposited on nanoporous alumina, in the flat geometry were also carried out.


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Abhishek Kumar
Mahendra K. Verma
11109062
Energy spectra and fluxes of buoyancy-driven turbulent flows
23/01/2017 (MON)
3:00 PM (Tea at 2:45 PM)
FB 382 (Physics seminar room)
Buoyancy-driven turbulent flows are often encountered in geophysics, astrophysics, atmospheric and solar physics, and engineering. An important unsolved problem in this field is how to quantify the small-scale quantities, e.g., spectra and fluxes of kinetic energy (KE) and potential energy (PE) of these flows. Using ideas of energy flux in direct numerical simulation (DNS) and shell model simulation, we show that the KE spectrum for stably stratified flows has −11/5 spectral exponent, while Rayleigh-Bénard convection (RBC) has Kolmogorov’s −5/3 spectral exponent. For RBC, we show the applicability of Taylor’s hypothesis in the presence of steady large-scale circulation in a cubical geometry. Our simulations indicate that a cubical geometry is better suited for spectral studies than a cylindrical one due to the steady nature of large-scale circulation in a cube. We also show a modified Bolgiano-Obukhov spectrum for the velocity field, and the presence of internal gravity waves at large-scale in two-dimensional stably stratified turbulence.


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Ambrish Pandey
Mahendra K. Verma
10109872
Scaling of large-scale quantities in Rayleigh-Bénard convection
06-Oct-2016
11:30 AM (Tea at 11:15 AM)
FB 382 (Physics seminar room)
Thermal convection plays an active role in many natural systems, e.g., in stars, Earth's outer core and mantle, atmosphere, etc. Convection has many applications in industrial processes as well. An idealized model of thermal convection is the Rayleigh-Bénard convection (RBC) in which a fluid, confined between two horizontal plates, is heated from below and cooled from top. Dynamics of the RBC is governed primarily by two nondimensional parameters – the Prandtl number (Pr) and the Rayleigh number (Ra). The response parameters are the transport of heat and momentum, which are quantified as the Nusselt (Nu) and the Reynolds numbers (Re) respectively. We study the scaling of Nu(Ra,Pr) and Pe(Ra,Pr) using direct numerical simulations and derive a formula to analytically compute the Péclet number for a given Ra and Pr that match well with simulation and experimental data. We observe that Nu and Pe follow different power laws in the viscous and the turbulent regimes. We also study the scaling of kinetic energy and entropy spectra for very large Prandtl numbers, and observe that the kinetic energy spectrum does not follow the Kolmogorov scaling, instead it scales with wavenumber (k) as k-13/3. The entropy spectrum exhibits dual branches in the inertial range. Moreover for very large Prandtl numbers, we observe that Nu and Pe, as well as the energy and entropy spectra scale similarly in three- and two-dimensions


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Mr. Ravi Pratap Singh
R K Thareja
10109068
Time resolved investigations of laser ablated single and colliding carbon plasma/plumes.
May 20, 2016 (Friday)
11:00 AM onwards
FB 382 (Physics Department Seminar Room)
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Shraddha Sharma
Amit Dutta
Y9209067
Fidelity and Loschmidt echo: quenches, non-analyticities and emergent thermodynamics
2nd May (Monday), 2016
10 AM
FB 382 (Physics seminar room)
Our work focuses on relating quantum information theoretic measures such as the ground state fidelity and the Loschmidt echo (LE) to quantum phase transitions (QPTs) and dynamics. Fidelity is the modulus of the static overlap of the two ground states of a system at different parameter values; whereas, the LE is the modulus of the overlap of two states evolved from the same initial state with two different Hamiltonians. Therefore, one can map a connection between the two; both the tools effectively detects the quantum critical point (QCP) and follows universal scaling behavior close to it.

We shall discuss the marginal behavior of fidelity and LE and explain how logarithmic scaling behavior enters in the scaling of fidelity in such situations. Furthermore, we shall relate LE to dynamical fidelity (obtained for a periodically driven quantum Hamiltonian), dynamical phase transitions. Exploring the finite temperature LE, we shall further study the emerging thermodynamics and analyze the behavior of the average irreversible work and the irreversible entropy in a many body quantum system following a sudden quench.


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Mr. Dushyant Kumar
Prof. R.C. Budhani and Prof. Z. Hossain
Y9109064
Spin and Electronic Charge Diffusion in Superconducting NbN and Disordered SrTiO3 Under the Influence of Magnetic Field and Light
April 28, 2016 (Thursday)
10:00 AM (Tea at 09:45 AM)
FB 382 (Physics Seminar Room)
Spin-based electronics generally called “spintronics” encodes and processes information in the quantum-mechanical spin of the electrons leading to reduced power consumption with faster operation. However, the generation of pure spin currents, its transport and detection remain a challenge. A comprehensive understanding of these phenomena requires study of spin diffusion in metals, semiconductors, and superconductors. It has been of fundamental interest to search for the sources of pure spin currents and the materials which can allow long spin diffusion lengths. In this thesis work we have addressed both these issues. We used the full Heusler alloy Co2MnSi (CMS) as a source of polarized spins. This Heusler compound is a half-metallic ferromagnet. Such a band structure suggests 100% spin polarization, thus making the ordered cubic phase of Co2MnSi a good candidate for spintronic devices. As for the material for efficient transport of spin current, we have concentrated on SrTiO3, a cubic band insulator well known for its useful dielectric and optoelectronic properties. The high mobility (∼2 x 103 cm2/Vs at 15 K) electron gas was induced in the few nanometer thickness of STO by irradiating its surface using Ar+-ions. The process converts ~4 nm thick surface layers of STO into a metal while over the remaining thickness of a 0.5 mm wafer remains insulating. The unperturbed base material has been used as a gate dielectric in a back gated geometry. For device perspective of the surface electron gas of reduced STO, it is desirable to study its response to photon and electrostatic fields. In this connection, we have studied the photoconductivity (PC) and photoluminescence (PL) of ion irradiated STO and tuned them using electrostatic gate field.


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Nikhil Kumar
Prof. Anjan Gupta
Y9109074
Controlling thermal hysteresis in superconducting weak links and micron size superconducting quantum interference device.
April 27, 2016
11:00 AM onwards
FB 382 (Physics Department Seminar Room)
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Gangadhar Behera
S. Anantha Ramakrishna
Y9109065
Nanostructured plasmonic thin films for enhanced optical properties
20 April 2016 (Wednesday)
4.00 p.m. (Tea will be served at 3.45 p.m.)
FB 382 (Physics department conference room)
TThe unique optical properties of nano-structured plasmonic films have attracted great attention due to their potential applications in solar cells, photo-detectors, sensors, nano-imaging devices, thermal emitters and many more. Surface plasmon polaritons (SPPs) are surface electromagnetic waves that exist at the interface between a metal and a dielectric material. In this work, the significant role of the surface plasmons in novel optical phenomena like enhanced transmission or enhanced absorption in nano-structured plasmonic thin films is investigated in detail. The extra-ordinary transmission of light through an array of holes in thin plasmonic films for use as conducting transparent electrodes with enhanced functionality due to large local fields is proposed. By experiments and simulations, these structured metallic electrodes are also shown to enhanced absorption for solar cell applications. Complementary layers of ladder-like plasmonic structures fabricated by laser interference lithography (LIL) are investigated for enhanced optical properties in the visible-IR bands. Possible applications as polarization dependent sensors at IR frequencies is also discussed.

Enhanced absorption from a trilayer plasmonic system consisting of structured hole arrays in gold film separated from the bottom gold layer by dielectric spacer is reported. The fabrication by LIL and optical characterization of these samples are presented. A new approach to design dual band perfect absorber in the visible to the NIR region with top metallic patches on a SiO2 coated Si substrate is reported. A physical model for these absorbers is presented. A combined structuring of metallic disc and grating arrays on glass substrates that give rise to triple-band perfect absorption at visible frequencies is also reported. The physics behind these enhanced phenomena are discussed. Further, a system consists of metal-dielectric multilayers grating structures that show polarization dependent and independent unit absorption at visible to near-infrared frequencies are also discussed. The results have good potential for realizing low-cost large area nanostructured plasmonic thin film devices for different device applications.


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Ms. Reeta Pant
Krishnacharya
Y9209066
Investigation of Tunable Wetting and Slippery Behavior on Hydrophilic Surfaces
519 April 2016 (Tuesday)
11:00 AM (Tea will be served at 10:45 AM)
FB382 (Physics Seminar Room)
Interaction of a liquid with a solid surface is one of the most fundamental surface/interfacial phenomenon which primarily governs the wetting behavior of the liquid-solid system. The surface wettability can be controlled by manipulating the surface free energy of the solid and/or liquid-solid interfacial energy with passive or active methods. Reversible switching of surface wettability using polystyrene/titania nanocomposite based responsive surfaces upon ultraviolet (UV) exposure and annealing will be discussed. As prepared nanocomposite coating shows hydrophilic behavior which become superhydrophobic upon annealing at 180°C for 90 mins. This happens due to increase in surface roughness during annealing. Subsequently upon UV exposure, the superhydrophobicity slowly decreases and the samples become superhydropholic with almost 0° water contact angle. The UV exposed superhydrophilic samples recover their superhydrophobicity upon annealing again at 180°C. Detailed investigation of static and dynamic wetting transition will also be presented. Subsequently superhydrophobic and lubricating fluid infused slippery surfaces will be discussed. Based on energy minimization, stability criteria for lubricating fluid infused slippery surfaces will be established. Initially we observed that slipping water droplets sink into lubricating silicone oil layer due to strong polar interaction between hydrophilic substrate and water molecules. This sinking can be prevented by using hydrophobic surfaces. Annealing silicone oil coated substrates makes the surface hydrophobic. Optimization of annealing parameters will be discussed in detail. Characterization of the stable slippery surfaces using contact angle hysteresis and slip velocity measurements will be presented. Subsequently, I will discuss the effect of substrate roughness on the stability of slippery surfaces. It was found that small nanoscale roughness enhances the stability of the lubricating fluid as well total slippery behavior. Fabrication of stable slippery surfaces based on nanostructured formed by nanoparticles enable us to get rid of the hydrophobic surface requirement. Hydrophilic silica and titania nanoparticles were used to cast nanostructured film which after coating with silicone oil show stable slippery behavior. Towards the end, I will discuss about the effect of surface wettability on the lubricating film thickness underneath a test drop. Electrical measurements were used to measure the thickness of the lubricating film underneath a test drop.


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Rohit Kumar
Prof. Mahendra K. Verma
10109875
Energy transfers in dynamos with small and large magnetic Prandtl numbers
03.11.2015
2:30 PM
FB 382 (Physics Seminar Room)
The presence of magnetic field in celestial bodies is explained by dynamo mechanism in which a conducting fluid in motion generates a self-sustained magnetic field. In dynamo, energy is transferred form velocity field to magnetic field and consequently the growth of magnetic energy takes place. The magnetic Prandtl number (Pm), the ratio of kinematic viscosity and magnetic diffusivity, is an important non-dimensional parameter for dynamos. We study energy transfers in dynamos with small- and large-Pm by performing direct numerical simulations (DNS) in a 3D periodic box on 10243 grid, using a pseudo-spectral solver Tarang. Energy fluxes and shell-to-shell energy transfers show that for dynamo with large-Pm, the growth of magnetic energy takes place due to nonlocal energy transfers from large-scale velocity field to small-scale magnetic field. For dynamo with small-Pm on the other hand, the magnetic energy grows due to local energy transfers from large-scale velocity field to large-scale magnetic field. We also use a shell model of dynamo to understand the energy transfers for extreme values of Pm, which are otherwise inaccessible to DNS due to the computational constraints. The energy fluxes in our shell model simulation are in qualitative agreement with the DNS results. We also construct a low-dimensional model to study the dynamo transition for very small to very large Pm and observe that the critical magnetic Reynolds number for dynamo transition, Rmc, saturates to constant values in the two limiting cases of Pm.


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Mr. Shubhankar Das
R.C. Budhani and Zakir Hossain
Y9109078
Magnetoresistance and Magnetothermopower studies of delta-doped 2-dimensional electron gas at the interface of LaTiO3/SrTiO3
1st April (Friday), 2016
8:45 AM
FB 382 (Physics Seminar Room)
Studies of the 2-dimensional electron gas (2DEG) at the interface of LaAlO3 (LAO) or LaTiO3 (LTO) and TiO2 terminated SrTiO3 (STO) has attracted much attention in recent years due to its interesting properties like metal-insulator transition, magnetism, superconductivity and strong spin-orbit interaction effects in the electronics transport. We have used a new approach to bring about modification in the electronic properties of this 2DEG. This involves delta (δ) doping at the interface of LTO/STO by an iso-structural antiferromagnetic perovskite (TN = 298 K) LaCrO3 (LCO). This δ-layer dramatically alters the properties of the 2DEG. The changes includes increase in room temperature sheet resistance (R□), drop in the sheet carrier density (n□) almost exponentially with the layer thickness, and emergence of new features in the temperature dependence of R□ at T ≤ 50 K. Our spectroscopic measurement along with density functional theorem (DFT) calculations show that the Cr-ions at the interface act like a trap for electrons which are transferred from the LTO to STO surface. Extensive measurements of out-of-plane and in-plane magnetoresistance (MR) have been carried out on all the samples to address issue such as weak antilocalization and Kondo scattering. We have also observed a gradual crossover from positive out-of-plane MR to negative in-plane MR when magnetic field is titled with respect to the film surface.

The MR measurements are augmented by the measurement of thermopower (S) which increases dramatically with δ-layer thickness at ambient temperature. The linear temperature dependence of S in the temperature range 100 to 300 K is indicative of diffusion thermopower. We also observed a large enhancement in thermopower in the temperature range where a minimum in R□ is observed. This enhancement is attributed to Kondo scattering. The thermopower is suppressed in the presence of a magnetic field and the suppression is isotropic with respect to the field direction.

We will also present a tunable Rashba S-O interaction in these interfaces by δ-doping with another iso-structural ferromagnetic perovskite LaCoO3 (LCoO). In LCoO-doped sample, the inelastic scattering time varies as 1/T and the S-O scattering time remains constant in temperature, which suggests that the spin relaxation follows the D’yakonov-Perel mechanism. The δ-doping also results in 3 orders of magnitude decrease in τso whereas the inelastic scattering time increases very slowly with doping. A detailed analysis of anisotropic MR when the field is applied in the plane of the sample displays the effect of mixing of spin-up d-states and spin-down d-state at Fermi level due to spin-orbit interaction.

Reference: 1. Shubhankar Das et al., Phys. Rev. B 90, 081107(R) (2014). 2. Shubhankar Das et al., Phys. Rev. B 90, 075133 (2014). 3. Shubhankar Das et al., Under review in Phys. Rev. B.


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Gopal
Pankaj Jain
Y9109066
Cosmological and Colliders Implications of Conformal Symmetry
21-3-2016 (Monday)
4.00 p.m. (Tea @ 3.45 p.m.)
FB 382 (Physics Conference room)
Symmetry has always played an important role in understanding Nature. At present most of our theoretical knowledge comes from the symmetry principles. In this talk I will discuss the implications of scale or conformal symmetry. In particular, I will propose a possible solution to the fine tuning problem of cosmological constant within the framework of softly broken conformally symmetric model. Cosmological constant is potentially a source of Dark energy, which constitutes about 70% of the energy density of the Universe. A major problem with the cosmological constant is that it gets very large quantum contributions from the matter sector at each order in perturbation theory. So we need to cancel these large contributions in order to maintain the small value of observed dark energy density, leading to an acute fine tuning problem. In our proposed solution we have shown that the matter sector will not contribute to the cosmological constant and hence we have solved the fine tuning problem. Here we have not considered the quantum gravity effects since quantum theory of gravity is not well formulated.


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Dheeraj Pratap
Prof. S. Anantha Ramakrishna
Y9109063
Anisotropic and Hyperbolic Metamaterials in the Cylindrical Geometry
15 Mar. 2016 (Tuesday)
11 a.m. (Tea will be served at 10:45)
L6, Lecture hall complex
Metamaterials are artificially designed composite materials that are structured on subwavelength lengthscales. They are often inherently optically anisotropic, having direction dependent properties, due to their structure. Acid anodization of an aluminium micro wire was used to fabricate a nanoporous alumina microtube with non-branching nanopores aligned along the radial direction. This is essentially a metamaterial fiber with anisotropy in the cylindrical geometry. These nanoporous alumina micro tubes show enhanced optical and thermal properties, of which a study of the optical properties is presented here.

Not only are these nanoporous alumina structures anisotropic, but they are radially inhomogeneous as well due to the radius dependent size of the nanopores. These metamaterials were homogenized to obtain the electromagnetic effective medium parameters (dielectric permitivity tensor)using techniques of transformation optics and Maxwell-Garnett homogenisation theory. Nanowires of silver were deposited electrochemically within the nanopores of the cylindrical nanoporous alumina to yield a metamaterial with hyperbolic dispersion. These metamaterial fibers exhibit novel optical modes that are described by Bessel functions of fractional and imaginary orders solely due to the anisotropy. Particularly for the high transverse momentum modes (whispering gallery modes), these modes are confined increasing near the centre of the waveguide in sheer contrast to conventional fibers where they localise to the edge. The inhomogeneous nature of the metamaterial fibers also cause extremely strong confinement of the modes near the center. The nanoporous alumina microtubes coated with active dye molecules of Rhodamine-6G show highly enhanced fluorescence, which is highly quenched in the presence of silver nanowires within the nanopores. Other studies, such as the possibility of manipulating the structure of the nanopores and a study of fluorescence from molecules deposited on nanoporous alumina, in the flat geometry were also carried out.


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Jhuma Dutta
Prof. S. Anantha Ramakrishna and Prof. Harshwardhan Wanare
Y9109069
Photonic and Plasmonic properties of Periodically Patterned Columnar Thin Films
19 Feb. 2016 (Friday)
11 a.m. (Tea will be served at 10:45)
L8, Lecture hall complex
Columnar thin films are known to be deposited by making incident a collimated vapor flux at large oblique angles to a substrate. Using a substrate that is periodically patterned at micro/nanoscales, new kinds of periodically patterned columnar thin films (PP-CTFs) have been fabricated. The extremely anisotropic optical properties offered by the nanocolumnar structure with larger scale structures for photonic bandgap or diffractive effects provide for new physical phenomenon. These structures were also found to be extremely reconfigurable giving rise to enormous flexibility in applications.

PP-CTFs are shown to have enhanced photonic and plasmonic properties. Dielectric PP-CTFs are shown to function as blazed diffraction gratings with large asymmetric diffraction efficiencies. A CTF made of plasmonicmetals like silver renders it an effectively biaxially anisotropic continuum. PP-CTFs of silver showed strong blazing action and unidirectionally coupled optical radiation to surface-plasmon-polariton(SPP) waves for both p- and s-polarizations.

The blazing effect of the gratings of dielectric CTFs as a result of the spatially linear phase shifts caused by prismatic air cavities was understood using Kirchhoff-Fresnel diffraction theory. Homogenization of the metallic CTFs using the Bruggeman formalism revealed them to display hyperbolic dispersion, and the dispersion of SPP waves on both 1D and 2D gratings of such anisotropic hyperbolic media was found to be adequately described thereby. Detailed electromagnetic simulations of the grating structures reveal large changes in the photonic properties with the slant angle such as diffraction efficiencies and the electromagnetic near fields. Furthermore, these slanted nanocolumnar structure can be uniformly reconfigured by ion beam irradiation method and gives rise to reconfigurable blazed gratings, thereby maximizing the diffraction efficiencies for different wavelength bands by changing the blazing (angle) condition. A novel application of CTFs to fingerprint visualization was developed. Visualization of latent fingerprints is enhanced by deposition of columnar thin films at large oblique angle of CaF2 and SiO2 on fingerprint marks on nonporous surfaces and further enhanced visualization is obtained by treating the deposited CTFs with a fluorescent dye and fluorescence imaging.


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Jagtap J Manihrao
Prof. V. Subhramanyam and Prof. Asima Pradhan
Y9109074
Mueller matrix spectroscopy imaging and image analysists techniques for human cervical pre-cancer discrimination.
17.12.2015
11:00 AM onwards
FB 382 (Physics Department Seminar Room)
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Vandana Yadav
Prof. V. Subrahmanyam
Y9109082
Steady state properties of nonequilibrium growth and transport processes: A boundary layer based approach
14 December, 2015
2:30 PM
FB 382
Microtubules are important components of the cytoskeleton of a cell. These are hollow,tube shaped biopolymers made of alpha, beta-tubulin heterodimers. Experimental studies reveal an interesting growth dynamics of microtubule which play an important role in various processes such as intracellular organization, transport, exerting pushing and pulling forces etc., it is crucial to understand the growth dynamics of microtubules and its regulations by various regulatory proteins. It has been predicted experimentally that motor proteins belonging to Kinesin-5 and Kinesin-8 family can regulate the dynamics of microtubule in a length dependent manner. Here I discuss a two-state model of microtubule polymerization with a length-dependent dynamics and obtain the length distributions of microtubules through a discrete formulation. These investigations suggest that the length dependent regulation leads to a much faster decay of distributions as compared to an exponential decay.

In the latter part, I shall discuss about the boundary layer technique and show how it can be used to understand phases and phase transitions of driven many-particle exclusion processes. We develop the fixed point based boundary layer method and apply this to two distinct driven exclusion models to show how the shape of the particle distribution profile, the location of the boundary layer etc can bepredicted from the stability properties of the fixed points of the boundary layer equation. Further, I shall discuss the applicability of boundary layer method in order to understand the polymerization dynamics in the presence of diffusing tubulins. Since various rates associated with the polymerization dynamics are expected to be influenced by the number of available tubulins, the growth dynamics and the diffusive dynamics of tubulins are coupled in a nonlinear way. The boundary layer analysis emerges as a powerful method that allows us to obtain analytical solutions for the length distributions in a systematic way.


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Mr. Sourabh Barua
Prof. K P Rajeev
Y7109075
Electrical Transport Studies of Topological Insulator Materials Bi2Te3, Sb2Te3 and Bi2-xSbxTe3-ySey
Monday, 30 Nov 2015
10 AM
FB 382
Topological insulators have generated a lot of interest because of the revolutionizing properties predicted for these materials. These materials are supposed to have a conducting surface and an insulating bulk and the surface carriers have spin locked to their momentum. The surface conduction is furthermore topologically protected from disorders as the surface bands traversing the bulk band gap cross at the time reversal invariant momenta and these crossings are guaranteed unless time reversal symmetry is broken by application of a magnetic field or magnetic impurities. However, experimental verification of the surface conduction from these topological surface states in electrical transport studies has been difficult because of the bulk conduction in these materials due to crystalline defects. The focus of this thesis is the electrical transport studies of single crystals of topological insulator materials: Bi2Te3, Sb2Te3 and Bi2-xSbxTe3-ySey. In case of Bi2Te3, even though the sample was metallic, we were able to obtain signatures of the topological surface states from the Shubnikov-de Haas (SdH) oscillations in the magnetoresistance, such as a non-zero Berry phase, magnitude of surface conduction similar to that expected for topological insulators and also large and linear magnetoresistance. We also performed angle dependent magnetoresistance studies which have revealed that the SdH oscillations do not show the angle dependence expected for a two dimensional Fermi surface of the topological surface states. Further, the angle dependence of the frequency of the oscillations does not adhere to the behavior expected for the conventional three dimensional bulk Fermi surface in this material. We also observe asymmetry in the magnetoresistance in positive and negative fields, which has been attributed to anisotropic bulk transport earlier, but we show that such asymmetry can arise due to the mixing of Hall signal with longitudinal resistance in this material. In Sb2Te3, we observed prominent SdH oscillations although the Berry phase extracted was inconclusive in determining whether these were from the surface states. In Bi2-xSbxTe3-ySey, the single crystals obtained were metallic and these showed linear magnetoresistance. We also made a simple model of the dependence of resistivity on thickness for a topological insulator and compared it to data for various topological insulators published in the literature.


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Anirban Bagui
Prof. S. Sundar Kumar Iyer
Y7109861
Effect of electric-field annealing during solvent drying step of
active layer in organic solar cell devices
5th August, 2015
12 noon
SCDT seminar room
The performance of organic electronic devices can often be improved by improving the nano-morphology of organic active layers in those devices. In this thesis, the role of thermal annealing of polymer layers during solvent drying in the presence of a constant electric-field to improve the the nano-morphology of the films thus formed was studied.

Comparative studies of morphological and electro-optical properties of the P3HT:PC61BM based blend film and bulk hetero-structure solar cell devices were carried out to understand the role of 'electric-field annealing'. The external quantum efficiency (EQE) and power conversion efficiency (PCE) of the solar cells made showed significant enhancement due to better charge transport in the case of electric-field annealed devices.

In order to study this process step on the hole mobility in the P3HT layer, devices with holes as the primary charge carriers were fabricated. The hole mobility in P3HT was found to increase monotonically for annealing field strengths up to 2000 V/cm. The current density – voltage (J–V) data corresponding to the space charge limited currents (SCLC) at various low temperatures for P3HT based hole-only devices were fitted with the empirical model and the Gaussian disorder model (GDM) to interpret the data. X-ray diffraction (XRD) measurements confirmed increase in crystallinity and crystallite size of the films for electric-field annealed samples. The observed changes in the charge transport properties due to electric-field annealing of the films at the time of their fabrication have been explained based on the above measurements and analysis.

Solar cells in inverted structure have also been made with appropriate modification in device structure to get better performing devices by electric-field annealing. Finally, the effect of electric–field assisted treatments was studied on devices fabricated with PTB7 (a new promising polymer for organic solar cells) as the device active layer. The experimental results presented in this thesis confirm that application of electric-field during annealing of organic films during their formation is a useful processing step to fabricate higher mobility polymer films for building improved organic electronic devices.


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Pranati Kumari Rath
Prof. Pankaj Jain
Y9109077
Large Scale Anisotropy in Cosmic Microwave Background Radiation and its Relationship with Primordial Power Spectrum
27th July, 2015
4 pm
FB-382
The Cosmological principle says that the universe at large distance scale is assumed to be statistically homogeneous and isotropic. The Cosmic Microwave Background Radiation (CMBR) is considered to be one of the best experimental evidence supporting this principle. However, there exists considerable evidence in cosmological data which suggests violation of this principle.

In this talk, I will revisit two of these anomalies observed in the CMBR temperature data provided by WMAP and recently by PLANCK team. These includes the alignment of the quadrupole (l=2) and octopole (l=3) and the hemispherical power asymmetry. To explain the low-l alignment, I will discuss some anisotropic inflationary models within the framework of the Big Bang cosmology. In these models, the anisotropy decays very quickly during the inflationary phase of expansion. The resulting direction dependent power spectrum in this anisotropic background leads to violation of isotropy and hence explain the alignment of the low l CMBR modes.

I will also discuss an inhomogeneous power spectrum model in order to explain the hemispherical power asymmetry, observed in the CMBR data. The hemispherical asymmetry can be parametrized in terms of the dipole modulation model. Alternatively, I will show that, an anisotropic dipolar imaginary primordial power spectrum, which is possible within the framework of noncommutative space-times, also provides a good description of the observed dipole modulation in CMBR data.


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Shail Pandey
Prof. Sudeep Bhattacharjee
Y7209063
Pulse modulated microwave plasmas : self-excited instability
and plasma states
3rd July, 2015
4.30 pm
L-5
Waves and instabilities are inherent to plasma and have been a widely studied subject owing to its importance in electron acceleration, wave absorption, and plasma heating. While most of the earlier studies discuss these in a preformed steady-state plasma, limited studies exist on excitation of waves and instabilities in pulsed plasmas. In the pulsed regime, large amplitude electromagnetic pulses can interact with a self-generated growing plasma, leading to the formation of instabilities. The present thesis carries out an experimental investigation of the plasma state created by pulsed electromagnetic waves in the microwave regime and excitation of instabilities due to counter streaming charged particles in the ionization front.

Pulsed microwaves of 2.45 GHz are launched in a low pressure argon (0.2 – 2.5 mTorr) gas to generate a temporally growing ionization front which subsequently develops into a full fledged plasma. Depending upon the peak power and time duration of the pulses, plasmas are characterized by an afterglow state (AGP) or an interpulse state (IPP). It is found that : (i) the AGP state characterized by a temporally decaying plasma with a low value of electron temperature ~ 1 eV, occurs at larger pulse duration > 50 microseconds and (ii) the IPP state, where the plasma grows beyond the pulse duration, often forming a quasi-steady state in the power off phase, is realized at shorter pulse duration. The distinction between the two is further investigated by measurements of the electron energy probability function (EEPF).

The IPP state is governed by plasma growth within the pulse, where strong interaction with the injected microwaves can be realized leading to formation of instabilities, which is investigated next. The measured particle current profile shows two phases of plasma development: (i) self-excited wave (SEW) in the ionization front, followed by (ii) rapid plasma growth. The growth rate of the SEW comes out to be ~ (1.0 – 4.5) × 10^7 s^-1, depending upon the pressure, which is further confirmed with optical emission spectroscopy measurements. Time – Frequency Analysis (TFA) indicates the presence of two frequency bands (~ 4 MHz) centered around 3.8 MHz and 13.0 MHz.

The profile of electron thermal velocity v_th(t) and relative charged particle drift velocities V_D(t) in the SEW, obtained from the measured EEPF, indicate V_D > v_th – a condition necessary for excitation of Buneman instability (BI). The calculated growth rate and oscillation frequency of BI further confirms its existence in the SEW. Finally, the energy transfer from the instability leading to the heating of the plasma during the pulse is investigated.


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Subhash Mahapatra
Prof. Tapobrata Sarkar
10109069
Applications of the Gauge/Gravity duality : Superconductivity,
Optics and Entanglement Thermodynamics.
1st June, 2015
10 am
L-6
A remarkable connection, in the context of string theory, has emerged in last few years between gravity and the strongly coupled field theories. This connection, which is generally called as the gauge/gravity duality or the AdS/CFT correspondence, maps a quantum theory of gravity in (D+1) dimension Anti de sitter (AdS) spacetime to D dimensional quantum field theory - living at the boundary of AdS spacetime. An important feature of this duality is that in certain approximation, the strongly coupled limit of field theory corresponds to the weakly coupled limit of the gravity theory. This strong-weak nature of the correspondence has provided a unique approach to address some questions in strongly coupled condensed matter systems which otherwise would be intractable.

In this thesis, we discuss important applications of the gauge/gravity duality, on three main directions. It includes the phenomenon of superconductivity, optics and entanglement thermodynamics.

In this thesis work, we have studied and developed various generalized models for holographic superconductors and have computed response functions of these generalized superconductors using the gauge/gravity duality. We have been able to show that the occurrence of negative refractive index is a generic feature of holographic superconductors for specified ranges of frequencies. For these superconductors, we also showed that the holographic entanglement entropy is a good measure to probe different phases of the system. We further found a indication of the first law of thermodynamics like relation in the context of entanglement entropy for holographic superconductors.


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Anukul Prasad Parhi
Prof. S. Sundar Kumar Iyer
Y5209061
Annealing in the presence and absence of electric field of copper phthalocyanine based thin films and their applications in organic solar cells
18th May, 2015
6 pm
Samtel seminar room
The ability to tailor nano morphology by influencing the growth and crystallization process of organic thin films is desired for various applications. Annealing is known to help in modifying thin film textures and properties and is easier to implement during device fabrication. In this thesis, films of copper phthalocyanine (CuPc) and their blends have been subjected to thermal annealing in the presence and absence of electric field. The films were then characterized by various methods including x-ray diffraction (XRD), scanning electron microscope (SEM) and by taking the film’s absorption spectrum. These were analysed to explain the effect of the annealing of the films.

Initially, pristine CuPc films were annealed at different temperatures, electric fields and annealing ambiance - in vacuum as well as in nitrogen. It was observed that there was enhanced re-crystallisation by annealing at higher temperatures, in higher electric fields and in vacuum.

Blends of CuPc with buckminsterfullerene (C60) films of different volume ratios (3:1, 1:1 and 1:3) were then annealed at different temperatures both in the presence and absence electric field. While the blending inhibited re-crystallisation (when compared to pristine films of CuPc), above certain temperatures, the two constitutes, CuPc and C60, were phase-segregated. The phenomenon of abrupt phase segregation was observed in 1:1 and 1:3 blends under electric field. Analyses of the absorption spectra carried out confirm blending and phase-segregation of the two constitutes in the film during annealing. Using density functional theory (DFT), the poloarisability of the constituent molecules were calculated and the trends in the intermolecular attraction and segregation were explained.

Bi-layer organic solar cells were fabricated with CuPc and C60 films. The effect of annealed CuPc films and CuPc-C60 bi-layer films on the solar cell performance and their current-voltage characteristics were analysed. It was observed that annealing of the bi-layer stack resulted in improved solar cell performance. However, the application of electric field during annealing of the bilayers does not significantly add to the improvement in performance of the solar cells observed due to thermal annealing.


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Bahadur Singh
Y9209061
First-principles investigations of topological phases in materials
11th May, 2015
11 am
FB-382
Prof. Rajendra Prasad
A new paradigm for classifying condensed matter systems by topology of bulk band structure has spurred the discovery of several new states of matter having exotic properties. In particular, topological insulators are such new states of matter that are distinct from conventional insulators. These materials support spin-polarized conducting states at the boundaries/surfaces while remaining insulating in the bulk. The surface states often exhibit Dirac-cone energy dispersion with helical or chiral spin-texture and are protected by time reversal symmetry. The topological protection ensures the backscattering free transport at the boundaries of topological insulators. The recent developments in the field show that these nontrivial states not only offer potential applications in quantum computing and spintronics, but also provide platforms for realizing novel quantum phenomena such as Weyl semimetals, and Majorana fermions in a condensed matter system.
The thesis presents an analysis of topological surface state properties of several selected materials and predict new materials or thin films of materials that realize the quantum spin Hall state, Dirac semimetal, Weyl semimetal, or Rasbha effect, using the ab-initio density functional theory framework and k.p theory. Through systematic analysis of bulk and surface electronic structures, it is shown that thallium based ternary III-V-VI2 series of compounds, TlMQ2 (where M= Bi or Sb and Q= S, Se, or Te), contain both topological and normal insulators. Therefore, it is possible to study the topological phase transition (TPT) in these compounds by tuning the lattice parameters as well as spin-orbit coupling (SOC). The electronic structure and spin-texture analysis of topological surface states show that the surface states form an unusual planer metal that is essentially half of an ordinary two dimensional (2D) conductor and carry nontrivial $\pi$ Berry phase. Furthermore, a possible TPT in Ge(BixSb1-x)2Te4 thin films is discussed as a function of layer thickness and Bi concentration x. The systematic examination of band topologies suggests that thin films of Ge(BixSb1-x)2Te4 are viable candidates for 2D topological insulators, which would undergo a 2D-TPT as a function of x. Finally, it is shown that the inversion asymmetric compound, Sb2Se2Te, harbors both a novel nontrivial band topology and giant Rasbha-type spin splitting in its native form driven by strong SOC. The Rasbha splitting in the bulk bands of Sb2Se2Te is the largest that has been found to date and attributed to large polarization field (electric field) and small band gap in the system.


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Samit Paul
Y8209868
Micron Focusing, Diagnostics and Interaction of Multi-Element
Ion Beams with Matter
7th May, 2015
11 am
L6
Prof. Sudeep Bhattacharjee
A focused ion beam (FIB) system is an inevitable tool for research and applications in nano-science and technology. However, the availability of only gallium (Ga) ions as projectile, in liquid metal ion source (LMIS) based FIB systems limits its applicability. Moreover, metallic Ga has contamination issues and provides less milling yield. In order to ameliorate the above problems, an emerging area in this field is development of focused ion beams of a variety of elements utilizing plasmas. A compact, high current density (~ 1 A/cm^2), microwave plasma based multi-element focused ion beam (MEFIB) system has been developed in the laboratory, that is capable of delivering beams of different ions (H_2 , Ar, Kr, Ne) of energies up to 18 keV and spot size of ~ 25 µm, using a 1 mm plasma electrode (PE) aperture. The beam has an emmittance of ~ 0.1 mm-mrad and brightness of ~ 10^5 Am^-2 sr^-1 V^-1, thereby making it an excellent candidate for FIB.
In this thesis our target has been to reduce the beam to submicron spot size and make it operate at higher energies (30 keV) comparable to that of conventional FIBs. In parallel, we developed a new diagnostics (spider probe) for measuring micron size beams and have investigated the time dependent physics of the interaction with matter, particularly, the milling capability to make micro-pores and microstructures on metallic foils and thin films.
In order to achieve the above, first the plasma has been optimized to maximize the extracted beam current density maintaining tolerable ion energy spread at the meniscus (~5 eV). The utilization of two Einzel lenses in conjunction with a PE and a beam limiting (BL) aperture, is found to have better control over the beam energy and current density. Controlled sculpting of micron scale pores in aluminum foil with Ar and Kr ion beams is demonstrated. The temporal evolution of the micro pores caused by beam interaction on metallic foils has been studied and smallest pore diameter of ~ 3 µm has been successfully created. This shows that with proper control of the irradiation time, submicron pores can be fabricated. Further experiments, using smaller PE apertures of 45 µm and 31 µm, created by MEFIB, have been used to obtain a focused beam size ~ 1 µm. A time-dependent exposure method to determine the exact location of the focal point of the beam has been invented that takes care of the over irradiation issue. Extensive use of Lab-View software is made for manipulating the beams to create several microstructures such as circular and rectangular trenches, gratings, symbols and alphabetical letters on 50 nm Cu thin films.
In order to reduce the beam spot size further, guiding and transmission of extracted Ar ion beams with the help of straight and tapered micro-glass-capillary have been investigated. The beam current through the capillary is found to have a threshold extraction voltage and observed to exhibit hysteresis, with a unique self-focusing capability. The temporal and dimensional dependence of the hysteresis have been studied. The guiding capability of the tapered capillary is found to be more effective where beam size reduction is desired without compromising total beam current unlike electrostatic beam limiters. For further understanding the phenomena, Particle-In-Cell simulations that solves Poisson's equation and equation of motion self consistently are carried out, and the experimental results are found to have a reasonable agreement with simulations.



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Prabwal Jyoti Phukon
Y8209867
Studies on the Physics of Brames from the bulk to the boundary
27th October 2014 (Monday)
3 pm
FB-382
Branes play an important role in our understanding of physics at different length scales: from black holes to condensed matter systems. They arise as a class of dynamical, extended objects in string theory and other closely related theories like M-theory and are specified by the number of spatial dimensions they possess (a brane in p spatial dimensions is called a p-brane). In this thesis, we are particularly interested in the D-p-branes of string theory and M-p-branes of M-theory. We primarily focus on exploring three important directions related to brane configurations: its bulk in the low energy limit,i.e supergravity theory, bulk to boundary correspondence and a class of conjectured world-volume theories on branes. The thesis addresses three illustrative issues in the context of each of these three directions in three portions.

To start with, we undertake an analysis of branes with reference to asymptotically AdS black holes and their thermodynamics. Here, we seek to understand logarithmic corrections to the Bekenstein-Hawking entropy for these black holes due to thermal fluctuations in the grand canonical ensemble.

Then, in the second part of the thesis, we carry out a bulk to boundary theory study of branes in the backdrop of gauge-gravity duality, focussing on R-charged black holes. We study the electromagnetic response functions of strongly coupled media whose dual gravitational descriptions are given by these R-charged black holes. In dimensions 4, 5 and 7, these correspond to rotating configurations of M2, D3 and M5-branes, respectively.

The third part of the thesis work attempts to understand the moduli space of a class of conjectured worldvolume theories on M2-branes. These are known as quiver gauge theories. As a specific example, we investigate the (2+1) dimensional N = 2 Chern-Simons (CS) quiver gauge theories which appear as worldvolume theories on a stack of M2-branes probing a special class of Calabi-Yau (CY) four folds, namely complex cones over smooth Fano 3-folds. In this portion, we put special emphasis on examining the Higgsing and unHiggsing of these theories via a "brane tiling" method.

We conclude by highlighting some novel results in black hole thermodynamics, holographic optic and Chern-Simons quiver gauge theories that have been obtained in course of this thesis work.


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Hemanadhan M.
Y8209866
Study of excited-state energy density functionals
constructed by splitting k-space for homogeneous electron gas
19th September 2014 (Friday)
9:30 am
FB-382  

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Prabhakar Tiwari
Y9109075
Observations of Large Scale Anisotropy and Cosmological Models
8th September 2014 (Monday)
12.00 pm
FB-382
The observable Universe is simply huge! ∼10^{26} meters in every direction, ~14 billion years old and contains ∼10 ^{80} hydrogen atoms. The modern cosmology is the science of the entire Universe. We, here on a small planet, can only assume that the Universe is knowable and physics is followed everywhere in the same manner. Furthermore it is reasonable to assume that the observable Universe is statistically same for all observers, located anywhere in the Universe. Today, we demand this uniformity as “Cosmological Principle” which assumes homogeneity and isotropy at large distance scales. This thesis is a critical examination of the cosmological principle. In the thesis we review the present observations of large scale anisotropy and discuss possible theoretical models to explain these observations.  


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Mr. Shyam Lal Gupta
Y6209062
Dynamics of Laser Ablated ZnO Colliding Plasma Plumes
19th August, 2014 (Tuesday)
03:00 pm
FB-382
Irradiation of the target material with a laser pulse having fluence higher than its ablation threshold fluence results in formation of its plasma. UV laser irradiation of target surface results in smaller/comparable thermal diffusion length compared to absorption length resulting in congruent ablation unlike the chunk ablation due to bulk target heating with longer wavelength irradiation. The properties of the plasma plume viz. temperature, electron number density, and ion velocities etc. affect the properties and quality of pulsed laser deposited thin films. The properties of the plasma plume can significantly be varied by using various configurations of colliding plasma plumes. The plasma plumes evolve with time due to the pressure difference with respect to the ambient and move due to the thermal kinetic energy gained by the plasma species from the ablating laser pulse. The velocity components of seed plasma plumes in the direction of the interaction contribute to processes occurring in the interaction zone, where as the forward plume front velocity components result in the movement of the interaction zone. The properties of interaction zone (interpenetration or stagnation) are tuned by varying the inter-plume distance and the ion-ion mean free path. The ion – ion mean free path depends on relative collision velocity of two seed plasmas, the ionization state, and the ion density of the plasma plumes. In the present work we report the dynamical behavior of collinearly colliding ZnO plumes and formation of interaction zone / stagnation layer. The plasma plume dynamics is studied using fast imaging and optical emission spectroscopy. Two seed plasmas initially evolve independently and then start interacting at ~15 ns along their lateral dimensions. The interaction of seed plasma plumes results in a stagnation layer having abundance of the higher ionic states, high electron number density and temperature with slower decaying rate as compared to the single plume that lasts for longer time delays as compared to single plume. At the later stages of plasma evolution the vapor phase is populated by nanoparticles /nanoclusters of ZnO. The presence of nanoparticles/clusters of ZnO in the vapor phase for longer time delays in colliding plumes is confirmed using dynamic laser light scattering (Rayleigh) and photoluminescence (PL) techniques. The configuration is used for deposition of ZnO thin films on glass substrates using 1064 nm irradiation. The deposited films are characterized using X-ray diffraction, AFM, optical transmission in the UV-visible range and photoluminescence measurements. IR ablated colliding plasma plumes results in deposition of a-plane ZnO thin films with better optical properties as compared to c-axis oriented thin films obtained using conventional UV (355 nm) single plume deposition. The observed differences in the quality and properties of thin films are attributed to the flux of mono-energetic plasma species with almost uniform kinetic energy and higher thermal velocity reaching the substrate from interaction/stagnation zone of colliding plasma plumes.

 

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Mr. SK Firoz Islam
Y8109070
Theoretical Study of Electronic, Transport and Thermoelectric Properties of Spin-Orbit Coupled Two-Dimensional Electron Systems in Presence of Magnetic Field.
3rd June, 2014 (Tuesday)
6 pm
FB-382

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M. Hemanadhan
Y8209866
Study of excited-state energy density functionals constructed by splitting k-space for homogeneous electron gas.
2nd June, 2014 (Monday)
4 pm
FB-382

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Abhishek Chowdhury
Y7209061
Multi-element focused ion beamlets for localized low energy ion matter interaction.
30 May, 2014 (Friday)
4 pm
FB-382
In this thesis a low energy multiple ion beamlet system (MIBS) is developed and utilized for non-destructive investigation of ion matter interactions at micrometer length scales. Since low-energy ion beams (1-5 keV) cannot penetrate deep inside the substrate, therefore the excitations are mainly limited to surface and sub-surface layers. Until now researchers had mainly considered single, broad ion beams (diameter ~ 0.1-2 cm) which made local perturbations challenging. The focused multiple ion beamlets can be employed for tailoring and creation of localized resistive, conducting or optically active regions in matter in a patterned manner.

The beamlets are extracted from a compact wave driven plasma source developed in the laboratory. Two types of beamlet "shower" electrodes are employed in the experiments: one with a honeycomb structure with notched apertures and another a 5×5 array of through apertures. Detailed experiments are performed to optimize the plasma source parameters for obtaining uniform beamlet current, as confirmed by angular measurements of the ion saturation current near the extraction electrode. A computer controlled switching technique is developed that can control motion of individual beamlets for patterned irradiation.

The MIBS is employed to systematically investigate low-energy ion irradiation induced changes in sheet resistivity and Debye temperatures in metallic nano-films of Ag, Cu and Al of thickness d/lo ~ 2-5, where d is the film thickness and lo is the bulk mean free path. The number of defects and impurities in the nano-film is varied in a controlled manner by varying the ionic mass number and beam fluence. Both atomic (Ne, Ar, Kr) and molecular (H2, N2) gases are employed in the investigation. Low temperature measurements are carried out for pristine and irradiated films to determine the residual sheet resistance (RRS). An empirical formula relating RRS with ionic mass number and beam fluence is proposed for the first time. Large diffusion of impurities into the film driven by thermal gradients at the interface is found to give rise to the unexpectedly large observed values of sheet resistance, as confirmed from energy dispersive x-ray spectroscopy. The Debye temperature as determined from Bloch-Grüneisen fitting of sheet resistance versus temperature data, is found to decrease with both ionic mass number and fluence, primarily due to the change in bulk modulus of the nano-film. The surface morphology of the films are investigated using atomic force microscopy (AFM) measurements. The experimental results are verified using well known ion matter interaction simulation tools such as SRIM, TRIM and TRIDYN_HZDR.

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Mihir Sarkar
Y7209062
Charge particle beam interaction with matter from a perspective of lithography.
27th May, 2014 (Tuesday)
2 pm
FB-382
The interaction of energetic charged particles with matter results in a variety of events which are interesting from the point of view of science, and offers exciting possibilities for exploring novel processing applications. In that context, we study two loosely connected aspects of charged particle matter interaction, one of which relates to the charge exchange process that occurs to high speed ions passing through the stripper medium of a tandem accelerator. The other one is electron and ion beam induced lithography which occurs by ionization of a target resist medium. The investigations are motivated by the unique possibilities of charge particle beam lithography for prototyping organic devices.

The charge exchange process is studied by using negative carbon ion (12C-) beam and analyzing the effect of the stripper gas pressure on the average charge (q_avg), charge state fractions, and transmission of the incident beam. It is shown that the equilibrium charge state istribution in the outgoing beam is slowly altered by increment in the stripper gas pressure. For particular incident ion energy the transmission shows a maximum with variation in stripper gas pressure due to the interplay between populations of charge changing target atoms and elastic scattering centers.

The charge particle based lithography process was taken up with MeV roton beams extracted from the tandem accelerator and keV electron beams of a dual-beam SEM/FIB. The effects of proton beam exposure dose and post exposure processing conditions on the quality of the developed micro-structures are analyzed. The exposure characteristics of 30 keV electron beam lithography in a thick chemically amplified resist SU-8 are discussed. The widening of the developed structures in excess of a particular equienergy deposition density contour follows non-monotonic variation. The sidewall profiles indicate that the crosslink profile of the resist proceeds in a reaction-diffusion environment in which the photoacid diffusion itself is controlled by crosslinking density.

Next, wetting behavior of an electron beam written 3D micro-structure pattern on ITO/Glass substrate by two functional inks used for inkjet printing material deposition is examined. The patterned substrate facilitates all additive deposition of an organic semiconductor over the micro-structure pattern. The possibility of convenient processing f array of thin film transistors and similar potential applications are discussed.

Finally, 30 keV electron beam lithography is investigated in a novel multilayer of resist which includes an intermediate metallic layer in the stack. The feasibility of the process and the possible line width resolution has been analyzed by obtaining the spatial distribution of electron energy deposition density profile through the multilayers of resist. As a test case application, the lithography process is shown to facilitate fabrication of a vertical organic transistor based on poly (3-hexylthiophene) (P3HT) as the active material.

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P. K. Rout
Y7209866
Interface superconductivity and giant proximity effect in La-based cuprate heterostructures
23rd May, 2014 (Friday)
3 pm
FB-382
A range of novel physical phenomena like high mobility two-dimensional electron gas, quantum Hall effect, magnetism, and interface superconductivity are observed at interfaces between complex oxides, which completely differ from the ones of the constituent compounds. Recent observation of superconductivity at the interface of various cuprate heterostructures is one such exciting phenomenon. Here, we will present the observation of interface superconductivity in the bilayer systems composed of various combinations of cuprates like La2-xSrxCuO4 and La2-x-yNdySrxCuO4. The nature of the interfacial layer in such composite systems will be discussed. The second part of the seminar will deal with the superconductor (S)-normal metal (N)-superconductor (S) Josephson junctions based on high temperature cuprate superconductors. Unlike conventional SNS junctions, the supercurrent in these junctions can flow through barriers as thick as 50 nm, which is order of magnitude larger than the coherence length. We will discuss our results in the context of "giant proximity effect".

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D. N. Patel
Y5209062
Spectroscopic investigations of brass plasma and synthesis of nanoparticles.
19th May, 2014 (Monday)
12 noon
FB-382
Laser ablation of materials has applications in thin films, laser etching, micromachining and nanotechnology. A comprehensive study of laser ablation of brass is carried out in different ambient conditions viz. vacuum, air and liquid to understand composite/alloy target plasma and formation of nano particles in plasma plumes. The brass plasma is created using Nd:YAG
laser operated at 10 Hz with 1064 nm and 266 nm. Optical emission spectroscopy (OES) measurement at threshold of brass shows the
preferential vaporization of Zn. The influence of magnetic field on plasma plume dynamics shows the splitting, oscillations and rotations of the
plume. The behavior of the expanding plume in various ambient; vacuum, air and liquid, shows distinct confinement of plume in water ambient. The high temperature, density and pressure of plume in water ambient facilitate the formation of metastable phases. Nanoparticles formed in the plumes show a compositional change with the size of nanoparticles in the vapor phase. The nanoparticles are characterized by scanning electron microscopy (SEM), energy dispersive X-ray (EDX) measurement, photoluminescence (PL) and Raman spectroscopy. The smaller size particles (nanoparticles size ~55 nm and size distribution FWHM ~ 7.5 nm) are reported in water ambient, however, the larger size particles (micron size particles ~ 2.5 µm and size distribution FWHM ~ 1.7 µm) are observed in air ambient. Smaller size particles are mostly Zn enriched. The photoluminescence (PL) measurement of the particles deposited in water ambient shows the peak at 380 nm and the PL peaks at 380 nm, 415 nm and 440 nm are observed when the particles are deposited in air ambient. The measurement of particles size shows the decrease in size away from the crater periphery in air however, in the case of ablation in water ambient nanosized particles are observed away from the crater periphery and nano rod shaped structures are reported at the crater periphery. The Raman measurement on the deposited structures confirms the ZnO rod at the crater periphery.

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Pabitra Mandal
Y6109067
Anomalous magnetic response of CaFe1.94Co0.06As2 superconductor and nonlinear response of the driven vortex state in NbS2 superconductor.
17th May, 2013
11:15 am
FB-382
In recent times studies on the new class of Iron-Pnictide superconductors have revealed a complex doping phase diagram. It is found that in a certain doping range, superconductivity in these system emerges via the suppression of magnetic order. As a part of the thesis, we have developed a high sensitivity magneto-optical imaging technique for sensitively imaging small changes local magnetic field distribution in underdoped 122- CaFe0.94Co0.06As2 single crystals. Our investigations reveal evidence of an unusual coexistence of magnetic and superconducting property in this material. Details of this will be discussed. From our work we suggest that there exists an the inherent coexistence of magnetism along with superconductivity may also be responsible for the anomalously strong pinning found in this class of superconductors.
I will also present our investigation into novel phase transformations in the driven vortex matter in NbS2 superconductor. We investigate the scaling of the current-voltage characteristic of the superconductor using universal scaling ansatz proposed by Meng-Bo Luo et al., PRL 98, 267002 (2007). This study attempts to understand the depinning transition and what is the nature of the driven vortex phase.

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Mr. Ajeet Kumar Sharma
Y8109061
Distributions of first passage times in stochastic processes in a living cell
8th May, 2014 (Thursday)
4 pm
FB-382

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Mr Suman Banerjee
Y5209865
Opto-electronic analysis of planar, bulk and hybrid planar-mixed heterojunction organic solar cells
8 May, 2014 (Thursday)
11 am
Samtel Centre Seminar Room
he electrical response to incident light on a thin film stack system was analysed with an opto-electronic model by considering interference effect in a thin film stack system and solving the basic semiconductor equations to describe various characteristics of organic solar cells. The model was then used to describe incident light wavelength dependent photocurrent response (spectral response) of Schoottky diode, planar and bulk-heterojunction solar cells. Important parameters of organic materials. Such as, exciton diffusion length, exciton dissociation
efficiency and exciton blocking efficiency are estimated. Quantitative analysis of the role of different thin film layers and optimised film
thicknesses of efficient solar cell devices are estimated using the model.

The model is used further to explain the current-voltage (I-V) characteristics of organic solar cell. A general model for planar mixed-heterojunction solar cell is developed and used for planar and bulk heterojunction solar cell by considering some special cases. The role of mobility and contact property (extraction rate) on the I-V characteristics of planar and bulk-heterojunction solar cells are explained using the model. Many devices were made and characterised using the experimental setup designed in the lab. Device characteristics were also explained using the model developed.

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Mr. Samar Layek
Y6209862
Structural and Magnetic Properties of Transition Metal based Oxides/hydroxides Synthesized Using Specific Methods
31st March, 2014 (Monday)
6 pm
FB-382
Transition metal oxides/hydroxides are important in many applications. Three oxide systems namely iron oxide/hydroxides, Nickel oxide and Barium ferrite are investigated. Interesting results are obtained as the properties can be controlled by the synthesis method and parameters.

Hematite nanoparticles synthesized by using different combustion methods show variety of magnetic properties depending of the particle size and specific choice of fuel used for preparation. Maghemite nanorods of length 20 nm show preferential magnetic ordering.

Mixed iron oxide/hydroxides are one of the potential candidates for the fluoride and heavy metal ions purification. The effect of dilute amount of Ca, Mg substitution on the phase formation, structural and magnetic properties is studied in details. Increasing Ca doping leads to phase formation. Simultaneous incorporation of Ca, Mg leads to completely amorphous phase.

The effect of transition metal and rare earth metal substitution on the structural and magnetic properties on NiO nanoparticles is studied. Mn
doping changes the antiferromagnetic ordering in NiO to completely ferromagnetic ordering whereas weak ferromagnetic nature is found for other dopants. The value of magnetization depends on the concentration of doping and the particular dopant.

Enhancement of the magnetic properties in single phase doped BiFeO3 ceramics is found by mechanical activation. The magnetization is seen to increase either by doping in Bi/Fe site or by decreasing crystallite size below a certain limit.

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Durgesh Chand Tripathi
Y5109066
Carrier Transport in Organic Semiconductor Diodes with Doped/Undoped Interfaces
29th March (Saturday), 2014
5 pm
SCDT Seminar Room
Organic semiconductors are currently being used for many large area electronic applications. With rising complexity of multilayer structures such as in organic light emitting diodes, the doped/undoped (high-low) organic interface has become an important determinant of device performance. However, underlying mechanisms of charge transport across the doped/undoped interface is not understood.

In this thesis, we study charge transport across the doped-undoped interface using capacitance-voltage (C-V) and temperature dependent current density-voltage characteristics. The specially designed device structures for this purpose have been fabricated using a state-of-the-art Cluster tool. The prototype materials m-MTDATA and F4-TCNQ are used as matrix and dopant, respectively and the doping is achieved by co-evaporation.

The carrier transport mechanism at doped/undoped interface is sensitive to the barrier height and carrier accumulation at the interface. It is found that the charge transport across homo-interfaces is controlled by tunneling through alignment of local density of states on the two sides; whereas at hetero-interfaces it is controlled by the offset.

The characteristic peak observed in C-V in such structures is found to depend on device configuration, and the peak height is sensitive to the layer thickness. A competition between the diffusion dominated and drift dominated transport is responsible for occurrence of the peak. A functional relation between small signal capacitance and voltage has been established in the diffusion regime prior to the C-V peak. It has also been shown that the measured mobility by impedance technique in space charge limited regime is dependent on device structures needing careful interpretation of its field dependence.

Electroluminescent transient technique has been used to measure diffusivity and mobility simultaneously in organic diodes. In contrast to mobility, field dependence of diffusivity showed a specific minimum at a critical field, which suggests a convenient normalization procedure, called "shift normalization", to annihilate temperature dependence. This method has been used for a unified description of mobility in prototypical small molecule Alq3 from which all the parameters of Gaussian disorder transport models can be inferred independently.

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Md. Shamim
Y6209861
Construction of exchange and correlation energy functionals for
excited-states in time-independent density functional theory.
27th December, 2013 (Thursday)
10:00 am
FB-382

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Debaprasad Sahu
Y7109067
Physics of negative ion containing plasmas: volume generation, measurement and wave induced phenomena.
16th December, 2013 (Monday)
10:30 am
FB-382In this thesis, a novel microwave plasma based volume negative ion source is developed and detailed experiments are performed to optimize negative ion generation for hydrogen (H-)and measurement of negative ion density. Electromagnetic waves of 2.45 GHz (angular frequency 1.53×1010 rad/s) are launched directly into a multicusp (MC) plasma device in the k perp B mode, where k is the wave vector and B is the magnetostatic field. Localized wave induced resonances are created at the periphery of MC (inner radius = 60 mm), depending upon the magnetic field and plasma density. These resonance zones are identified experimentally. The electron cyclotron resonance (ECR) occurs close to the boundary at r = 42 mm, whereas the upper hybrid resonance (UHR) lies r = ~ 22 mm away from MC boundary.The H- source is divided into two sections namely production (PR) and attachment (AR), separated by a transverse magnetic filter, which deflects the hot electrons (7 - 15 eV) trying to enter from PR and allows only the cold electrons (~ 1 - 2 eV) into AR. The hot electrons in PR are favorable for the generation of vibrationally excited molecules of hydrogen, which pass into AR, where negative ions are produced by their dissociative attachment with the cold electrons. The plasma frequency at the center is ~ (1.2 - 1.4)×1010 rad/s and is comparable to the angular wave frequency indicating sustenance of close to cutoff density plasmas (~ 7.44×1010 cm-3) at the wave frequency. The electron mean free path lies in the range ~ 25 - 30 cm and is comparable to the system size, therefore the plasma is almost collisionless. Measurements of electron energy distribution function (EEDF) in both AR and PR confirm the nature of the distribution and suitability for the aforementioned processes. The H^- density measured using two methods, viz. the extracted current and the second derivative beat method is found to be ~ 5×109 cm-3. The source has also been operated in the pulsed mode. Here, temporal filtering generates negative ion rich plasmas in the afterglow phase where cold electrons (~ 1 eV) are generated. The H- density in the afterglow is measured to be ~ 2×1010 cm-3, using the saturation current ratio method and is higher than that of the continuous mode case.Finally, a steady state and a time dependent model, using particle and charge balance equations are developed to estimate the negative ion density in both the downstream region (continuous mode) and in the temporal afterglow (pulsed mode). The results of the two models agree reasonably well with the experimentally measured H- density. The compact negative ion source would be useful in a variety of physics studies and applications such as dusty plasma with negative ions, neutral beam injection in fusion, modification of material surfaces, semiconductor fabrication, and ion beam lithography.

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Amit Banerjee
Y7209864
Resonance behavior of FIB grown nanomechanical systems and the role of microstructure.
28th October, 2013 (Monday)
8:00 AM
FB-382
In recent time discovery of novel material properties at nanodimension has motivated studies pertaining to investigating the mechanical properties of materials at nanometer dimension. Due to limitations in detection capability, most of the studies on anomechanical systems have been on systems fabricated out of semiconducting materials. In this thesis we discuss ways to fabricate metallic nanomechanical systems and also develop ways to detect resonance behaviour in these systems. We have fabricated using the focused ion beam induced milling process metallic nanocantilevers by milling self-supporting polycrystalline thin films of Au and Ag. The resonance frequencies and the amplitude of vibration of the metallic nanocantilevers have been experimentally measured. We study the resonance behaviour of our nanocantilever systems within the Euler-Bernoulli formulation and investigate the deviations from the expected behavior. We show the existence of a characteristic dimension in the metallic nanocantilever system at which the dissipation in the system is minimum and the resonance frequency approaches the ideal Euler-Bernoulli limit. We shall discuss issues related to this characteristic dimension and its relationship with the novel microstructure observed in the metallic thin films from which the nanocantilevers are fabricated. We have also fabricated a set of elastically coupled Au nanocantilever system and studied their resonance behaviour. In the second part of this presentation, we shall demonstrate the fabrication and resonance behaviour of a focused electron and ion beam grown C-Pt core-shell type bilayer nanowire resonator system. The resonance frequency of a bilayer system depends on densities and Young's moduli of the constituting materials. We have experimentally measured the densities and Young's moduli of the materials that we have used for the present study. The resonance frequency behaviour of the bilayer system is compared with the theoretical predictions. Finally towards the end of the presentation we discuss a few novel method we have developed for sensing nanomechanical vibration.

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Nitul Singh Rajput

Studies of issues involved in Focused ion beam based Nanostructures and developing newer methodologies.
19th June, 2013 (Monday)
11:00 am
FB-382
FIB technology is gradually becoming one of the most popular techniques for micro/nano engineering materials. In FIB, the maskless processes (deposition & milling) are used as bottom-up and top-down fabrication tool to meet the challenges at nanoscale. Despite its successful venture from precise engineering to manufacturing for the study of basic physics, the ion irradiation processes have number of secondary effects. However, in some cases, the secondary effects viz., implantation, damage formation can be exploited fruitfully to modify the sample locally.

In this thesis work, the FIB processes are used for novel structure fabrication and several newer methodologies are developed. A metallic cantilever, fabricated using a very unique technique, tends to change its shape and geometry on ion irradiation.

Investigation shows that ion-matter interaction at nanoscale has a prime role in the bending process. The phenomenon is exploited to create several 3D micro/nano structures as well as used to sense mass of the order of femtogram, lifting a heavy mass... etc.

FIB is also used to fabricate metallic nanowires and nanogap electrodes and their characterizations are made. It is found that insulating base substrate has the prime role in the failure of the nanogap structures. Using milling and deposition, several overhanging 3-D structures are created and their thermal characterization is made.

And lastly we will discuss about the role substrate and the ion beam heating in the FIB-CVD process. It is found that, the ion beam heating is not significant to cause any disrupt change in the growth of the structure.

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Dhirendra Kumar Sinha
Y4109071
Carrier Transport and Electroluminescence in Polyfluorene embedded with CdSe/ZnS Quantum Dots
17th June, 2013 (Monday)
12:00 Noon
Samtel Centre for Display Technologies (Seminar Room)
We simultaneously study the carrier transport and luminescence processes in the model system of a blue-emitting conjugated polymer, Polyfluorene (PFO), embedded with core-shell type CdSe/ZnS Quantum Dots (QDs). The test devices used for these studies have conventional structure of ITO (transparent anode)|PEDOT:PSS (hole-injecting layer)|PFO:QDs
(active layer)| Ca-Al (Cathode), wherein the concentration of the Qds varies between 0-80% by weight. The study of the electroluminescence transients at varying temperatures (10-300 K) is the principal technique which we use in this study.
The thesis focuses primarily on the following three issues:

•The simultaneous determination of the field and temperature dependence of
the diffusivity and the mobility of the charge-carriers in PFO;

•The differences between the mechanisms of PL and EL at QDs and the role
of the charge-carrier mobility in controlling the EL;

•Modifications in the mechanisms controling charge-carrier mobility for a
wide range of QD-loading conditions, and field and temperature variation.

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Gorky Shaw
Y6209863
Static and driven phases of vortex matter in superconductors with intrinsic and nanopatterned pins
06-05-2013 (Monday)
10:00 AM
FB-382
Preamble: The vortex state in superconductors exhibits competition and interplay between effects of interactions, thermal fluctuations and quenched random disorder (natural or artificially generated). Consequently the static and dynamic phases of the vortex state are quite complex. In this seminar, I will present results of my studies on the static and driven phases of vortex matter in high-Tc and low-Tc superconductors with intrinsic as well as artificially patterned pinning centers.
Static phases of vortex matter in sample with intrinsic pinning centers: Using high sensitivity magneto-optical imaging (MOI) on a Bi2Sr2CaCu2O8 (BSCCO) single crystal we have identified signatures of a novel interaction driven freezing transition from a dilute vortex liquid to solid phase along with a solid-liquid phase coexistence regime. We have constructed a phase boundary for the low field vortex liquid - solid transition line in the H - T vortex phase diagram. We have studied the effect of pinning strength on this phase boundary. We have also evaluated the entropy change associated with the low field phase transformation.
Driven phases of vortex matter: By measuring the velocity time series response of a vortex state in response to a continuous drive applied via a transport current, we have uncovered a new non-equilibrium driven jammed vortex state into which the vortex state organizes. The entry into a jammed vortex state we find is driven either by attempting to steadily accelerate the vortex state or by waiting for long time at constant drive. De-pinning of the jammed vortex state is also highly unusual, and is associated with giant vortex-velocity fluctuations with life-times exhibiting critical divergence on approaching the threshold depinning force value. Our study is compared with recent studies on similar effects found in driven colloidal systems.
Effects of artificial pinning on static phases of vortex matter: We have used Focused Ion Beam (FIB) to generate array of nanopins (blind holes) on surfaces of single crystals of 2H-NbSe2 and BSCCO. Bulk magnetization measurements in these samples provide evidence of a driven weak to strong pinning crossover. Using these artificial pinning centers we show how the local static configuration of vortices can be significantly altered with nanopatterning.

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Neeraj Shukla
Y6109861
Ion beam induced nanostructuring and magnetic ordering
22nd January, 2013
11:00 AM
FB-382
Ion irradiation has a variety of effects on various substrates such as sputtering of materials, adhesion of thin films, intermixing at the interface of multilayer's, scattering effects, ripple formation, defect formation. Low energy ions(few 10s of keV) are utilized for surface modification purposes and the high energy ions (few MeV) are utilized for ion implantation and for inducing defects of various kinds.
In the present work, we have used low energy (10-30 keV) focused Ga ion beam to fabricate and study micro/nano structures on a substrate and high energy(1-2 MeV) 1H+ and 12C+ ion beams to modify properties of highly oriented pyrolytic Graphite(HOPG).
By using focused ion beam (FIB) of 30 keV Ga ions, a variety of 1-D and 2-D micro/nano structures are prepared. A 2-D array of metallic patches has been fabricated using improved adhesion of metallic film on glass substrate caused by the ion beam bombardment. Similarly 1-D gratings with nanometer sized grating element have been fabricated by milling selective areas on silver film using Ga FIB. The forward and backward scattering of the incident Ga ions from different material structures is used to modify the nearby FIB fabricated structure. The effect has been investigated in three different geometries/material combinations.

Ferromagnetic ordering has been shown to occur in Highly Oriented Pyrolytic Graphite (HOPG) due to proton irradiation. By varying ion beam parameters, magnetic moment of the order of 10-6 emu has been obtained in optimum conditions. We have obtained 2 orders higher magnetic moment in HOPG by bombarding 12C+ ions. The results are analyzed in terms of defect densities induced in different layers along the thickness traversed by ions. Magneto-resistance studies have been made on selected irradiated HOPG sample at 20 K in the in-plane and out-of-plane orientations of unirradiated and 1H+ and 12C+ ion irradiated HOPG flakes. The results correlate well with the
findings of the VSM studies of irradiated sample. 

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Dibyendu Hazra
Y5109065
Hysteresis in superconducting weak links and micron size superconducting quantum interference devices.
31 January, 2012
11 am
FB-382

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Mr. Victor Mukherjee
Y6209864
Non-Equilibrium Quantum Critical Quenches: Defects, Entropy and Fidelity
Wednesday, 9th November, 2011

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Indranuj Dey
Y5209863
Wave Interaction with Plasmas Confined in Multicusp Magnetic Fields
02-11-2011 (November 2, 2011 Wednesday)

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Mr. Devendra Kumar
Y280962
Non equilibrium features in the solid state: a case study of the transition metal oxide NdNiO3
Friday, 21 Oct, 2011

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Mr. Sunil Kumar Mishra
Y4109075
Size-dependent magnetization fluctuations and slow dynamics in NiO nanoparticles
Wednesday July 27,2011

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Jose V Mathew
Y5109069
Multi-element focused ion beams from intense microwave plasmas
22-12-2010 (December 22, 2010 Wednesday)

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Mr. Bhaskar Kamble
Y4809061
An effective quantum expansion parameter for metallic ferromagnets: Role of orbital degeneracy, hund's coupling, and quantum corrections.
Thursday, 4th November 2010

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Jaivardhan Sinha
Y4209862
Properties of magnetic materials under extreme conditions
07-10-2010

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Uma Divakaran
Y4209864
Slow quenching dynamics in quantum critical systems
4th June 2010 (Friday)

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Mr Bhupendra Nath Tiwari
Y220961
Correlations, Stability and Black Holes in string theory and M-Theory
18 May 2010

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Mr. Sarvesh Kumar Tripathi
Y6109861
Investigation of focused ion beam induced processes and their utilization for nanofabrication
15th April (Thursday), 2010

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Mr. Prasanta Kumar Muduli
Y3109063
Spin Polarized Transport in Planar Structures and Tunnel Junctions of Perovskite Oxides
12th April (Monday), 2010

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Mr. Ashok Garai
Y4109078
Low Molecular motor transport and traffic: effects of individual mechanochemistry and steric interactions
6 April, 2010

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Mr. Udai Raj Singh
Y4109077
Low Temperature Scanning Tunneling Microscopy and Spectroscopy Studies of Magnetoresistive Manganite Thin Films
3rd February, 2010

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Vinod Chandra
Y3109068
Hot QCD equations of state and quark-gluon-plasma in heavy ion collisions
Tuesday, Nov. 24, 2009

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Shyam Mohan
Y4109074
Instabilities in the vortex state of a type II superconductor
16th November, 2009

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Subhayan Mandal
Y2809066
Response functions of quark-gluon plasma
24 September, 5 PM, 2009

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Prasanta Kumar Muduli
Y3109063
Spin Polarized Transport in Planar Structures and
Tunnel Junctions of Perovskite Oxides
12th April, 2010

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Soumen Mandal
Y3809061
Magnetic interactions and electron transport in hole-doped
manganite-superconducting cuprate heterostructures
March 20, 2009

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Rajib Saha
Y280963
Angular Power Spectrum Estimation using Linear
Combination of Multi-Frequency CMB Maps
August 20, 2008

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Dasari Durga Bhaktavatsala Rao
Y210963
Spin Decoherence in Quantum Dots: A model Study
August 18, 2008

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Akhilesh Ranjan
Y020961
Response functions of quark-gluon plasma
August 4, 2008