Department of Physics
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Item Metal oxide Graphene oxide nanocomposite thin film for optoelectronic applicationsDe, Sukanta; Gayen, Rabindra NathThis thesis deals with the study of the solution-processed wide band gap metal oxide (TiO2) - graphene oxide (GO) nanocomposite materials in thin film form for their optoelectronic applications, such as UV-photodetector and dye-sensitized solar cells (DSSCs). Here, we demonstarte the fabrication of individual metal oxide (TiO2) - graphene oxide (GO) nanocomposites, as well as hybrid nanostructures (ZnO NW/TiO2), with GO incorporation using an easy, cost-effective and simple sol-gel spin coating technique. The formation of GO-composited highly transparent nanocomposite thin films, comprised of the rutile phase of TiO2 nanoparticles, as well as hybrid nanostructures (ZnO NW/TiO2), has been confirmed through microstructural, morphological, optical, and electrical characterizations. Modification of optical and electrical characteristics with a small amount of GO reinforcement into the host TiO2, as well as hybrid nanostructures (ZnO NW/TiO2), is also examined. Due to the incorporation of a small amount of GO into metal-oxide films, as well as hybrid nanostructures, the optical band gap values of those nanostructures are slightly reduced. At room temperature, DC bias dependent impedance spectroscopic analysis of TiO2 as well as hybrid nanocomposites (ZnO NW/TiO2) with GO, was performed for various external bias voltages in the frequency range of 4 Hz to 5 MHz. To evaluate and analyze the various contributions originating from the core grains and grain boundaries, the experimental Nyquist plot derived from the bias-dependent impedance spectra was fitted with an appropriate model electrical circuit consisting of two parallel RC circuits combined with a series resistance. The modification of grain boundary and its consequential effect on charge transport in individual metal oxide semiconductors, as well as hybrid nanocomposites, were confirmed by the variation of relaxation times (τ = RC) with an external bias and its modification after graphene oxide (GO) reinforcement. This demonstrates that a conducting graphene oxide (GO) network is capable of modifying the grain boundaries of individual metal oxides, as well as hybrid nanocomposites, and facilitates better charge transport through it, which may be beneficial for numerous optoelectronic applications. The fabricated nanocomposite thin films are used as efficient photoanodes for dyesensitized solar cells (DSSCs). In this study, the ruthenium-based N3 dye, widely employed as a prominent photosensitizer for the absorption of solar energy, is utilized along with the iodinebased redox pair (I- /I3-) mediator serving as the electrolyte. A transparent and conductive FTOcoated glass substrate is used as the counter-electrode. Under the irradiation of a 1.5 AM solar spectrum and with a power density of about 100 mW/cm2 , the current density-voltage (J–V) characteristic curves of TiO2, TiO2-GO, ZnO NW, ZnO NW/TiO2, and ZnO NW/TiO2-GO–based DSSCs are evaluated and analyzed. In the TiO2-GO (10 %) based DSSC structure, the maximum value of power conversion efficiency (η ~ 0.48 %) is obtained. The inclusion of GO in the TiO2 photoanode leads to a notable increase in the open-circuit voltage (Voc), short-circuit current density (Jsc), fill factor (FF), and power conversion efficiency (η). This improvement is ascribed to the less recombination of photogenerated charge carriers and improved charge transport enabled by the conductive GO network. The hybrid nanostructure that comprises ZnO NW/TiO2-GO (10 %) exhibits the highest level of performance, with an optimized power conversion efficiency value of η ~ 0.72 %. This observed enhancement in performance is due to the combined effects resulting from the improvement in charge transport along the axis of ZnO nanowires (NWs), an increase in contact area, less recombination of photogenerated charge carriers, and the modification of the conductivity of TiO2 by the incorporation of a conductive GO network. Therefore, high transparency with enhanced photocurrent density and large power conversion efficiency make TiO2-GO, ZnO NW-GO, and hybrid ZnO NW/TiO2-GO nanocomposite thin films more efficient photoanodes for dye-sensitized solar cells (DSSCs). Enhanced UV photoresponse properties of fabricated nanocomposite thin films were studied through UV photoconductivity measurements. Here, we present an investigation into the modification of UV photo-response properties of transparent TiO2-GO nanocomposite thin films, as well as hybrid ZnO NW/TiO2-GO nanocomposites. Schottky-enabled enhanced UV-light detection with high photoresponsivity, rapid response, and recovery time were observed by the incorporation of small amounts of GO into TiO2, as well as hybrid nanostructures (ZnO NW/TiO2). Under UV irradiation of wavelength ~ 365 nm with a power of ~ 8.07 µW, better photoresponsivity, sensitivity, specific detectivity, and external quantum efficiency were achieved by the inclusion of a small amount of GO into TiO2 thin films and hybrid ZnO NW/TiO2 nanostructures. Under the same circumstances, a maximum photocurrent of 1.78 mA/cm2 and a maximum UV photo-responsivity of 6.93 A/W, as well as a shorter rise time (tON ~ 0.28 sec) and recovery time (tOFF ~ 0.41 sec), were observed for TiO2–GO (15 %) thin film. These observed phenomena may be related to the modification of the space charge region produced by the incorporation of GO. This modification results in the passivation of interface defect states, which enhances the transport of charge carriers while simultaneously reducing the recombination of photo-generated charge carriers. When subjected to UV irradiation at a wavelength of 365 nm and an incident light power of 8.07 μW, the potential barrier height dropped from 0.54 eV to 0.49 eV, and the ideality factor decreased from 9.40 to 8.71 in ZnO NW/TiO2 films with 15 % GO incorporation. These results are beneficial for the development of optoelectronics applications and are advantageous for the film's overall performance. Also, for ZnO NW/TiO2-GO (15 %) nanostructures, a notable increase in photocurrent density (3.47 mA/cm2 ) and an impressive photoresponsivity of 13.52 A/W at a low bias (0.5 V), along with a speedy rise time of 0.89 sec and a quick recovery time of 1.66 sec, were achieved. Impedance spectroscopic analysis has been adapted to describe the transport mechanism in those prepared nanocomposite films in dark and UV-irradiated conditions. Therefore, the construction of a good Schottky junction with an Ag electrode, effective UV-stimulated charge separation, and improved transportation of charge via a conducting GO network all contribute to the fact that TiO2-GO thin films, as well as hybrid nanostructures (ZnO NW/TiO2-GO), are excellent UV response devices even when the external bias voltage is relatively low.Item Development of suitable ferroelectric nanocomposites in thin film form for energy applicationsGayen, Rabindra NathThis thesis provides information about the development of highly flexible ferroelectric nanocomposite materials in thin film form and their applications in energy storage, conversion and detection process. Polyvinylidene fluoride (PVDF) is synthesized in flexible thin film form, which is dielectric, ferroelectric, piezoelectric and pyroelectric in nature. A cost effective chemical solution based method is adopted to fabricate PVDF nanocomposite membranes with nanofillers like zinc oxide (ZnO), graphene oxide (GO) and both ZnO and GO loaded into that. PVDF polymer membrane possesses energy density of 11.3 × 104 J/m3 which increases to 65.5×104 J/m3 when ZnO nanoparticles are added to it. Inclusion of GO nanofiller completely destroys its energy density to 0.06 × 104 J/m3. Addition of both nanofillers increases energy density to 61.7 × 104 J/m3. Polarization of PVDF at an external electric field of 15kV/cm is 5.94 μC/cm2 which increases greatly to 15.98 μC/cm2 by ZnO incorporation into it. For tri-phase PVDF/ZnO/GO nanocomposite membrane, the polarization value is 14.14 μC/cm2. A great improvement in dielectric constant is observed from 47 for PVDF to 261 for PVDF/ZnO composite. For PVDF/ZnO/GO composite dielectric constant is 151. Dielectric loss for PVDF is 2.31 which reduce to 0.46 for PVDF/GO. PVDF/ZnO/GO tri-phase composite shows higher open circuit voltage (Voc) and short circuit current density (Jsc) resulting in highest output power delivering composite in different modes like finger tapping, periodic bending and repetitive stretching. PVDF membrane shows responsivity of 0.004 (µA/cm2/W) in presence of solar spectrum at an external bias of 10 V with rise time of 44 sec. Introduction of increasing GO amount upto 15% (v/v) gradually increases the photocurrent to 4.0 µA/cm2 with shorter response time of 21 sec. Further, in mechanically bend condition, all of these composites show higher dark and photocurrent due to formation of piezo-potential. Impedance spectroscopic study is conducted to gain proper insight of these occurrences along with effect of filler inclusions. Also, effect of external stimuli like mechanical stress and light irradiation on those composites are explained from the changes in various parameters obtained from the fitting of Nyquist plots drawn from impedance spectroscopic measurements.Item Many-body physics of interacting bosons in optical latticeChakrabarti, BarnaliThis thesis addresses the many-body physics of ultra-cold bosons loaded in optical lattices. Ultracold atoms in an external trap has received most attention from both experimental and theoretical research as it is considered as the ideal testbed to study the rich many-body physics. The existing mean-field Gross-Pitaevskii method is inadequate to probe the highly correlated and strongly interacting atoms in optical lattice. To solve the many-body Schrödinger equation, we use multi configurational time-dependent Harte method for bosons (MCTDHB) which is an ab-initio many-body calculation. This thesis can be divided into three different parts. A quick overview of optical lattices is provided in the first section. In addition, the formulation of the MCTDHB method is explained in detail. The next section of the thesis comprises time-independent many-body calculations of bosons in an optical lattice. Bosons exhibit distinct phases in an optical lattice. Because of quantum fluctuations, quantum phase transitions can occur even at absolute zero. We have characterized those phases utilizing Shannon information entropy and Glauber normalized correlation measures. The time-dependent many-body theory is the subject of the last part of the thesis. MCTDHB is an efficient theoretical tool to study the dynamical evolution of the interacting bosons in the optical lattice. Here we utilize MCTDHB to study the quench dynamics and to address whether thermalization and statistical relaxation is ubiquitous in nature. To study the time dynamics, we have quenched the system by manipulating the interaction strength or the lattice depth and studied different dynamical measures. Bosons tunnelling in a double well potential are also discussed in this thesis. Self-trapping is seen at a moderate interaction regime, whereas Josephson oscillation is observed at non-interacting limit. Furthermore, in the strong-interaction regime, we found a unique phase of tunneling dynamics which was not observed in the mean-field scenario. Additionally, we have addressed the path from the SF phase to the MI phase by directly monitoring the change in order, disorder, and complexity of the system. We demonstrated that complexity is a more comprehensive measure that may be used as a ‘figure of merit’ to accurately estimate the time-scale during time dynamics.Item Statistical studies of metastable Lifetimes in magnetic model SystemsAcharyya, MuktishIn my thesis, I have focused on metastable behaviours of a ferromagnetic system. The studies have been carried out particularly on Ising and Blume-Capel ferromagnet in presence of externally applied magnetic field using Monte Carlo simulation technique based on Metropolis algorithm. The metastable lifetime is found to decrease in presence of quenched random field compared to that in presence of uniform field only. The strength of the random field is observed to play a similar role as played by temperature on metastable lifetime. Becker-D¨oring theory of classical nucleation (originally proposed for the spin-1/2 Ising system), as well as Avrami’s law, have been verified in the random field Ising model. However, the nucleation regime is found to be affected by the stronger random field. The metastable lifetime of both the surface and bulk have been studied separately by introducing a relative interfacial interaction strength (R). Surface reversal time is found to be different from the bulk reversal time. Depending on R, temperature and applied field, a competition between the surface reversal and bulk reversal is noticed. The effect of anisotropy (D, both positive and negative) on the metastable lifetime has been investigated. The linear dependency of the mean macroscopic reversal time on a suitably defined microscopic reversal time has been observed. The saturated magnetisation Mf, after the reversal, is noticed to be strongly dependent on D. That Mf, D, and h (field) are found to follow a proposed scaling relation. The metastable behaviours under the influence of graded and step-like variation of both the applied field and anisotropy have been explored. Motion of an interface, arising due to the presence of a stronger gradient of either field or anisotropy, has been studied. A competitive reversal by the field and anisotropy is observed when I consider the similar spatial modulation of both the field and anisotropy.Item Assessment of atmospheric carbon dioxide and aerosols from ground spectroradiometry and hyperspectral image analysisRaychaudhuri, BarunThe role of anthropogenic carbon dioxide (CO2) in global warming is a serious issue of climate change. Developing precise monitoring systems for greenhouse gases including CO2 has got tremendous impetus during the last two decades. The physical principle of modern CO2 sensors is to quantify the extent of absorption of solar radiation depending on the gaseous concentration at specific wavelengths of molecular absorption. The techniques of hyperspectral remote sensing, popularly known as ‘imaging spectroscopy’ have achieved immense significance in this connection. The present work has investigated on the assessment of atmospheric CO2 profile using a combination of ground-based solar radiance measured with spectroradiometer of high spectral resolution, radiance spectra retrieved from airborne hyperspectral images and the statistics derived from the database produced by satellite-borne hyperspectral global CO2 sensors. The studies are conducted from different points of view: at local level, particularly in the Indian context and at global level. Both the atmospheric column average and the ground level concentrations of CO2 and their relationships are explored. The spatial as well as the temporal variations of the column concentration are addressed. In brief, attempts are made to divulge all the facets of the present CO2 situation and its relationships with human activities. Along with CO2, the urban aerosol is considered to be related closely to man-made events and the aerosol optical depths are estimated from hyperspectral images. The main highlights of the work are spectral calibration of CO2 absorption profile under the influence of water vapour in the tropical atmosphere of India, new techniques for estimating CO2, aerosol and water vapour concentrations, spatial variation of CO2 with detection of point source, correction for the airborne sensor height, seasonal and temporal changes of CO2 and aerosol and the present trends of CO2 in Indian and Global context.Item Characterization of Chemical Vapor Deposition Grown Thin film Semiconductor Nanostructure for Optoelectronic ApplicationsNayak, ArabindaThis work first represents a detailed high-resolution X-ray reflectivity (XRR) and rocking curve (XRC) analysis of a spontaneous superlattice structure in MOCVD grown AlxGa1-xAs epilayer on (001) GaAs. A model-independent Distorted Wave Born Approximation formalism was applied to the XRR results to obtain the compositional information as a function of depth. A kinematical model was then used successfully to fit the sharp peaks observed in the XRC using the characteristic length and compositional parameters obtained from the XRR simulation. The thermal stability of such superlattice was analyzed with the help of X-ray rocking scans at elevated temperatures. The ordered (spontaneous) structure irreversibly transformed to the disordered one retaining its single-crystalline nature at 900o C. For photo detection, the (Au-Ge) alloy was deposited on the as-grown and the disordered sample to realize an (MSM) photodetector (400-900 nm). Furthermore, a comparative analysis of the two devices was also discussed. In the next part, a symmetric coplanar MSM structure with Nickel as metal contacts was fabricated on quaternary (AlxGa1-x) yIn1-yP epitaxial film on (100) SIGaAs for visible photodetection. The crystallinity and alloy composition (y=0.66 and x=0.67) of the grown material was determined by high-resolution X-ray rocking curve and photoluminescence experiment. The photo-response of the device was studied in the wavelength range of 300-1000 nm which exhibited a peak at 610 nm. The photodetector showed a fast rise time of 91 μs however, the decay characteristics exhibited two channels with decay times of 83 μs and 282 μs, respectively. In our final work, X-ray, SEM, and photoluminescence analysis were employed to understand the band-edge related excitonic lasing features from the vertically oriented ZnO nanowires grown by CVD on sapphire. The nanowires exhibited high degree of crystallinity, strong preferred orientation, and relative inplane alignment having 6-fold hexagonal symmetry of the Wurtzite phase. A 325 nm laser source was employed to optically pump the nanowires. The nanowires showed the onset of lasing below 150 K being prominent at 3.4 K. The threshold pump power was found to be 14.5 mW at 3.4 K. Such nanowires are suitable for lasing in the ultraviolet region of the electromagnetic spectrum below a characteristic temperature of 150 K at a low threshold optical pumping power.Item X-ray Properties of Galaxy Groups and ClustersChatterjee, SuchetanaActive Galactic Nuclei (AGN) plays an important role in the evolution of large scale structure in the universe. A large number of observations, ranging from X-ray studies of galaxy clusters to observing correlations between the central supermassive black hole (SMBH) with their host galaxies, point to its important role in structure formation. In this thesis, we explore the influence of feedback from AGN on their surrounding medium and study the cosmological evolution of AGN along with their host dark matter halos and galaxies. For this purpose, we use a cosmological volume simulation that includes dark matter dynamics, hydrodynamics, star formation along with a subgrid modeling of black hole formation and their associated feedback (Di Matteo et al. 2008; Khandai et al. 2015). We investigate various correlated properties of AGN-host galaxies and explain them in terms of feedback from the AGN. From the temperature and density maps of the gas surrounding the central SMBH, we statistically show that AGN feedback effectively evacuates the gas in the ambient medium as well as energize them, resulting in a deficit of the cold gas supply, which in turn suppresses the black hole growth and halts star formation. Moreover, we model the theoretical X-ray emission from galaxy groups and clusters and develop a promising technique of synthetic X-ray observation to compare the theoretical results with actual observations. We find that the stacked X-ray emission from the diffuse gas surrounding the black holes in the presence and absence of AGN feedback reveals a suppression in the X-ray surface brightness profile in the vicinity of the AGN producing feedback. This also shows agreement with the results from real observations. Finally, we aim to establish the robustness of our simulated results by comparing them with the results of a different cosmological simulation (Dav´e et al. 2019, SIMBA). We show that galaxy, black hole and X-ray properties of individual systems in both the simulations agree well with each other, indicating their ubiquity. We further show that different modes of feedback might have substantially – 6 – different effect on the galaxy and gas properties for AGN host galaxies. Finally, we propose new observations with current and upcoming X-ray telescopes to investigate the effect of AGN feedback in galaxies and clusters in the high redshift universe.Item Development, Synthesis and Characterization of II-IV-V Chalcopyrite Thin Film for Photovoltaic Device ApplicationNayak, ArabindaDuring last few years much research attention has been devoted for the development and design of new highly efficient economic optoelectronic devices to replace Silicon (Si) based devices for commercial application in various field. The major disadvantage for Si based devices is the indirect electronic bandgap which limits the devices efficiency. This work exhibits the growth, characterization and device application of a novel lead (Pb) free material ZnSnP2 as a replacement of Si. Here, we have successfully grown the ZnSnP2 thin film on p-type silicon (001), sapphire and glass substrate by physical vapor deposition technique for the very first time. It is also evident that during growth the deposition rate plays a significant role to determine the bandgap. At higher growth rate the bandgap of ZnSnP2 thin film decreases due to increasing cationic disorder. Temperature and power dependent photoluminescence properties indicate that the peaks have originated due to donor-acceptor pair related transition. For photodetector application Mg/ZnSnP2/Sn structure is fabricated to operate in the UV-VIS-NIR region of the electromagnetic spectrum. The observed current-voltage characteristics show roll-over like features which was successfully modeled considering two Schottky diodes connected back to back. The maximum values of responsivity, photosensitivity and detectivity are found to be of 29.58 mAW-1, 1.0x102 cmW-1 and 9.42x1010 cmHz1/2W-1 in the forward bias and 0.0076 AW-1, 1.2x102 cm2W- and 15.53x1010 cmHz1/2W-1 in the reverse bias, respectively at illumination of wavelength 850 nm and it offers very fast response speed of 47 μs. The performance of the photodetector is investigated from energy band diagram and to improve it the Sn contact was annealed at 250o C for one hour to reduce barrier height and the respective junctions were treated individually. After annealing the obtained values of the responsivity and detectivity are found to be of 0.2 AW-1 and 4.62x1012 Jones for the Mg/ZnSnP2 and 0.03 AW-1 and 1.62x1011 Jones for the Sn/ZnSnP2 junctions, respectively when they were operated without bias. Under a bias of 2.5 V, these values enhance to 4.7 AW-1 and 1.19x1012 Jones in case of Mg/ZnSnP2 and 12.7 AW-1 and 4.78x1011 Jones for Sn/ZnSnP2 junctions, respectively. The variation of photocurrent is super-linear with the illumination power with the values of the power exponent for the Mg/ZnSnP2 and Sn/ZnSnP2 junctions of 1.75±0.19 and 1.42±0.21, respectively.Item 2D nano materials and their hybrids as supercapacitor electrodes for energy storage applicationsDe, SukantaTwo dimensional (2-D) nanomaterials have drawn considerable attention to the evolution of supercapacitor electrodes for high performance flexible energy storage device applications due to their high electro-active sites, high specific surface area, macro mechanical flexibility, wellbehaved electrical properties etc. The thesis entitled “2-D Nano Materials and their Hybrids as Supercapacitor Electrodes for Energy Storage Applications” encompasses a detail report on the development of some 2-D nanomaterials through different synthesis routes and their applications in supercapacitor energy storage. More precisely, this dissertation highlights the successful synthesis of some 2-D transition metal oxides (TMOs) like MoO3, MnO2, RuO2, metal dichalcogenides (MoS2), BiOCl nanoplates, vanadyl phosphate (VOPO4) and their composites with graphene, single and multi-wall carbon nanotubes (SWCNT, MWCNT) etc. and their applications as supercapacitor energy storage devices. The prepared samples have been thoroughly characterized by FESEM, TEM, XRD, XPS, AFM BET, Raman and etc. In this work, a facile large scale production of TMOs nanosheets by exfoliation in ethanol-water mixed solvent instead of using highly toxic organic solvents has been demonstrated. After optimization, this method gives high concentration (0.42 mg/ml, 0.47 mg/ml and 0.40 mg/ml for MoO3, MnO2 and RuO2 respectively) stable dispersion of TMOs nanosheets. Thin flexible solid state supercapacitors based on SWCNT /TMOs nanosheets composites showing a large specific capacitance and high energy density. In addition, a planner supercapacitor based on SWCNT/MnO2 shows an improved electrochemical performance with excellent stability under different bending condition. Thin flexible films of BiOCl/MWCNT composites have been prepared with varying the weight percentage. Among various compositions, 60% BiOCl loaded electrodes delivers superior electrochemical performance. Instead of gold we have used SWCNT network as a current collector which will be helpful to make transparent flexible supercapacitor with high energy density for future application BiVO4/rGO hybrid nanostructure based symmetric supercapacitor exhibits better performance in solid electrolyte. MoS2/rGO hybrids thin film supercapacitor and MWCNT/VOPO4 hybrid supercapacitor are also deliver improved energy storage performance.Item Thermodynamics and Statistical Fluctuations for the Trapped Interacting Bose GasChakrabarti, BarnaliWe study the condensate fluctuation and several statistics of weakly interacting Bose gas by a correlated many-body approach -correlated potential harmonics method (CPHEM). We include all two-body correlations and realistic van der Waals interaction to probe real experimental situation which considers the mesoscopic condensate and go beyond the scope of mean-field and existing many-body theoretical methods like Diffusion Monte Carlo and Faddeev approach. We prescribe an ab initio but approximate many-body calculation which accurately calculates energy spectrum of the condensate. We consider attractive Bose gas of 7Li in harmonic trap and repulsive Bose gas of 87Rb both in harmonic and anharmonic trap. Experimental observation of atom number fluctuation for repulsive condensate of 87Rb by M. Kristensen et al. [144], increases the hope that in near future the same for attractive condensate will also be performed. We theoretically study the fluctuations properties like condensate fraction, root-mean-square fluctuation and different orders of central-moments of the condensates for both repulsive and attractive condensate. We observe the lambda-structure in all the fluctuation measures near the transition temperature which remain same qualitatively in all types of traps. We define three characteristic temperatures calculated from the point of inflexion of the lambda-structure which jointly confirm that the achievement of condensation is more favourable in anharmonic trap. The possibility of true phase transition is ruled out for all traps for the mesoscopic Bose Einstein condensation. The effect of interaction (repulsive and attractive) on the energy fluctuations of BEC in the mesoscopic regime is thoroughly studied and observed the effect of interatomic correlation for the attractive condensate. The possibility of phase transition is ruled out in the mesoscopic regime irrespective of the type of interactions.Item Study of Electronic Structure, Optical and Magnetic Response of Disordered SolidsNayak, ArabindaMy Ph.D program includes the theoretical studies of the electronic structure, optical and magnetic response of ordered compounds and disordered alloys. We have studied the optical conductivity and dielectric response of disordered, bulk NixPt1−x alloys and disordered, two dimensional SixC1−x. This study is based on the density functional theory (DFT) approximation and the tight binding linear muffin tin orbitals (TB-LMTO) method. Disorder is taken into account by the “Augmented Space Formalism” which allows us to accurately estimate the effect of disorder both homogeneous disorder as well as inhomogeneous disorder related to short-ranged ordering, clustering and segregation. This can be done both in bulk, surfaces and interfaces. We have overcome the defeciency of the local density approximation (LDA) and incorporated the vLB-corrected exchange correlations. Our estimation reasonably, successfully agrees with the initial experimental observations. This encourages me to go forward in this area. The proposed formalism opens up a facile way to band-gap engineer material for optoelectronic application. In parallel, we have generated the electronic structure of disordered Graphene and Graphinic materials with random vacancies and random doping have also been investigated. We have shown how the topology of the Dirac dispersion changes with disorder. We have studied disorder induced lifetime effects in 2D materials. Our study will provide a reference and detailed useful insight for building interesting nano materials. Specifically, I have worked on the magnetization dynamics and excitation spectra of disordered binary FexCo1−x alloys. We have studied the magnon softening for chemically disordered Fe20Co80 alloys in the linear spin wave regime. Our proposed formalism on magnetic response can be extended to study the spin transport, magnetic properties, excitation spectra and spin dynamics of real complex materials, ferromagnetic semiconductors and Heisenberg spin chains in disordered alloys. In this thesis, we have not only been able to study the electronic structure and response function of a few disordered systems but also successfully analyzed and interpreted the experimental results based on our theories.Item Nonequilibrium Phase Transition in Ferromagnetic Model SystemsAcharyya, MuktishIn this thesis, I have mainly discussed the dynamical responses and behaviours of a ferromagnetic system under different kinds of magnetic field variations that are varied in time as well as in space, especially in the form of propagating magnetic waves and standing magnetic wave using Monte-Carlo simulation. These magnetic field waves cause the ferromagnet to undergo nonequilibrium phase transition; depending on the temperature, amplitude and wavelength of the magnetic field and the strength of anisotropy. In an Ising ferromagnet driven by standing magnetic field wave the high temperature phase is found to be quite different from that observed when propagating magnetic wave was passing through the Ising ferromagnet. Dynamical phases having similar attributes are formed in Blume-Capel (S = 1) ferromagnet when placed under these kinds of waves. However, in BC ferromagnet the strength of anisotropy affects the transition temperature as well as the morphological structure of the ferromagnetic spins. The studies regarding general characteristics of spin-S Ising ferromagnet excited by magnetic field waves revealed that the nature of dynamic phase transition is similar to the earlier studies but the transition temperature is found to decrease towards a limiting value with increase in the value of S. The values of critical exponents of spin - 1/2 Ising ferromagnet driven by propagating magnetic field waves are found to be very close to those obtained in Onsager’s solution for equilibrium ferro-para phase transition. This indicates that the nonequilibrium phase transition in an Ising ferromagnet driven by propagating magnetic wave belongs to the same Universality Class of equilibrium Ising ferromagnet.