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Browsing Physics by browse.metadata.institutetitle "Central Department of Physics"
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Item Electronic Transport in Bulk and Quantized Low Dimensional Semiconductor Systems(Department of Physics, 2005) Shrestha, SanjuIt is very interesting and important to study electronic transport parameters of bulk and low dimensional systems. Bulk material, such as GaN, a III+V compound semiconductor is a very useful material for electronic device applications due to its direct and wide band gap and also due to the strong bond strength. However, a non availability of single crystalline form of GaN and perfectly matched substrates are always problems for GaN. Hence, GaN, grown by various chemical vapor deposition techniques on a substrate such as sapphire is having large dislocations at the interface, the layer between the bulk layer of GaN and the sapphire due to the mismatch in the lattice constants. Such interfacial layer significantly affects the transport parameters of the material, where the transport properties are high dominated by scattering due to dislocations. The author calculated various transport parameters such as ac/dc mobilities, Seebeck coefficient, thermal conductivity and the thermoelectric figure of merit considering two layer model formulated by Look and the results are also found to be agreement with the experimental values. In a junction between high and low band gap semiconductors, the carriers are transferred from higher band gap to the lower band gap due to which a space charge is generated in the higher band gap. At the same time, charge gets accumulated at the lower band gap forming an accumulation of charges. It causes the band bending and the formation of a triangular quantum well. The carriers accumulated at the interface of the junction get quantized into various energy levels. Such system behaves as quantum-two dimension (Q2D) because the carriers are free to move on a plane, perpendicular to the junction. The mobility of such a system is very high due to reduction of the scattering mechanisms as well as due to charge getting separated from the parent donors. Further, by confining the one more dimension of the heterojunction by taking an infinite potential well causes the carriers to move only on one dimension and the system is called quasi-one dimensional (Q1D) system. Such Q1D system is also called quantum well wire (QWW) because; the carries are restricted to move on only one dimension instead of a plane, which was in the case of Q2D system. The author has carried out comparative studies on Q2D and Q1D ac/dc mobilities in the heterojunctions of AlGaAs/GaAs and GaInAs/InP. The effect of various low temperature phonon and nonphonon scattering mechanisms on the systems are also observed. In addition, the Q1D system formed by magnetically confined system is also attracting attention of researchers in device application because it is capable of over looking the various techniques of fabrication difficulties and defects created by such fabrication techniques. In the presence of a high magnetic field, the transverse component of the energy dispersion relation gets quantized into various equally spaced energy levels called Landau levels and the motion of the carriers is completely restricted. However, the longitudinal component along the field is still free to move. The mobility of such system is enhanced when a low effective mass semiconductors n-HgCdTe (Mercury Cadmium Telluride) is used. The band structure of n-HgCdTe is found to be nonparabolic due to its low band gap according to Kane [Phadke and Sharma, 1975]. Recent publications, based on experimental verifications of transport coefficient of n-HgCdTe of Chen and Sher [Chen and Sher, 1982] show that the band structure of MCT (Mercury Cadmium Telluride) is more hyperbolic in nature rather than nonparabolic. The author has compared the effect of band structures on the various transport properties of MCT such as mobility, Seebeck coefficient, thermal conductivity, figure of merit ( )Z etc. The figure of merit is a very important property of a material to be used in thermoelectric devices, such as cooler, refrigerator etc. Emission of photoelectron takes place when a monochromatic photon of energy ωh is incident on a sample. These photoelectrons will have the information about the quantized states they left. Depending upon the ii strength of the application of the photon energy, the transverse components of the energy dispersion relation with various quantized energy levels in the case of magnetic confinement take part in the photoemission process. Hence, the photoemission causing photo electric current density is found to be step like nature due to the participation of various energy levels. Comparative study of the photo emission due to geometrically confined system of dimension exactly as that of the magnetically confined structure is another interesting study presented in the thesis. When such a magnetically confined Q1D system be replaced by a thin film of thickness equal to that of the deBroglie wave length, the longitudinal component of the energy dispersion relation also gets quantized and the system behaves as quantum-zero dimensional (Q0D) system. The comparative studies of such magnetically and geometrically confined systems have also been formulated in the thesis. Due to the presence of impurities in semiconductors quantized energy levels get broadened. Hence, the photoelectric current density gets modified due to broadening. The author has observed and compared the effect of broadening of the quantized energy levels on the photoelectric emission due to magnetically and geometrically confined systems. It is found that the characteristic nature of the photoelectric current density remains the same but the values of the current density decreases with the inclusion of the broadening parameter. Phadke U. P. and Sharma S., J. Phys. Chem. Sol. 36, 1 (1975). Chen A.B. and Sher A ,J. Vac. Sci. Technol. 21, 138 (1982).Item First Principles Study of Properties of Functionalized MXENE (Ti2N) and Defect onMXENE Mono-Layers(Department of Physics, 2021) Limbu, YogendraThe evolution of structural and electronic properties of graphene-like titanium nitride MXene has been studied with different functional groups (-F, -O, -H, and -OH) employing first principles electronic structure calculations. The calculated formation and cohesive energies reveal the chemical stability of all MXenes and MAX phase. The bare MXene exhibits same hexagonal symmetry as its parent bulk MAX phase. The surface terminated species are distributed randomly on the surface of bare MXene with preferred locations between Ti atoms. The MAX phase and all the studied defect free functionalized MXenes are metallic in nature except for oxygen terminated one, which is found to be 100% spin polarized half-metallic. Additionally, the bare MXene is nearly half-metallic ferromagnet. The spin orbit coupling (SOC) only significantly influences in the bare MXene and trivially influences with O and N defected MXenes. The strain effect influences the Fermi level thereby shifting towards the lower energy state under compression and toward higher energy state under tensile strain in Ti . These properties are reversed in the case of Ti 2 N, Ti 2 NF 2 , and Ti . The halfmetallic nature changes to semi-metallic under 1% compression and completely destroyed under 2% compression. The variable topological phenomena have been studied in pristine, strained, and defected MXenes. Interestingly, the band structure of Ti 2 N(OH) remarkably transforms from half-metallic to semi-conducting (with large band gap of 1.73 eV) in 12.5% Ti, weakly semi-conducting in 5.5% Ti, and topological semi-metal in 12.5% oxygen. The N defect with 25% converts from half-metallic to metallic with certain topological features. Further, the 12.5% Co substitution in Ti preserves the half-metallic character, whereas Mn substitution allows to convert half-metallic into weak semi-metallic preserving ferromagnetic (FM) character. However, Cr substitution converts half-metallic FM to half-metallic anti-ferromagnetic (AFM) character. 2 2 NO 2 2 NH 2 NO 2 2Item First-principles study of structural, electronic and magnetic properties of defected (monovacant) hexagonal boron nitride sheet(Department of Physics, 2021) Khatri, KisanThe first-principles calculations based GGA functionals was implemented to study the structural, electronic and magnetic properties of pure and defected hexagonal boron nitride (h-BN) monolayer sheet using Quantum ESPRESSO (QE) package, 6.5 version. The pure h-BN is found to be non-magnetic insulator with band gap of 4.64 eV. The calculated values of formation energy reveals the structural stability of defected system. The formation energies for B and N vacant system are found to be 16.45 eV and 12.84 eV respectively which predicts that N vacant system is more preferable with lower formation energy. The defect on a system abruptly changes the electronic and magnetic properties of h-BN system. The 6.25 % B-vacancy and 6.25 % N-Vacancy defects are found to be half metallic ferromagnet with total magnetization of 2.74 /cell and magnetic semiconductor with total magnetization 1.00 B /cell respectively. BItem Fluxon Dynamics In Couple Long Josephson Junction Based on Two-Gap Superconductors Like MgB2 And Iron-Prictides(Department of Physics, 2021-09) Chimouriya, Shanker PrasadIn the present work, the fluxon dynamics in long Josephson junction based on two-gap superconductors like MgB and iron-pnictides has been studied. The procedure has been started by establishing the microscopic BCS Hamiltonian of the junction system in terms of fermionic field operators. In order to enter in the long route of path integral formalism, the Hamiltonian has been introduced into the quantum mechanical partition function through the definition of Lagrangian density and then action functional. Some important steps such as Hubbard-Stratonovich transformation for bosonization, Nambu representation, reciprocal space transformation, saddle-point (mean-field approximation), Goldston mode etc. have been followed in order to simplify the action and hence Lagrangian density. The Lagrangian density, which are of solely the function of phase differences across the junction barrier, is minimized to derive the system of perturbed sine-Gordon equations which help to explain the phase dynamics of the junction system. In the present work, the system of sine-Gordon equations are established for the stack of long Josephson junction based on multi-gap superconductors and then applied for two-gap superconductor like MgB 2 . The generalized sine-Gordon equations for stack of LJJ are used to explain the phase dynamics in the single and double (coupled) LJJ. The system of perturbed sine-Gordon equations, for single and double junctions, have been solved numerically using the finite difference approximation, assuming the solution of unperturbed sG equation as the initial condition. The Neumann boundary condition has been maintained so that the kink or anti-kink can reflect at the boundary. The dynamics of phase differences have been observed for different layer and junction thicknesses and found that the motion of kink or anti-kink, which also represents the fluxon or anti-fluxon, is found to be more complicated as time goes on. During the motion, it has been observed that fluxons and anti-fluxon are created and superposed to each other. As a result, phase locked and anti-locked situation have been observed. The Josephson part of Lagrangian density has been computed and minimized in the domain length at each time step. In order to study the phase frustration, the minimized energy is plotted as the function of time in addition to the corresponding phase differences. It is found that 2 the phase frustration occurs quickly for higher layer and junction thicknesses. It is also observed that the phase frustration occurs at low time for higher tunnel voltage. The current-voltage characteristics have also been studied by computing the average current flowing out across the junction system at different tunnel voltages. The current-voltage characteristics in coupled LJJ is found to be linear at very low tunnel voltage, slightly non-linear with positive differential resistance up to certain tunnel voltage depending on the junction geometry and completely non-linear for higher ones irrespective to the junction geometry. But the nature of the non-linearity solely depends of junction geometry i.e. thicknesses of the barrier and superconducting layer. In some non-linear regions, the negative differential resistances are observed which confirms that the device behaves as source or radiation chamber. The negative resistance may arise due to the non-dissipative transition of quasi-particles (i.e. fluxons or anti-fluxons) in the system. Due to this peculiar nature, the device is applicable to the low temperature electronic devices which demands the negative resistance.Item Kinetic Trajectory Simulation Model for Magnetized Plasma Sheath(Department of Physics, 2020) Adhikari, Bhesha RajPlasma wall interaction is an important phenomenon in all applications, where plasma comes into contact with a material wall. The understanding of plasma sheath, a thin layer that is formed between the core plasma and the wall, is crucial in understanding as well as controlling the particle and energy fluxes reaching the surface. Because of sharp gradients in the sheath region the fluid approach, which is usually used to explain the core plasma and presheath region, does not yield accurate results in the sheath region. In order to understand the characteristics of the sheath formed in a magnetized plasma the method of kinetic trajectory simulation (KTS) has been used and also the coupling of presheath-sheath has been extended. Assuming that, at the sheath entrance distribution functions of ion and electron to be cut-off Maxwellian the final self consistent states are obtained by solving the governing kinetic equations, iteratively. In order to satisfy the Bohm-Chodura condition, initial velocity of ions entering the sheath region is taken equal to the ion acoustic velocity. As the plasma sheath region is small compared to the collision mean free path and also the particle densities are less compared to the presheath region, the sheath region is assumed to be collisionsless. It has been observed that the presheath electron temperature affects the Child-sheath thickness and the space charge density reaching the wall. Furthermore, improved distribution of electrons, i.e., consideration of cut-off by the negative wall yields our simulation result to deviate from earlier works without cut-off by about 3%. The coupling scheme, developed, provides a basis for smooth transition of plasma parameters in presheath-sheath interface. Ion velocity at presheathsheath boundary is also affected by obliqueness of the field. Velocity of ion increases towards the wall as the obliqueness of the field increases. In the presence of oblique magnetic field in the plasma sheath, the damping of ion velocity with time has been observed. It is shown that the velocity waves are damped in plasma without collisions in the time scale of the order of seconds. As the obliqueness of the field changes the separation of the mean values as well as the maximum amplitude of all the three components of the velocity also change. Mean value of Gcomponent of velocity is nearly equal to zero for all angles at magnetic field 2.5 mT, but the mean value of H and Icomponents of velocity varies with nearly equal amplitudes. When the field is greater than zero, the mean value of Gcomponent is almost equal to zero, but the mean values of H and Icomponents of velocities are both nearly equal to 10 m/s at zero field. But for the greater value of magnetic field, the two components of velocities are split with different mean values. The frequency of oscillations of three component of velocity of ions changes as the magnetic field changes. The maximum amplitude of Gcomponent of velocity is almost independent of the magnetic field but the maximum amplitudes of H and Icomponents of velocity change and show oscillating nature as the magnetic field changes also. As the obliqueness of the field changes the mean values, beat frequency as well as the maximum amplitude of the three components of the velocity also change but frequency of oscillation almost remains same at magnetic field 2 mT. Modulation frequency of ions in a magnetized plasma sheath has been observed for different angles at constant magnetic field 6 mT. Also, by varying angle average values, maximum amplitude, damping factor as well as frequency of oscillation of the velocities are studied. For H and Icomponents of velocity maximum amplitudes change but for Gcomponent the maximum amplitude remains constant. However, there is a quite change observed in the values of damping factor and modulation frequency for all component of velocity. Frequency of oscillation of all velocity components of ions remains same. At 30 and 60 , shoulder is not seen but at 75 , shoulder in the velocity profile is obtained around 0.05 second for each component of velocity. The amplitude of the oscillating velocity decreases with time and the mean values of different component of velocity changes as well. The computed and fitted values of the vector sum of oscillatory part of all three components of velocities match. At angle 30 , damping rate of vector sum of oscillatory part of total velocity increases with the magnetic field increases from 1 mT to 5 mT. On the other hand vector sum of oscillating part of initial velocity is almost equal for magnetic field 1 mT, 3 mT and 5 mT at the same angle 30 . But for different obliqueness of the magnetic field, the initial velocity of the vector sum of oscillating part at 90 is greater than that at 30 and 60 . The input physical parameters taken are consistent with earlier previous works and comparison reveal that our results agree well. The present magnetized plasma wall transition study provides a basis for proper understanding of the plasma sheath, which has useful applications in fusion devices as well as in many industrial applications of plasma.Item Molecular Dynamics Study of Diffusion of Argon in Water at Different Temperatures(Department of Physics, 2010) Pandey, AnupMolecular dynamics study of a binary mixture of argon and SPC/E water, with argon as solute and water as solvent, at argon mole fraction of 0.023 have been accomplished at temperatures 293 K, 298 K, 303 K, 308 K and 313 K. The solvent-solvent, solute-solute and solute-solvent radial distribution functions (RDFs) have been estimated. The water-water radial distribution function has been found to agree with the experimental values within 5%. Self-diffusion coefficients of both solvent and solute have been determined by means of mean-squared displacement (MSD)curves using Einstein’s relation. The evaluated selfdiffusion coefficient at temperature 298 K has been found to agree with the experimental value within 4%. Also the values of self-diffusion coefficient of water have been found to agree with the available experimental values within 8% at maximum. Then, the Darken’s relation has been invoked in order to determine the mutual/binary diffusion coefficients at the respective temperatures. The temperature dependence of the diffusion coefficients has also been analyzed. The estimated values of self-diffusion coefficients of water and argon as well as the mutual diffusion coefficients of argon in water have given the linear Arrhenius plot, which indicates that the self-diffusion coefficients have an Arrhenius dependence on temperature. Moreover, the temperature dependence of the diffusion coefficients has been found to be consistent with the nature of RDF’s at the respective temperaturesItem Nature of the Molecular Interaction in DNA-Protein Complexes(Department of Physics, 2021) Koirala, Rajendra PrasadDNA base methylation at the O6 point of guanine is a major cause of cancer. This methylation transits from G:C to A:T mutation pairs during the DNA replication and translation process. The O6-alkylguanine-DNA alkylguanine (AGT) serves as a non methylating agent, which repairs methylation damage at the O6 point of guanine and O4 point of cytosine by direct damage reversal mechanism. Despite the identification of its role in the methylation damage repair process in the human body, a detailed study is necessary to unlock the underlying mechanism during this methyl transfer process. The present work is focused on the microscopic investigation of DNA-AGT interaction to explore the more insight on DNA damage repair mechanism. Molecular dynamics (MD) simulation has been carried out to investigate structural basis of the DNA methylation damage repair mechanism, modeling three basic structures: pre-methyl transfer condition (complex-I), transient intermediate state (complex-II), and post-methyl transfer condition (complex-III). Complex-I represents the formation of the DNA-AGT complex and complex-III represents the deformation of the complex AGT from DNA. Complex-II is a close representation of the transient intermediate state of complex-I and complex-II. The structural and thermodynamic stability of each complex was examined with several physical aspects. Formation of hydrogen bonds as well as energy contributions due to electrostatic as well as van der Waals interactions were taken into consideration to investigate favorable binding of the molecules in all three complexes, which agree with the findings of Daniels et al., 2004. Steered Molecular Dynamics (SMD) results showed that the force in pre-methyl transfer process was greater than that of the post-methyl transfer condition referring to a more favorable binding between DNA and AGT in complex-I. Our findings on force of binding between DNA and AGT agree with AFM experiment of Tessmer and Fried, 2014; and DFT results of Jena et al., 2009. The study was further extended to investigate the changes in the free energy during the interaction of pre- and post-methyl transfer: the former with methylated GUA7 in DNA and the later with methylated CYS145 in AGT. The umbrella sampling method was utilized to calculate the free energy and the results suggested that the change in free energy during the pre-methyl transfer process is greater than that of the post-methyl transfer by 1.3 kcal/mol thereby demonstrating the stronger binding affinity of methylated GUA with AGT than that of the complex in which methylation lies at AGT. Our free energy result agrees with the outcomes of Hu et al., 2007.Item Ordering and Segregation in Liquid Alloys(Faculty of Physics, 2018) Yadav, Shashit KumarThe design and fabrication of new alloys is usually approached by a method that combines theoretical analysis and experimental observation (composition−structure− property), but in many cases, due to the experimental difficulties related to high temperatures, the theoretically predicted property values are of key importance. This is particularly relevant for all industrial processes, such as casting, joining, crystal growth, etc. that involves the presence of the liquid phase, i.e. liquid metals and alloys. Alloying processes have been evolved as one of robust tool to achieve desired materials with required characteristics. The thermal treatments of materials, chemical compositions and operating parameters (temperature, pressure and working atmosphere) overrule the microstructure of an alloy. The in−depth knowledge of thermodynamics, kinetics and thus, the energetic of the alloy prototypic process has great significance in metallurgical science and engineering. In view of the aforementioned, we have studied and explained the mixing behaviour of two Al−based (Al−Fe at 1873 K and Al−Mg at 1073 K) as well as two Bi−based (Bi−Tl at 750 K and In−Bi at 900 K) liquid binary alloys with the help of theoretical modeling. The thermodynamic properties, such as free energy of mixing (G ), activity (a), enthalpy of mixing (H ) and entropy of mixing (S vii ), structural properties, such as concentration fluctuation in long wave length limit (S (0)), chemical short range order parameter (α ) and ratio of diffusion coefficients (D ) for above mentioned liquid alloys at chosen temperatures have been analyzed in the fame work of regular associated solution model. For this purpose, we have determined the model parameters which are the interaction energy parameters (ω , , ω , and ω /D , ), equilibrium constant (k), mole fractions of complex (x ) and free monomers (x and x ). The compositional contribution from the heat associated with the formation of the complex and the heat of mixing of species to the net enthalpy change has been studied for each system. The comparative studies of the thermodynamic properties of these systems reveal that the Al−Fe being the most interacting system followed by the Bi−Tl, the Al−Mg and the In−Bi, in which the viii ABSTRACT interactions are the weakest. Theoretical investigations of structural properties show that all the preferred liquid alloys show complete ordering nature at least close to their respective melting temperatures. The interaction energy parameters are found to be temperature dependent. The surface properties of the chosen liquid alloys have been explained with the help of Renovated Butler model. The trends of surface segregations in these liquid binary alloys have been studied by computing surface tension (σ) and surface concentrations (x and x ). Theoretical investigations confirm the segregation of the alloy component having a lower surface tension, i.e. the extent of segregation of Bi−atoms at the surface layer, which is more pronounced in In−Bi melts with respect to that of liquid Bi−Tl alloys. Moreover, in case of two Al−based melts, the Al−atoms segregate on the Al−Fe surface phase whereas they remain in the bulk region of Al−Mg. Additionally, the regular associated solution model has been extended to study and predict the thermodynamic and structural properties of concerned liquid alloys at different temperatures. For this purpose, ω , , ω , and ω for each of the system have been computed at different temperatures keeping x , , x , x and k invariant. The modeling equations obtained by the polynomial fitting of different orders along with the values of parameters to forecast these properties have also been included in this work. Theoretical computations indicate that the excess Gibbs free energy of mixing (G ) of the alloys gradually decreases with an increase in temperature above melting temperatures. Accordingly, at higher temperatures, the ordering or the compound formation tendencies of these alloy systems gradually decrease and sometimes show segregating nature. These findings are further supported by decrease in deviations between the computed and observed values of S (0) at higher temperatures. The liquid alloys thus show the maximum tendency towards complex formation at their respective melting temperatures, however, these tendencies significantly decreases at elevated temperatures. Thermodynamic properties have been then correlated with the Renovated Butler model to explain the surface properties at different temperatures. The computed values for the surface concentrations of the segregating components of the liquid alloys approach respective ideal values at higher temperatures. Similarly, the surface tension of metallic melts metals and alloys, decrease at elevated temperatures.Item Spatial Orientation of Angular Momentum of Galaxies in the Clusters and Superclusters(Department of Physics, 2016) Yadav, Shiv NarayanAvailable with full textItem Study of Cross Section for Ionization and Electron Capture Process(Department of Physics, 2018) Gupta, Suresh PrasadCollision of electron and ions with atoms and molecules is common technique for extracting information from such small entities. Electron impact ionization and excitation have been actively studied by many research groups. In spite of successes of using different quantal approximations in the case of light atoms, there exist difficulty in the calculation of electron impact single and multiple ionization cross sections for heavy atoms due to mathematical complexities. The binary encounter approximation (BEA) for the investigation of single and multiple ionizations of atoms by electron and heavy charged particles impact is found to be suitable. The approximation gives reliable results consistent with the experiments. Vriens (1966) derived a more reliable classical formalism of electron impact ionization including effect of exchange and interference. Various theoretical approaches have contributed in the development of binary encounter approximation for electron-atom, ion-atom collision processes. Double ionization of atoms and ions is a four particle interaction and hence it is still impossible to carry out exact calculations for these processes. Gryzinski and Kune (1999) have derived general analytical expression for electron impact double ionization cross sections in binary encounter model to describe the direct double ionization. In the first case the two electrons may be ejected from target atom by two successive encounters of the incident particle and secondly the incident particle may knock out one target active electron and the second electron of the target is removed by first elected electron. In this work heavy charged particles impact single and double ionization cross section for Cu and Fe atoms have been calculated in binary encounter approximation using Hartree-Fock momentum distribution for the target electrons. Electron impact single ionization cross sections for Kr, Xe and single and double ionization cross sections for Fe has been carried out. Our theoretical results for electron impact single ionization of Kr, Xe and Fe using binary encounter approximation are in good agreement with the experimental data. About 94.7 % in the case of Kr, 71 % in the case of Xe, and 93.9% in the case of Fe are within ratio factor of two and hence results are in close agreement with experimental data in the given energy range. In the case of proton impact single ionization cross section of Cu about 72 % results have ratio factor less v than 1.12 and 50% have less than 1.1. Same nature is observed in the case of Fe and about 94.7 % fall under validity region of ratio factor 2. In the case of He vi 2+ impact single ionization cross section of Cu and Fe the results agree well in intermediate and high energy region. For the He 2+ impact double ionization cross section of Cu and Fe about 75 % and 76.9 % of calculated results are in agreement with experimental results respectively. The direct double ionization of Fe is considered to be due to the ejection of loosely bound 3d and 4s electrons and also considered ionization of 3s electron to lead an excited state which results double ionization through auto ionization. Alpha particle impact double ionization of Fe and Cu have random fluctuations in the experimental observations in low energy range which are not observed in theoretical results. Further investigations are required both in experimental and theoretical methods. In different cases of single and double ionization by electron and heavy charged particles the calculated results are found to be in satisfactory agreement with the available experimental data. Theoretical knowledge of ionization cross section and collision dynamics find wide application in different fields of science. Phenomena involving electron collisions have important roles such as astrophysics, upper atmosphere of Titan, electron driven chemistry, low temperature plasma diagnostics, modeling of plasma in Tokomak, plasma processes in cometary, radiation effects, biomedical applications, display technology, astrophysics, Stellar model, radiative process in the earth’s upper atmosphere and medical application. Using a technique of Monte Carlo simulations track structure is usually used in micro and nano dosimeter to find radiation transport index in medical science. Better the results of cross sections used as simulation codes better the results of treatment in medical science. Projectile particles of ions like protons and helium deposit a large amount of their energy in a volume of a few micrometers or even nanometers and cause extensive damage to the microscopic structure of matter and results cell death in the DNA. With different suitable theoretical models one can predict reliable values of cross sections of different atoms/ions.Item Study of Electronic Structure of Clusters and Disordered Solids(Faculty of Physics, 2014) Kaphle, Gopi ChandraWe have carried out the first-principles calculation of Pd (n = 2-19) clusters with plane augmented wave (PAW) based Density Functional Theory (DFT) using the Perdew, Burke, Ernzerh of (PBE) exchange correlation functional implemented in Vienna abinitio Simulation Package (VASP) to understand the structural evolution, electronic and magnetic properties of the clusters. Our findings show that the highly symmetric structures like Icosahedral, Buckle Bi-planner, Cube-Octahedral and Hexagonal closed pack do not represent the minimum energy configurations for all the clusters. Present calculations show that the enhanced stabilities for clusters size (n) = 2, 8, 13 and 18 indicating that pristine Pd n n clusters follow the magic number effect. The highest occupied molecular orbital (HOMO)-Lowest unoccupied molecular orbital (LUMO) gap is higher for these highly stable clusters in comparison to their neighbors. Interestingly, even though bulk structure of Palladium is nonmagnetic, some of the finite size clusters possess significant magnetic moment. The highest value of magnetic moment is found to be 6.57µ for n = 13 in Icosahedral structure. To get further insight into the effect of Mn and Mn B 2 doping on magnetic properties of Pd clusters, calculations have been carried out to study the magnetic properties of Pd n (n−1) Mn and Pd for n = 2-13. For Pd (n−1) (n−2) Mn, the cluster of size (n) = 4, 7, 10 and 12 are more stable than their neighbors and magnetic moments for all the clusters increase due to Mn doping. The highest magnetic moments 9.64µ B is for Pd 12 Mn clusters. In case of Pd , the clusters of sizes n= 3, 7, 9 are more stable than their neighbors. The magnetic moments enhance due to Mn 2 doping on Pd (n−2) Mn 2 except for Pd clusters. It may be due to the fact that in Pd 11 Mn 2 11 cluster the spin of two Mn atoms align antiparallel. To perform adsorption and dissociation properties of hydrogen and nitrogen molecules on Ta n and Nb (n = 2-7) clusters, we have carried out structural stability, charge transfer, chemisorption energy as well as HOMO-LUMO gap. Our findings show that Ta n and Nb n n clusters favor the dissociation of both hydrogen and nitrogen except TaH 2 and NbH 2 . This indicates that Ta n and Nb clusters can be used as catalyst for the dissociation of hydrogen and nitrogen molecules which is necessary for the synthesis of ammonia. Present result agrees well with results of Yadav and Mookerjee for the case of Ta n H 2 for n ≥ 4. It is also found that Ta n n and Nb clusters bind nitrogen more strongly than hydrogen. It it mainly due to the fact that binding of nitrogen with triple bond which requires large amount of force to break the bond than single bonded hydrogen atom. It is also concluded that Ta n and Nb n n nano-structures may be used for the hydrogen storage materials. This will be the important task for future generation. We have performed density functional based ab-initio calculations through VASP to carry out the stability and magnetic properties of ZnO nanosystems with different morphologies like nanosheet, nanotube and fullerene type structures in pristine form. Our findings show that nanosheet favors most energetic than nanotube and fullerene like structures. All the morphologies in elemental(ZnO) form do not bear any magnetic properties. Further to get insight into the magnetic properties after doping of TM elements (Mn, Fe, Co, Ni, Cu) in pristine system, we perform near and far dop case in all the systems. Present study showed that ZnO:Mn always favors the near dop AFM alignment in all three morphologies. In case of ZnO:X (X=Fe, Co, Ni), the AFM alignment favors for sheet and this alignment changes while moving sheet to fullerene like vii Mn Mn 2 2 (n−2) Mn 2 structure. Our findings also show that there is lack FM alignment in ZnO:X (X = Mn, Co) indicating that these are not suitable candidate for spintronics applications at low temperature. Such properties agrees well with the previously reported data for bulk. This trends found to be changed while we are moving from tube to fullerene-like structures. To analyze the band gap properties, we used both PBE (GGA) and HSE06 version of hybrid functionals through VASP. We found there is no change in magnetic moments after the inclusion of Heyd-Scuseria- Ernzerhof (HSE). After the inclusion of HSE we found that Ni and Cu doped ZnO sheet show blue shift where as Mn, Fe and Co dope ZnO sheet show blue shift. Further ZnO:Ni tube shows blue shift with band gap 3.98 eV. We found red shift for all cases in ZnO doped TM fullerene like structures. We have discussed electronic and magnetic properties of disordered NiMn experimentally as well as theoretically. For the theoretical work, we used self consistent ASR code and performed calculation on different concentrations of Ni 1−x Mn with 15%, 20%, 25%. 30%, 35% and 30% of Mn by atom. For that we used lattice parameters 3.572 A x , 3.583A 0 , 3.595A 0 , 3.615A 0 , 3.654A 0 and 3.670A 0 coming through XRD analysis for increasing concentrations. It is observed that our theoretical and experimental phase diagram exactly matched with phase diagram of Montecarlo calculations. Further we used spin dynamics code to get more insight in to the spin glass behavior. From the present study we found that Ni 75 Mn shows anomalously slow relaxations which is one of the signature of spin glass phase. Further we studied the electronic and magnetic behavior of disordered Pt-Mn, Pd-Mn and Ni-Mn alloys. For this we used non-collinear version of TB-LMTO-ASR for the electronic and magnetic properties as well as linear muffin-tin orbital green function (LMTOGF) based on CPA code for the exchange pair energy interactions. We found SG behaviors in all the three systems. The magnetic phase diagrams of Pd-Mn and Pt-Mn are found to be more or less same kinds. In case of Pd-Mn SG phase stretches from 0.0 to 0.17 atomic concentration of Mn with tri-critical point around 200K and around 7% atomic concentration of Mn. Similarly, in Pt-Mn SG phase stretches from 0 to 0.2 atomic concentration of Mn with tri-critical points around 150K and 10% of Mn concentration. For Ni-Mn, the phase diagram is different from Pd-Mn and Pt-Mn on which random ferromagnet and anti-ferromagnet flanking either side (both Mn as impurity or Ni). The spin-glass phase stretches from 0.1 to 0.3 atomic concentration of Mn. The Ni-Mn phase diagram qualitatively agrees with experiment. 25 Keywords: Transition metal clusters, Nanosystems, Magnetic phase diagram, Augmented space recursion technique, Disordered alloys.Item Study of Photoproduction on the Reaction γp → ηp by Using Monte Carlo Simulation(Department of Physics, 2017) Tripathee, DevshreeMonte Carlo simulation provides virtual experimental set up for the study of photo production reaction γp→ηp→γγp in Linux based system. Within the range of threshold energy (710 MeV) for eta meson, photo production events were generated. The generated event includes generation of eta meson, proton, two photons, beam and target vertex. datfiles were created in Linux which include the energy of gamma photon, length of target etc for the energy range 711 ≤ E iv γ ≤ 1510 MeV. Then the dat files were run by using the mkin software which generates events. Thus the generated events were identified by using cbsim software. The detected events include identification of position of eta mesons, outgoing photons, proton, vertex identification, detection of position of elements in Crystal Ball (NaI) and TAPS (BaF 2 ) crystal and detection of energy and momentum. We found the invariant mass of eta meson and proton are 550 MeV and 938 MeV respectively according to Particle Data Group. Keywords: Monte Carlo simulation, eta meson, proton, photons, photo productionItem A Study on Image Quality In Teledermatology and Telepathology of Telemedicine Using a Digital Camera(Department of physics, 2007) Rai, Krishna BahadurThis dissertation entitled “A STUDY ON IMAGE QUALITY INTELEDERMATOLOGY AND TELEPATHOLOGY OF TELEMEDI-CINE USING A DIGITAL CAMERA” is based on the rating method in image of patient characteristics that affects whether the disease can bediagnosed or not. Telemedicine (Teledermatology and Telepathology) are being everincreasing popular and advanced. The use of teledermatology andtelepathology for clinical diagnosis are not always without risk becausesufficient images quality may not be there in images. To get sufficientdiagnostic information, sufficient quality of images must be used anddetection display and interpretation should be optimal. An image takendepends upon various factors and specially has an effect on diagnostic ofdisease. An optimized image procedure is one in which image quality ofpatient characteristics is properly balanced. In this context, a generalstudy of the image quality of image formation in diagnosticteledermatology and telepathology are the major works performed atDepartment of dermatology, TUTH, T.U., Kathmandu; KathmanduMedical College, K.U. Kathmandu and Health Net Maharajgunj,Kathmandu Nepal. In the test carried out to study the image quality of teledermatologyand telepatholgy (telehistology and telecytology), there were found thatthe gold standard for any specialist referred traditional way incase ofpathology viewing through microscope and in case of dermatology faceto face consultation remainsame with the image of diseased patient usedto diagnosis. 5 Here it was found that the area under the ROC curve fordermatology is0.922 means that a randomly selected individual from thepositive group has a test value larger than that for a randomly chosenindividual from the negative group by92% of the time. And for histologyand cytology area under ROC curve are 0.909 and 1.000 respectively.These larger area support high diagnostic accuracy through the highimage quality.Again the P-valuefor dermatology, histology and cytologyare respectively0.000,0.000 and0.001. Such that P is low i.e. P < 0.05which is statistically significant. Therefore, there is evidence that imagehas an ability to detect disease. Andwe also found that the correct test capacity of expert doctor inthe diagnosis of cases through image being high quality. For this, the P-value from SPSS output is0.000 such that P<0.005 which is statisticallysignificant again. So that for high accuracy of diagnostic test supports thatthe image quality is good. Finally this research says that the good image quality is necessary for correct diagnosis in teledermatology and telepathology. However,probably most important for the most part of telemedicine is far frombeing a mature discipline, and much work remains to be done to establishits place in health care delivery.Item Study on the characteristic emission loading from the public transport and its contribution for air quality degradation over the Kathmandu valley : Grided emission estimation and dispersion modeling(Department of physics, 2021) Shrestha, ChiranjibiThe Kathmandu valley is witnessing an extremely poor air quality from years. The degradation of air quality over Kathmandu is often found to be associated with the spontaneous urbanization, haphazard industrial expansions and vehicular fleet. Present study was conceived to understand the contribution of transport sector in degrading the air quality of Kathmandu valley. We developed a comprehensive gridded emission inventories of potential pollutants and performed particulate pollutant dispersion modelling using a comprehensive Chemical Transport Modelling (CTM) at the horizontal grid resolution of 1 km 2 over an area of 70 km x 70 km that covers the Kathmandu valley and its immediate surroundings, initialized with the Weather Research and Forecasting (WRF) simulated meteorological fields. The gridded emission inventory showed about 1918.17 kg km −2 of TSP, 12872.74 kg km −2 of CO, 6925.82 kg km −2 of NOx and 708.41 kg km −2 of SO2 are currently loaded into the atmosphere of the valley per day from the public transport over the Kathmandu valley. CTM simulation shows the ambient concentration of PM2.5 due to public transport fleets appears a minimum during the day time and remains maximum during the night and morning times. The pollutants released over the valley are transported to the east and are flushed out into the eastern neighbouring valley. The contribution of the public transport fleet to the deterioration of ambient air quality of the valley appears significant compared to total emission from transport sector.Item Super Substorm Related Signatures at Middle and Low Latitude Ionosphere(Faculty of Physics, 2022) Pandit, DrabindraAvailable With Full TextItem The Theoretical Study of Multiphoton Ionization of Atom by Multiple Laser Beams(Faculty of Physics, 2014) Shrestha Pradhan, NilamMultiphoton processes occur and are important for many aspects of matter-radiation interaction that include the efficient ionization of atoms and molecules, and more gen- erally atomic transition mechanism, system-environment couplings, and dissipative quantum dynamics, laser physics, optical parametric processes and interferometry. A single review can not account for all aspects such an enormously vast subject. Here we concentrate our attention on atomic transition due to presence of multiple beams in non-linear media. The interference of the amplitudes of atomic transition from an initial state |g⟩ to the given final state |f ⟩ is one of the major consequences of the validity of quantum mechanics. In the case of radiative transitions, new possibilities for the illustration and investigation of this phenomenon are created in the simultaneous interaction of an atom with the fundamental frequency of laser. The time dependent Schrodinger equation (TDSE) for non relativistic, semi-classical in dipole approximation, based under the framework of high order perturbation theory is solved by using Dalgarno-Lewis technique. We have evaluated multiphoton ionization of ground state hydrogen atom due to the simultaneous interaction with multiple laser beams of the same frequencies, direction of propagation but different polarization. We could able to separate the radial and angular part of transition amplitudes and obtain the analytical expression for the various multiphoton tran- sition rates as a function of polarization, phase shift, intensities and frequency of the incident photon. In this formulation we can use any number of incident beams with arbitrary polarization. Based on our analytical expression, we have shown numerical results the change in angular distribution of ejected photo- electrons by varying intensity, phase, and polarization of different beams. The results illus- trate the influence of laser frequency on angular distribution of ejected electron by varying phases and polarization on the differential ionization cross-section of the MPI.Item Ultrasonographic And Computational Insight Into The Thyroid Disorders(Department of Physics, 2021-06) Lamichhane, Tika RamObjective of this research is to evaluate thyroid dysfunction states (euthyroidism, hypothyroidism, hyperthyroidism and resistance to thyroid hormones) by using thyroid function tests (TFTs) of the hospital based subjects in central Nepal followed by thyroid ultrasonographic (USG) examinations and to explore the related impacts on the molecular levels by means of nanoscale molecular dynamics (NAMD) simulations of thyroid hormone receptors (THRs). To develop the complex correlations among thyroid variables and to find the conformational changes, energy distributions and thermal properties such as heat capacity, diffusion and conduction of THRs in the cases of overt thyroid disorders are the major physics questions which have been solved in this research. Euthyroidism, hypothyroidism and resistance to thyroid hormones (RTH) are associated with the structural and physical properties of liganded, unliganded and mutated THRs, respectively. In a total of 3425 thyroid patients, there were 70.45% euthyroid, 18.95% subclinical hypothyroid, 3.30% overt hypothyroid, 5.11% subclinical hyperthyroid and 2.19% overt hyperthyroid subjects. An isolated group of 48 patients fell in the category of RTH who have elevated serum free triiodothyronine (FT3) and thyroxin (FT4) but not suppressed thyroid stimulating hormone (TSH) affected by point mutation on THR- gene. Thyroid USG and TFTs of euthyroid subjects recommend age- and gender-specific reference levels of thyroid lobe volume, FT3, FT4 and TSH. Normal thyroid size first increases and then decreases whereas FT4 first decreases and then slightly increases with aging. Right thyroid lobe of males has greater size than that of females in all age-groups except in menarche and menopause. There is a linear relation between FT3 and FT4 and their correlation increases from euthyroidism (r = 0.1) to overt thyroid dysfunctions (r = 0.91, p <0.001). The four parameter logistic (4PL) model has been better fitted(R 2 = 0.97, p <0.001) supporting non-linear relation between ln(TSH) and FT4 in both hypothyroidism and hyperthyroidism on the basis of euthyroidism. The fitted non-linear curves are response trajectories of pituitary TSH by the deviation in serum FT4. The negative correlations and slopes of the linear fits in ln(TSH) vs FT4 or FT3 are indicatorsn of negative feedback mechanism in the thyroid cycle. Hypothyroid patients suffer from goitre, thyroiditis, benign type of nodular and/or cystic lesions, diffused echogenicity and vascularity. Some patients with normal TFTs also have cystic and/or hypoechoic and even malignant lesions in the thyroid gland. The patients with THR- mutations suffer from RTH and goitre. Thyroid dysfunction is a consequence of THR-malfunctions due to its unliganded and/or mutated forms. In both constrained and unconstrained dynamics, temperature auto-correlations and echo-curves show distinct characteristics of T3 liganded and unliganded THRs. Under the effect of anharmonicity on the phase coherent state of normal modes, the dephasing time lies in a range of 0.6-0.8 ps when the THR-systems are perturbed suddenly. T3 makes some increase in heat capacity upon binding to THR- ligand binding domain (LBD) in both anhydrous and hydrous states that signifies the effect of receptor-ligand interactions, and hydrophobic, vibrational and conformational changes. The specific heat of the THR isoforms in solution ranges from2000 to 2200 Jkg 1K 1 at 310 K which is about 20% higher than that in anhydrous state in agreement with the experimental results. Providing temperature relaxation from 310 K to 200 K across protein-water interface in nanodroplets, the thermal diffusivity of THRs ranges from 1.28 10 7 to 1.57 107m 2/s which is around 1.46 10/s for water. The thermal conductivity of THRs lies in the range of 0.26–0.30 Wm which is about half the value, 0.64 Wm 1 K 1 for water at 310 K. In euthyroidism, the THRT3 isoforms with lower thermal conductivity are responsible for the regulation of body scale temperature through thermogenesis and metabolism. The hypothyroid patients with lower hormone-receptor interactions suffer from cold intolerance. The point mutations like L330S on THR- LBD make changes on Ramachandran plots, solvent accessible surface area, radial distribution functions, root mean square deviations and fluctuations, and interaction and internal energies in comparison with its wild type. These physical parameters reflect that L330S-mutant causes steric hindrance while binding T3 into THR- LBD resulting RTH in the thyroid patients. Thus, prevalence of thyroid disorders, correlations among thyroid variables and physical properties of THRs responsible for thyroid dysfunction states are explored clinically and computationally.