Somrita Ray

Study of non-Markovian dynamics of a charged particle in presence of a magnetic field in a simple way

In this paper, we have investigated the dynamics of a Brownian particle in the presence of a magnetic field. The present investigation is generalized considering different kinds of force fields, magnetic field, and non-Markovian thermal bath. The properties of the Brownian particle have been calculated based on the multi-dimensional Fokker-Planck description of stochastic processes. It leads to the study of non-Markovian dynamics of a Brownian particle in the presence of a magnetic field in a simple way. Using the present simple method, we have identified several important signatures of magnetic field and non-Markovian thermal bath in the dynamics.

Tuning of barrier crossing time of a particle by time dependent magnetic field

We have studied the effect of time dependent magnetic field on the barrier crossing dynamics of a charged particle. An interplay of the magnetic field induced electric field and the applied field reveals several interesting features. For slowly oscillating field the barrier crossing rate increases remarkably particularly at large amplitude of the field. For appreciably large frequency a generically distinct phenomenon appears by virtue of parametric resonance manifested in multiple peaks appearing in the variation of the mean first passage time as a function of the amplitude. The parametric resonance is more robust against the variation of amplitude of the oscillating field compared to the case of variation of frequency. The barrier crossing time of a particle can be tuned para-metrically by appropriate choice of amplitude and frequency of the oscillating magnetic field.

Hole diffusion at the recombination junction of thin film tandem solar cells and its effect on the illuminated current–voltage characteristic

Computer simulation of experimental current density–voltage (J–V) and quantum efficiency characteristics of thin film p1-i1-n1-p2 structures and of double junction solar cells (p1-i1-n1-p2-i2-n2), has been used to understand the hole transport mechanisms near the np “tunnel” junction between two subcells of a multijunction structure. Two different types of p layers at the junction have been studied: (i) hydrogenated microcrystalline silicon (μc-Si:H) and (ii) hydrogenated amorphous silicon carbide (a-SiC:H). There is a striking difference between the experimental J–V characteristics for the p1-i1-n1-p2 structures, with case (i) having a fairly high fill factor (FF) and conversion efficiency (η), as against a very low FF and η in case (ii). Although the difference is much smaller for double junction cells employing these two types of materials as the p layer at the junction, the fill factor of the cell employing μc-Si:H is about 8% higher. Analysis of transport properties as a function of position by compu...

Resonant activation in a colored multiplicative thermal noise driven closed system

In this paper, we have demonstrated that resonant activation (RA) is possible even in a thermodynamically closed system where the particle experiences a random force and a spatio-temporal frictional coefficient from the thermal bath. For this stochastic process, we have observed a hallmark of RA phenomena in terms of a turnover behavior of the barrier-crossing rate as a function of noise correlation time at a fixed noise variance. Variance can be fixed either by changing temperature or damping strength as a function of noise correlation time. Our another observation is that the barrier crossing rate passes through a maximum with increase in coupling strength of the multiplicative noise. If the damping strength is appreciably large, then the maximum may disappear. Finally, we compare simulation results with the analytical calculation. It shows that there is a good agreement between analytical and numerical results.

On exact hydrogenic Compton profiles

Using nonrelativistic Coulomb functions in the spherical polar coordinates, atomic Compton profiles are calculated in the form factor or exact hydrogenic (EH) approximation. A three‐term recurrence relation is shown to facilitate a straightforward evaluation of the Gaussian hypergeometric functions which characterize the bound–free matrix elements under consideration. Numerical data for EH L‐shell profiles of elements from 10Ne to 14Si and 18Ar are compared with those obtained in the impulse approximation. While the impulse profile is symmetric about the profile center, the EH profile exhibits energy defect. Based on analysis of the present data, the dynamical origin of such defects is investigated.

Resonance behavior of a charged particle in presence of a time dependent magnetic field

In this article, we have explored the resonance behavior of a particle in the presence of a time dependent magnetic field (TDMF). The particle is bound in a harmonic potential well. Based on the Hamiltonian description of the system in terms of action and angle variables, we have derived the resonance condition for the applied TDMF along z-direction which is valid for arbitrary frequencies along x and y directions of the two dimensional harmonic oscillator. We have also derived resonance condition for the applied magnetic field which is lying in a plane. Finally, we have explored resonance condition for the isotropic magnetic field. To check the validity of the theoretical calculation, we have solved equations of motion numerically for the parameter sets which satisfy the derived resonance condition. The numerical experiment fully agrees with the theoretically derived resonance conditions.

Synchronization of Nonidentical Coupled Phase Oscillators in the Presence of Time Delay and Noise

We have studied in this paper the dynamics of globally coupled phase oscillators having the Lorentzian frequency distribution with zero mean in the presence of both time delay and noise. Noise may be Gaussian or non-Gaussian in characteristics. In the limit of zero noise strength, we find that the critical coupling strength (CCS) increases linearly as a function of time delay. Thus the role of time delay in the dynamics for the deterministic system is qualitatively equivalent to the effect of frequency fluctuations of the phase oscillators by additive white noise in absence of time delay. But for the stochastic model, the critical coupling strength grows nonlinearly with the increase of the time delay. The linear dependence of the critical coupling strength on the noise intensity also changes to become nonlinear due to creation of additional phase difference among the oscillators by the time delay. We find that the creation of phase difference plays an important role in the dynamics of the system when the intrinsic correlation induced by the finite correlation time of the noise is small. We also find that the critical coupling is higher for the non-Gaussian noise compared to the Gaussian one due to higher effective noise strength.

Microscopic theory of heat transfer between two fermionic thermal baths mediated by a spin system.

In this paper we have presented the heat exchange between the two fermionic thermal reservoirs which are connected by a fermionic system. We have calculated the heat flux using solution of the c-number Langevin equation for the system. Assuming small temperature difference between the baths we have defined the thermal conductivity for the process. It first increases as a nonlinear function of average temperature of the baths to a critical value then decreases to a very low value such that the heat flux almost becomes zero. There is a critical temperature for the fermionic case at which the thermal conductivity is maximum for the given coupling strength and the width of the frequency distribution of bath modes. The critical temperature grows if these quantities become larger. It is a sharp contrast to the Bosonic case where the thermal conductivity monotonically increases to the limiting value. The change of the conductivity with increase in width of the frequency distribution of the bath modes is significant at the low temperature regime for the fermionic case. It is highly contrasting to the Bosonic case where the signature of the enhancement is very prominent at high temperature limit. We have also observed that thermal conductivity monotonically increases as a function of damping strength to the limiting value at the asymptotic limit. There is a crossover between the high and the low temperature results in the variation of the thermal conductivity as a function of the damping strength for the fermionic case. Thus it is apparent here that even at relatively high temperature, the fermionic bath may be an effective one for the strong coupling between system and reservoir. Another interesting observation is that at the low temperature limit, the temperature dependence of the heat flux is the same as the Stefan-Boltzmann law. This is similar to the bosonic case.

Magnetic field induced dynamical chaos

In this article, we have studied the dynamics of a particle having charge in the presence of a magnetic field. The motion of the particle is confined in the x-y plane under a two dimensional nonlinear potential. We have shown that constant magnetic field induced dynamical chaos is possible even for a force which is derived from a simple potential. For a given strength of the magnetic field, initial position, and velocity of the particle, the dynamics may be regular, but it may become chaotic when the field is time dependent. Chaotic dynamics is very often if the field is time dependent. Origin of chaos has been explored using the Hamiltonian function of the dynamics in terms of action and angle variables. Applicability of the present study has been discussed with a few examples.

Adiabatic to Diabatic Transformation and Nuclear Dynamics on Diabatic Hamiltonian Constructed by Using Ab Initio Potential Energy Surfaces and Non-adiabatic Coupling Terms for Excited States of Sodium Trimer

The non-adiabatic coupling terms (NACTs) among the electronic states 22 E ′ and 12 A 1 ′ of Na3 system demonstrate the numerical validity of so called “Curl Condition” and thus such states closely form a sub-Hilbert space. For this subspace, we employ the NAC terms to solve the “Adiabatic–Diabatic Transformation (ADT)” equations to obtain the functional form of the transformation angles and pave the way to construct the continuous and single valued diabatic potential energy surface matrix. Nuclear dynamics has been carried out on those diabatic surfaces to reproduce the experimental spectrum for system B of Na3 cluster and thereby, to explore the numerical validity of the theoretical development on beyond Born–Oppenheimer approach for adiabatic to diabatic transformation.