Publications (132)258.05 Total impact
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ABSTRACT: The interplay of coherence and decoherence is a vexing issue in contemporary condensed matter physics, quantum optics and quantum information theory. We present an overview of this important topical subject, in terms of three different paradigms, in which the 'noisy' effect of the environment on small quantum subsystems is analysed.  [Show abstract] [Hide abstract]
ABSTRACT: Fluctuation theorems have a very special place in the study of non equilibrium dynamics of physical systems. The form in which it is used most extensively is the GallavotiCohen Fluctuation Theorem which is in terms of the distribution of the work $p(W)/p(W)=\exp(\alpha W)$. We derive the general form of the fluctuation theorems for an arbitrary Gaussian Markov process and find conditions when the parameter $\alpha$ becomes a universal parameter $1/kT$. As an application we consider fluctuation theorems for classical cyclotron motion of an electron in a parabolic potential. The motion of the electron is described by four coupled Langevin equations and thus is nontrivial. The generalized theorems are equally valid for nonequilibrium steady states.  [Show abstract] [Hide abstract]
ABSTRACT: We study domain kinetics in ferroelectric films, subsequent to a rapid quench below the critical temperature. We model the kinetics via the timedependent GinzburgLandau (TDGL) equation for the GinzburgLandauDevonshire (GLD) free energy, which is a functional of the electric polarization vector (P) over right arrow = (Px, Py). The nature of ordering dynamics depends on the strength of coupling (alpha) between Px and Py. For alpha = 1, domain growth is mediated by the annihilation of vortex defects with the growth law L(t) similar to (t/ln t)(1/2). For alpha not equal 1, the corresponding defects are sharp interfaces between fourfolddegenerate ground states. The resultant growth law is purely diffusive: L(t) similar to t(1/2). Copyright (C) EPLA, 2013  [Show abstract] [Hide abstract]
ABSTRACT: The coupling of a mesoscopic system with its environment usually causes total decoherence: at long times the reduced density matrix of the system evolves in time to a limit which is independent of its initial value, losing all the quantum information stored in its initial state. Under special circumstances, a subspace of the system's Hilbert space remains coherent, or "decoherence free", and the reduced density matrix approaches a nontrivial limit which contains information on its initial quantum state, despite the coupling to the environment. This situation is called "partial decoherence". Here we find the conditions for partial decoherence for a mesoscopic system (with $N$ quantum states) which is coupled to an environment. When the Hamiltonian of the system commutes with the total Hamiltonian, one has "adiabatic decoherence", which yields N1 timeindependent combinations of the reduced density matrix elements. In the presence of a magnetic flux, one can measure circulating currents around loops in the system even at long times, and use them to retrieve information on the initial state. For N=2, we demonstrate that partial decoherence can happen only under adiabatic decoherence conditions. However, for $N>2$ we find partial decoherence even when the Hamiltonian of the system does not commute with the total Hamiltonian, and we obtain the general conditions for such nonadiabatic partial decoherence. For an electron moving on a ring, with $N>2$ singlelevel quantum dots, nonadiabatic partial decoherence can arise only when the total flux through the ring vanishes (or equals an integer number of flux quanta), and therefore there is no asymptotic circulating current.  [Show abstract] [Hide abstract]
ABSTRACT: An effort based on the kinetic model allowed by LandauGinzburgDevonshire theory is presented here. The formation of vortex, antivortex states in ferroelectric films are reported and theoretically explained. Phase transition in toroidal moment is observed. Vortexantivortexvortex (VAVV) triplet is generated by applying inhomogeneous transverse static localized fields in xy plane. A specific boundary condition is used to keep the spirit of vortex state i.e. the net polarization across the boundary is always zero. %This is neither a periodic nor an open circuit electric boundary condition.  [Show abstract] [Hide abstract]
ABSTRACT: We consider a prototypical system of an infinite range transverse field Ising model coupled to a bosonic bath. By integrating out the bosonic degrees, an effective anisotropic Heisenberg model is obtained for the spin system. The phase diagram of the latter is calculated as a function of coupling to the heat bath and the transverse magnetic field. Collective excitations at low temeratures are assessed within a spinwave like analysis that exhibits a vanishing energy gap at the quantum critical point. We also consider another limit where the system reduces to a generalized spinboson model of two interacting spins. By increasing the coupling strength with the heat bath, the twospin wavefunction changes from an entangled state to a factorized state of two spins which are aligned along the transverse field. We also discuss the possible realization and application of the model to different physical systems.  [Show abstract] [Hide abstract]
ABSTRACT: The qubit (or a system of two quantum dots) has become a standard paradigm for studying quantum information processes. Our focus is Decoherence due to interaction of the qubit with its environment, leading to noise. We consider quantum noise generated by a dissipative quantum bath. A detailed comparative study with the results for a classical noise source such as generated by a telegraph process, enables us to set limits on the applicability of this process vis a vis its quantum counterpart, as well as lend handle on the parameters that can be tuned for analyzing decoherence. Both Ohmic and nonOhmic dissipations are treated and appropriate limits are analyzed for facilitating comparison with the telegraph process.  [Show abstract] [Hide abstract]
ABSTRACT: The time evolution of a qubit, consisting of two singlelevel quantum dots, is studied in the presence of telegraph noise. The dots are connected by two tunneling paths, with an AharonovBohm flux enclosed between them. Under special symmetry conditions, which can be achieved by tuning gate voltages, there develops partial decoherence: at long times, the offdiagonal element of the reduced density matrix (in the basis of the two dot states) approaches a nonzero value, generating a circulating current around the loop. The flux dependence of this current contains full information on the initial quantum state of the qubit, even at infinite time. Small deviations from this symmetry yield a very slow exponential decay towards the fullydecoherent limit. However, the amplitudes of this decay also contain the full information on the initial qubit state, measurable either via the current or via the occupations of the qubit dots.  [Show abstract] [Hide abstract]
ABSTRACT: We employ the Ginzburg–Landau–Devonshire free energy theory to construct a set of timedependent ordinary differential equations in order to model the interaction dynamics of the tip of an atomic force microscope (AFM) with the surface of a ferroelectric thin film. Using this method we obtain (i) domain profile and (ii) domain nucleation which starts under the AFM tip. Since ferroelectrics are piezoelectric, the electromechanical coupling considered in our equations causes polarization to oscillate with the same frequency with which the AFM tip vibrates.  [Show abstract] [Hide abstract]
ABSTRACT: Growing colonies of bacteria on the surface of thin agar plates exhibit fractal patterns as a result of nonlinear response to environmental conditions, such as nutrients, solidity of the agar medium and temperature. Here, we examine the effect of glucose on pattern formation by growing colonies of Bacillus thuringiensis isolate KPWP1. We also present the theoretical modeling of the colony growth of KPWP1 and the associated spatiotemporal patterns. Our experimental results are in excellent agreement with simulations based on a reactiondiffusion model that describes diffusionlimited aggregation and branching, in which individual cells move actively in the periphery, but become immotile in the inner regions of the growing colony. We obtain the Hausdorff fractal dimension of the colony patterns: D(H.Expt)=1.1969 and D(H, R.D.=)1.1965, for experiment and reactiondiffusion model, respectively. Results of our experiments and modeling clearly show how glucose at higher concentration can prove to be inhibitory for motility of growing colonies of B. thuringiensis cells on semisolid support and be responsible for changes in the growth pattern.  [Show abstract] [Hide abstract]
ABSTRACT: In this article we consider the nonlinear dynamics of domain growth and dynamics under the influence of an external electric field and an intrinsic pinning field due to disorder. The theoretical framework is based on a finite timedifference method as applied to a timedependent Ginzburg–Landau–Devonshire equation. The domain growth is seen to be of fractal nature, the fractal dimension of which is in good agreement with experiments. When it comes to dynamics we compute the areal velocity of the domain growth as a function of the applied field and find different regions of the nonlinearities that are also in qualitative agreement with experiments. 
Article: Peierls’ elucidation of Diamagnetism
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ABSTRACT: This article summarizes the contribution to the phenomenon of Diamagnetism made by Rudolf Peierls, as Quantum Mechanics was triumphantly unfolding in the 1930’s. KeywordsDiamagnetismboundary effectsdissipationde Haasvan Alphen oscillations 
Article: Role of quantum heat bath and confinement in the lowtemperature thermodynamics of cyclotron motion
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ABSTRACT: In this Brief Report we show how the lowtemperature thermodynamics of the dissipative motion of an electron in a magnetic field is sensitive to the nature of the spectral density function, J(omega), of the quantum heat bath. In all cases of couplings considered here the free energy and the entropy of the cyclotron motion of the electron fall off to zero as power law in conformity with the third law of thermodynamics. The power of the power law however depends on the nature of J(omega). We also separately discuss the influence of confinement.  [Show abstract] [Hide abstract]
ABSTRACT: We present a detailed study of the quantum dissipative dynamics of a charged particle in a magnetic field. Our focus of attention is the effect of dissipation on the low and hightemperature behaviors of the specific heat at constant volume. After providing a brief overview of two distinct approaches to the statistical mechanics of dissipative quantum systems, viz., the ensemble approach of Gibbs and the quantum Brownian motion approach due to Einstein, we present exact analyses of the specific heat. While the lowtemperature expressions for the specific heat, based on the two approaches, are in conformity with powerlaw temperature dependence, predicted by the third law of thermodynamics, and the hightemperature expressions are in agreement with the classical equipartition theorem, there are surprising differences between the dependencies of the specific heat on different parameters in the theory, when calculations are done from these two distinct methods. In particular, we find puzzling influences of boundary confinement and the bathinduced spectral cutoff frequency. Further, when it comes to the issue of approach to equilibrium, based on the Einstein method, the way the asymptotic limit (t>infinity) is taken seems to assume significance.  [Show abstract] [Hide abstract]
ABSTRACT: A Landaulike theory of phase transition and its timedependent generalization are shown to be sufficient for describing the formation and kinetics of 180°, 90°, and vortex (toroidal) domains in ferroelectric thin films. The theory relies only on the choice of boundary conditions and does not require at the outset the presence of either anisotropy or strain fields. An ingredient in the calculational scheme is the incorporation of finite element methods in the kinetic equations for the ferroelectric order parameter.  [Show abstract] [Hide abstract]
ABSTRACT: A qubit (containing two quantum states, 1 and 2), is coupled to a control register (state 3), which is subject to telegraph noise. We study the time evolution of the density matrix $\rho$ of an electron which starts in some coherent state on the qubit. At infinite time, $\rho$ usually approaches the fully decoherent state, with $\rho^{}_{nm}=\delta^{}_{nm}/3$. However, when the Hamiltonian is symmetric under $1\leftrightarrow 2$, the element $\rho^{}_{12}$ approaches a nonzero real value, implying a partial coherence of the asymptotic state. The asymptotic density matrix depends only on ${\rm Re}[\rho^{}_{12}(t=0)]$. In several cases, the information stored on the qubit is protected from the noise. Comment: 4 pages, 2 figures  [Show abstract] [Hide abstract]
ABSTRACT: In this work, low temperature thermodynamic behaviour in the context of dissipative diamagnetism with anomalous coupling is analyzed. We find that finite dissipation substitutes the zerocoupling result of exponential decay of entropy by a power law behaviour at low temperature. For Ohmic bath, entropy vanishes linearly with temperature, $T$, in conformity with Nernst's theorem. It is also shown that entropy decays faster in the presence of anomalous coupling than that of the usual coordinatecoordinate coupling. It is observed that velocityvelocity coupling is the most advantageous coupling scheme to ensure the third law of thermodynamics. It is also revealed that different thermodynamic functions are independent of magnetic field at very low temperature for various coupling schemes discussed in this work.  [Show abstract] [Hide abstract]
ABSTRACT: We revisit here the effect of quantum dissipation on the much studied problem of Landau diamagnetism and analyze the results in the light of the third law of thermodynamics. The case of an additional parabolic potential is separately assessed. We find that dissipation arising from strong coupling of the system to its environment qualitatively alters the lowtemperature thermodynamic attributes such as the entropy and the specific heat.  [Show abstract] [Hide abstract]
ABSTRACT: This review deals with the dynamics of quantum systems that are subject to high frequency external perturbations. Though the problem may look hopelessly timedependent, and poised on the extreme opposite side of adiabaticity, there exists a ‘Kapitza Window’ over which the dynamics can be treated in terms of effective timeindependent Hamiltonians. The consequent results are important in the context of atomic traps as well as quantum optic properties of atoms in intense and highfrequency electromagnetic fields.  [Show abstract] [Hide abstract]
ABSTRACT: The “springdefect model” developed by us earlier to discuss ferroelasticity, exhibited in the orthorhombic phase of YBa2Cu3O7–δ (YBa–Cu–O), is used to analyze anelastic relaxation around the paraelastic to ferroelastic phase transition under the influence of an external inhomogeneous stress. The kinetics of the underlying Hamiltonian representing strainstrain interactions is treated within meanfield theory. The relaxationresponse relation of linear response theory is employed to derive explicit expressions for the anelastic strain, the frequencydependent compliance and the internal friction in terms of the basic parameters of the springdefect model.
Publication Stats
1k  Citations  
258.05  Total Impact Points  
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Institutions

20042015

Jawaharlal Nehru Centre for Advanced Scientific Research
Bengalūru, Karnataka, India


20082012

Indian Institute of Science Education and Research Kolkata
Kolkata, West Bengal, India


20002006

S.N. Bose National Centre for Basic Sciences
Kolkata, Bengal, India


19902006

Jawaharlal Nehru University
 School of Physical Sciences
New Dilli, NCT, India


1998

Forschungszentrum Jülich
Jülich, North RhineWestphalia, Germany 
University of Cambridge
 Department of Earth Sciences
Cambridge, England, United Kingdom


19841987

University of Hyderabad
 School of Physics
Hyderābād, State of Andhra Pradesh, India


1986

IBM
Armonk, New York, United States


19751977

Carnegie Mellon University
 Department of Physics
Pittsburgh, Pennsylvania, United States


1976

University of Alberta
 Theoretical Physics Institute
Edmonton, Alberta, Canada


1974

St. John's University
 Department of Physics
New York City, New York, United States
