arXiv subjects

A. M. Jayannavar

Publications and source records attributed to A. M. Jayannavar.

At least 73 records · Page 4Linked to original sources

arXiv2008

Stochastic resonance and heat fluctuations in a driven double-well system

We study a periodically driven (symmetric as well as asymmetric)double-well potential system at finite temperature. We show that mean heat loss by the system to the environment(bath) per period of the applied field is a good quantifier of stochastic resonance. It is found that the heat fluctuations over a single period are always larger than the work fluctuations. The observed distributions of work and heat exhibit pronounced asymmetry near resonance. The heat losses over a large number of periods satisfies the conventional steady-state fluctuation theorem, though different relation exists for this quantity.

cond-mat.stat-mech↗

arXiv2007

Nonequilibrium Work distributions for a trapped Brownian particle in a time dependent magnetic field

We study the dynamics of a trapped, charged Brownian particle in presence of a time dependent magnetic field. We calculate work distributions for different time dependent protocols. In our problem thermodynamic work is related to variation of vector potential with time as opposed to the earlier studies where the work is related to time variation of the potentials which depends only on the coordinates of the particle. Using Jarzynski identity and Crook's equality we show that the free energy of the particle is independent of the magnetic field, thus complementing the Bohr-van Leeuwen theorem. We also show that our system exhibits a parametric resonance in certain parameter space.

cond-mat.stat-mech↗

arXiv2007

Generation and Measurement of Non Equilibrium Spin Currents in Two Terminal Systems

Generation and measurement of non-equilibrium spin current in two probe configuration is discussed. It is argued and shown that spin current can be generated in two terminal non-magnetic system. Further it is shown that these spin currents can be measured via conductance in two probe configuration when the detector probe is ferromagnetic.

cond-mat.mes-hall↗

arXiv2007

Fluctuation theorems and orbital magnetism in nonequilibrium state

We study Langevin dynamics of a driven charged particle in the presence as well as in the absence of magnetic field. We discuss the validity of various work fluctuation theorems using different model potentials and external drives. We also show that one can generate an orbital magnetic moment in a nonequilibrium state which is absent in equilibrium.

cond-mat.stat-mech↗

arXiv2007

A charged particle in a magnetic field - Jarzynski Equality

We describe some solvable models which illustrate the Jarzynski theorem and related fluctuation theorems. We consider a charged particle in the presence of magnetic field in a two dimensional harmonic well. In the first case the centre of the harmonic potential is translated with a uniform velocity, while in the other case the particle is subjected to an ac force. We show that Jarzynski identity complements Bohr-van Leeuwen theorem on the absence of diamagnetism in equilibrium classical system.

cond-mat.stat-mech↗

arXiv2007

Two simple models of classical heat pumps

Motivated by recent studies on models of particle and heat quantum pumps, we study similar simple classical models and examine the possibility of heat pumping. Unlike many of the usual ratchet models of molecular engines, the models we study do not have particle transport. We consider a two-spin system and a coupled oscillator system which exchange heat with multiple heat reservoirs and which are acted upon by periodic forces. The simplicity of our models allows accurate numerical and exact solutions and unambiguous interpretation of results. We demonstrate that while both our models seem to be built on similar principles, one is able to function as a heat pump (or engine) while the other is not.

cond-mat.stat-mech↗

arXiv2007

Work Fluctuations and Stochastic Resonance

We study Brownian particle motion in a double-well potential driven by an ac force. This system exhibits the phenomenon of stochastic resonance. Distribution of work done on the system over a drive period in the time asymptotic regime have been calculated. We show that fluctuations in the input energy or work done dominate the mean value. The mean value of work done over a period as a function of noise strength can also be used to characterise stochastic resonance in the system. We also discuss the validity of steady state fluctuation theorems in this particular system.

cond-mat.stat-mech↗

arXiv2006

Supersymmetry and Fokker-Planck dynamics in periodic potentials

Recently, the Fokker-Planck dynamics of particles in periodic potentials $\pm V$, have been investigated by using the matrix continued fraction method. It was found that the two periodic potentials, one being bistable and the other metastable give the same diffusion coefficient in the overdamped limit. We show that this result naturally follows from the fact that the considered potentials in the corresponding Schrödinger equation form supersymmetric partners. We show that these differing potentials ${\pm}V$ also exhibit symmetry in current and diffusion coefficients: $J_{+}(F)=-J_{-}(-F)$ and $D_{+}(F)=D_{-}(-F)$ in the presence of a constant applied force F. Moreover, we show numerically that the transport properties in these potentials are related even in the presence of oscillating drive.

cond-mat.stat-mech↗

arXiv2006

Giant coherence in driven systems

We study the noise-induced currents and reliability or coherence of transport in two different classes of rocking ratchets. For this, we consider the motion of Brownian particles in the over damped limit in both adiabatic and non-adiabatic regimes subjected to unbiased temporally symmetric and asymmetric periodic driving force. In the case of a time symmetric driving, we find that even in the presence of a spatially symmetric simple sinusoidal potential, highly coherent transport occurs. These ratchet systems exhibit giant coherence of transport in the regime of parameter space where unidirectional currents in the deterministic case are observed. Outside this parameter range, i.e., when current vanishes in the deterministic regime, coherence in transport is very low. The transport coherence decreases as a function of temperature and is a non-monotonic function of the amplitude of driving. The transport becomes unreliable as we go from the adiabatic to the non-adiabatic domain of operation.

cond-mat.stat-mech↗

arXiv2006

Stoke's efficiency of temporally rocked ratchets

We study the generalized efficiency of an adiabatically rocked ratchet with both spatial and temporal asymmetry. We obtain an analytical expression for the generalized efficiency in the deterministic case. Generalized efficiency of the order of 50% is obtained by fine tuning of the parameter range. This is unlike the case of thermodynamic efficiency where we could readily get an enhanced efficiency of upto 90%. The observed higher values of generalized efficiency is attributed to be due to the suppression of backward current. We have also discussed briefly the differences between thermodynamic, rectification or generalized efficiency and Stoke's efficiency. Temperature is found to optimize the generalized efficiency over a wide range of parameter space unlike in the case of thermodynamic efficiency.

cond-mat.stat-mech↗

arXiv2005

Phase time for a tunneling particle

We study the nature of tunneling phase time for various quantum mechanical structures such as networks and rings having potential barriers in their arms. We find the generic presence of Hartman effect, with superluminal velocities as a consequence, in these systems. In quantum networks it is possible to control the `super arrival' time in one of the arms by changing the parameters on another arm which is spatially separated from it. This is yet another quantum nonlocal effect. Negative time delays (time advancement) and `ultra Hartman effect' with negative saturation times have been observed in some parameter regimes. In presence and absence of Aharonov-Bohm (AB) flux quantum rings show Hartman effect. We obtain the analytical expression for the saturated phase time. In the opaque barrier regime this is independent of even the AB flux thereby generalizing the Hartman effect. We also briefly discuss the concept of "space collapse or space destroyer" by introducing a free space in between two barriers covering the ring. Further we show in presence of absorption the reflection phase time exhibits Hartman effect in contrast to the transmission phase time.

quant-ph↗

arXiv2005

Enhanced thermodynamic efficiency in time asymmetric ratchets

The energetic efficiency of an overdamped Brownian particle in a saw tooth potential in the presence of time asymmetric forcing is studied in the adiabatic limit. An error made in the earlier work on the same problem in literature is corrected. We find that asymmetry in potential together with temporal asymmetry in forcing leads to much enhanced efficiency without fine tuning of parameters. The origin of this is traced to the suppression of backward current. We also present a comparative study between the role of continuous and discontinuous ratchet forces on these measurable quantities. We find that the thermal fluctuations can optimize the energy transduction, the range of parameters, however, being very small. This ratchet model also displays current reversals on tuning of parameters even in the adiabatic regime. The possible relationships between nature of currents, entropy production and input energy are also addressed.

cond-mat.stat-mech↗

arXiv2005

Noise induced currents and reliability of transport in frictional ratchets

We study the coherence of transport of an overdamped Brownian particle in frictional ratchet system in the presence of external Gaussian white noise fluctuations. The analytical expressions for the particle velocity and diffusion coefficient are derived for this system and the reliability or coherence of transport is analysed by means of their ratio in terms of a dimensionless P$\acute{e}$clet number. We show that the coherence in the transport can be enhanced or degraded depending sensitively on the frictional profile with respect to the underlying potential.

cond-mat.stat-mech↗

arXiv2004

Phase delay time and superluminal propagation in barrier tunneling

In this work we study the behaviour of Wigner phase delay time for tunneling in the reflection mode. Our system consists of a circular loop connected to a single wire of semi-infinite length in the presence of Aharonov-Bohm flux. We calculate the analytical expression for the saturated delay time. This saturated delay time is independent of Aharonov- Bohm flux and the width of the opaque barrier thereby generalizing the Hartman effect. This effect implies superluminal group velocities as a consequence. We also briefly discuss the concept called "space collapse or space destroyer".

quant-ph↗

arXiv2004

Hartman effect and non-locality in quantum networks

We study the phase time for various quantum mechanical networks having potential barriers in its arms to find the generic presence of Hartman effect. In such systems it is possible to control the `super arrival' time in one of the arms by changing parameters on another, spatially separated from it. This is yet another quantum nonlocal effect. Negative time delays (time advancement) and `ultra Hartman effect' with negative saturation times have been observed in some parameter regimes.

quant-ph↗

arXiv2004

Diffusion accompanying noise induced transport in frictional ratchets

We study the noise induced transport of an overdamped Brownian particle in frictional ratchet systems in the presence of external Gaussian white noise fluctuations. The analytical expressions for current and diffusion coefficient are derived and the reliability or coherence of transport are discussed by means of their ratio. We show that frictional ratchets exhibit larger coherence as compared to the flashing and rocking ratchets.

cond-mat.stat-mech↗

arXiv2004

Engines at molecular scales

In recent literature there has been a lot of interest in the phenomena of noise induced transport in the absence of an average bias occurring in spatially periodic systems far from equilibrium. One of the main motivations in this area is to understand the mechanism behind the operation of biological motors at molecular scale. These molecular motors convert chemical energy available during the hydrolysis of ATP into mechanical motion to transport cargo and vesicles in living cells with very high reliability, adaptability and efficiency in a very noisy environment. The basic principle behind such a motion, namely the Brownian ratchet principle, has applications in nanotechnology as novel nanoparticle separation devices. Also, the mechanism of ratchet operation finds applications in game theory. Here, we briefly focus on the physical concepts underlying the constructive role of noise in assisting transport at a molecular level. The nature of particle currents, the energetic efficiency of these motors, the entropy production in these systems and the phenomenon of resonance/coherence are discussed.

physics.bio-ph↗

arXiv2004

Brownian rectifiers in the presence of temporally asymmetric unbiased forces

The efficiency of energy transduction in a temporally asymmetric rocked ratchet is studied. Time asymmetry favours current in one direction and suppresses it in the opposite direction due to which large efficiency ~ 50% is readily obtained. The spatial asymmetry in the potential together with system inhomogeneity may help in further enhancing the efficiency. Fine tuning of system parameters considered leads to multiple current reversals even in the adiabatic regime.

cond-mat.stat-mech↗