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A. M. Jayannavar

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

At least 109 records · Page 6Linked to original sources
arXiv2001

Quantum current enhancement effect in hybrid rings at equilibrium

Current enhancement- a novel quantum phenomena is found to occur in a mesoscopic hybrid ring at equilibrium. The hybrid system is described by a ring with bubble which is in turn coupled to a reservoir. In the system the ring encloses a magnetic flux $Φ$ while the bubble does not enclose any flux. The novelty of this work lies in the fact that while earlier current enhancement was observed in non-equilibrium systems (e.g., a ring coupled to two reservoirs at different chemical potentials $μ_{1}$ and $μ_{2}$), herein we prove that current enhancement can also arise in equilibrium. In addition, we show that the closed system analog of our chosen open hybrid ring system violates parity effects. Finally, we bring to focus the discrepancy between the equilibrium magnetic moment (obtained via energy eigenvalues) and that calculated from the currents in the system.

cond-mat.mes-hall↗
arXiv2001

Inhomogeneous Systems and their Rectification Properties

We explore the possibility of obtaining unidirectional current in a symmetric (periodic) potential system without the application of any obvious (apparent) externally applied bias. There are many physical models proposed to accomplish this nonequilibrium effect. In the present work we consider inhomogeneous systems so that the friction coefficient and/or temperature could vary in space. We find out a model with minimal conditions that the inhomogeneous system assisted by fluctuating forces must satisfy, in order to obtain unidirectional current. In the process we discuss about thermal and frictional ratchets that are of current interest. We argue that different models of frictional ratchets work under the same basic principle of alteration of relative stability of otherwise locally stable states in the presence of temperature inhomogeneity. We also discuss in detail the nature of currents in rocked frictional ratchets. In particular we analyse a novel phenomenon of multiple current reversals and the efficiency of the energy transduction in these systems.

cond-mat.stat-mech↗
arXiv2001

Effect of quantum entanglement on Aharonov-Bohm oscillations, spin-polarized transport and current magnification effect

We present a simple model of transmission across a metallic mesoscopic ring. In one of its arm an electron interacts with a single magnetic impurity via an exchange coupling. We show that entanglement between electron and spin impurity states leads to reduction of Aharonov-Bohm oscillations in the transmission coefficient. The spin-conductance is asymmetric in the flux reversal as opposed to the two probe electrical conductance which is symmetric. In the same model in contradiction to the naive expectation of a current magnification effect, we observe enhancement as well as the suppression of this effect depending on the system parameters. The limitations of this model to the general notion of dephasing or decoherence in quantum systems are pointed out.

cond-mat.mes-hall↗
arXiv2001

Landau diamagnetism revisited

The problem of diamagnetism, solved by Landau, continues to pose fascinating issues which have relevance even today. These issues relate to inherent quantum nature of the problem, the role of boundary and dissipation, the meaning of thermodynamic limits, and above all, the quantum-classical crossover occasioned by environment-induced decoherence. The Landau Diamagnetism provides a unique paradigm for discussing these issues, the significance of which are far-reaching. Our central result is a remarkable one as it connects the mean orbital magnetic moment, a thermodynamic property, with the electrical resistivity, which characterizes transport properties of materials.

cond-mat.mes-hall↗
arXiv2001

Aharonov-Bohm oscillations and spin transport in a mesoscopic ring with a magnetic impurity

We present a detailed analysis of the Aharonov-Bohm (AB) interference oscillations manifested through transmission of an electron in a mesoscopic ring with a magnetic impurity atom inserted in one of its arms. The spin polarization transport is also studied. The electron interacts with the impurity through the exchange interaction leading to exchange spin-flip scattering. Transmission in the spin-flipped and spin-unflipped channels are explicitly calculated. We show that the entanglement between electron and spin-flipper states lead to a reduction of AB oscillations in spite of absence of any inelastic scattering. The spin-conductance (related to spin-polarized transmission coefficient) is asymmetric in the flux reversal as opposed to the two probe conductance which is symmetric under flux reversal. We point out certain limitations of this model in regard to the general notion of dephasing in quantum mechanics.

cond-mat.mes-hall↗
arXiv2001

Enhancement of mobility by periodically modulating the slanting slope of a washboard potential

Average mobility of very feebly damped particles in tilted periodic potentials is considered. Under the combined action of thermal fluctuations and small temporal modulation of the tilt of the potential the particles, in the small tilt range, become more mobile than without modulation. The enhancement of mobility depends (nonmonotonically) on the frequency of modulation. For small modulations the enhancement shows a peak as a function of frequency. This has an obvious implication on the measured voltage across a Josephson junction driven by a small amplitude alternating current of suitable frequency.

cond-mat.stat-mech↗
arXiv2001

Loss of interference in an Aharonov-Bohm ring

We study a simple model of dephasing of Aharonov-Bohm oscillations in the transmission of an electron across a mesoscopic ring. A magnetic impurity in one of the arms of the ring couples to the electron spin via an exchange interaction. This interaction leads to spin flip scattering and induces dephasing via entanglement. This is akin to the models evoked earlier to explain destruction of interference due to which-path information in double-slit experiments. Total transmission is found to be symmetric under flux reversal but not the spin polarization.

cond-mat.mes-hall↗
arXiv2001

AC driven thermal ratchets

We consider the motion of a overdamped Brownian particle in periodic asymmetric potential with space dependent friction coefficient. In the presence of external time periodic forcing, the system shows multiple current reversals on varying the amplitude of the external forcing and the temperature of the thermal bath. In the adiabatic regime we find a single reversal of current as a function of noise strength which can only be accounted due to the presence of space dependent friction coefficient. For very large forcing term, the current does not go to zero, instead it asymptotically tends to a limiting value depending on the phase shift between the potential and the friction. This fact plays an important role in obtaining multiple current reversals.

cond-mat.stat-mech↗
arXiv2000

Role of quantum entanglement due to a magnetic impurity on current magnification effect in mesoscopic open rings

We study the current magnification effect in presence of exchange scattering of electron from a magnetic impurity placed in one arm of an open mesoscopic ring. The exchange interaction causes entanglement of electron spin and impurity spin. Earlier studies have shown that such an entanglement causes reduction or loss of interference in the Aharonov-Bohm oscillations leading to decoherence. We find however, that this entanglement, in contradiction to the naive expectation of a reduction of current magnification, leads to enhancement as well as suppression of the effect. We also observe additional novel features like new resonances and current reversals.

cond-mat.mes-hall↗
arXiv2000

Multiple current reversals in forced inhomogeneous ratchets

Transport properties of overdamped Brownian paricles in a rocked thermal ratchet with space dependent friction coefficient is studied. By tuning the parameters, the direction of current exhibit multiple reversals, both as a function of the thermal noise strength as well as the amplitude of rocking force. Current reversals also occur under deterministic conditions and exhibits intriguing structure. All these features arise due to mutual interplay between potential asymmetry,noise, driving frequency and inhomogeneous friction.

cond-mat.stat-mech↗
arXiv2000

Dephasing of Aharonov-Bohm oscillations in a mesoscopic ring with a magnetic impurity

We present a detailed analysis of the Aharonov-Bohm interference oscillations manifested through transmission of an electron in a mesoscopic ring with a magnetic impurity atom inserted in one of its arms. The electron interacts with the impurity through the exchange interaction leading to exchange spin-flip scattering. Transmission in the spin-flipped and spin-unflipped channels are explicitly calculated. We show that the spin-flipper acts as a dephasor in spite of absence of any inelastic scattering. The spin-conductance (related to spin-polarized transmission coefficient) is asymmetric in the flux reversal as opposed to the two probe conductance which is symmetric under flux reversal.

cond-mat.mes-hall↗
arXiv2000

Motion in a rocked ratchet with spatially periodic friction

We present a detailed study of the transport and energetics of a Brownian particle moving in a periodic potential in the presence of an adiabatic external periodic drive. The particle is considered to move in a medium with periodic space dependent friction with the same periodicity as that of the potential but with a phase lag. We obtain several results, most of them arising due to the medium being inhomogeneous and are sensitive to the phase lag. When the potential is symmetric we show that efficiency of energy transduction can be maximised as a function of noise strength or temperature. However, in the case of asymmtertic potential the temperature may or may not facilitate the energy conversion but current reversals can be obtained as a function of temperature and the amplitude of the periodic drive. The reentrant behaviour of current can also be seen as a function of phase lag.

cond-mat.stat-mech↗
arXiv2000

Novel interference effects and a new Quantum phase in mesoscopic systems

Mesoscopic systems have provided an opportunity to study quantum effects beyond the atomic realm. In these systems quantum coherence prevails over the entire sample. We discuss several novel effects related to persistent currents in open systems which do not have analogues in closed systems. Some phenomena arising simultaneously due to two non-classical effects namely, Aharonov-Bohm effect and quantum tunneling are presented. Simple analysis of sharp phase jumps observed in double-slit Aharonov-Bohm experiments is given. Some consequences of parity violation are elaborated. Finally, we briefly describe the dephasing of Aharonov-Bohm oscillations in Aharonov-Bohm ring geometry due to spin-flip scattering in one of the arms. Several experimental manifestations of these phenomena and their applications are given.

cond-mat.mes-hall↗
arXiv2000

A study of disordered systems with gain: Stochastic Amplification

A study of statistics of transmission and reflection from a random medium with stochastic amplification as opposed to coherent amplification is presented. It is found that the transmission coefficient $t$, for sample length $L$ less than the critical length $L_c$ grows exponentially with $L$. In the limit $L \to \infty$ transmission decays exponentially as $\avg{lnt} = -L/ξ$ where $ξ$ is the localization length. In this limit reflection coefficient $r$ saturates to a fixed value which shows a monotonic increase as a function of strength of amplification $α$. The stationary distribution of super-reflection coefficient agrees well with the analytical results obtained within the random phase approximation (RPA). Our model also exhibits the well known duality between absorption and amplification. We emphasize the major differences between coherent amplification and stochastic amplification where-ever appropriate.

cond-mat.dis-nn↗
arXiv2000

Modelling of Stochastic Absorption in a Random Medium

We report a detailed and systematic study of wave propagation through a stochastic absorbing random medium. Stochastic absorption is modeled by introducing an attenuation constant per unit length $α$ in the free propagation region of the one-dimensional disordered chain of delta function scatterers. The average value of the logarithm of transmission coefficient decreases linearly with the length of the sample. The localization length is given by $ξ~ = ~ ξ_w ξ_α/ (ξ_w + ξ_α)$, where $ξ_w$ and $ξ_α$ are the localization lengths in the presence of only disorder and of only absorption respectively. Absorption does not introduce any additional reflection in the limit of large $α$, i.e., reflection shows a monotonic decrease with $α$ and tends to zero in the limit of $α\to\infty$, in contrast to the behavior observed in case of coherent absorption. The stationary distribution of reflection coefficient agrees well with the analytical results obtained within random phase approximation (RPA) in a larger parameter space. We also emphasize the major differences between the results of stochastic and coherent absorption.

cond-mat.dis-nn↗
arXiv1999

Efficiency and current reversals in spatially inhomogeneous ratchets

Efficiency of generation of net unidirectional current in an adiabatically driven symmetric periodic potential system is studied. The efficiency shows a maximum, in the case of an inhomogeneous system with spatially varying periodic friction coefficient, as a function of temperature. The ratchet is not most efficient when it gives maximum current. The direction of current may also be reversed as a function of noise strength when, instead, an asymmetric periodic potential is considered.

cond-mat.soft↗
arXiv1999

Quantum Stochastic Absorption

We report a detailed and systematic study of wave propagation through a stochastic absorbing random medium. Stochastic absorption is modeled by introducing an attenuation constant per unit length $α$ in the free propagation region of the one-dimensional disordered chain of delta function scatterers. The average value of the logarithm of transmission coefficient decreases linearly with the length of the sample. The localization length is given by $ξ~ = ~ ξ_w ξ_α/ (ξ_w + ξ_α)$, where $ξ_w$ and $ξ_α$ are the localization lengths in the presence of only disorder and of only absorption respectively. Absorption does not introduce any additional reflection in the limit of large $α$, i.e., reflection shows a monotonic decrease with $α$ and tends to zero in the limit of $α\to\infty$, in contrast to the behavior observed in case of coherent absorption. The stationary distribution of reflection coefficient agrees well with the analytical results obtained within random phase approximation (RPA) in a larger parameter space. We also emphasize the major differences between the results of stochastic and coherent absorption.

cond-mat.dis-nn↗
arXiv1999

Successive crossover from ordinary Born scattering to multiple scattering to localization - A delay time analysis in electronically random systems

We have studied the reflection delay time distribution from a one-dimensional electronically random system of finite length. We show that the average delay time is a non-monotonic function of the strength of the disorder and exhibits three qualitatively different regimes. In the weak disorder limit the scattering is dominated by the ordinary Born scattering. For the intermediate strengths of disorder a well defined regime of multiple scattering emerges followed by the localization domain.

cond-mat.dis-nn↗