Showing papers on "Einstein relation published in 1997"

Dispersive aspects of the high-field hopping mobility of molecularly doped solids with dipolar disorder

Abstract: The time-of-flight mobility of photoinjected charges in molecularly doped polymers obeys a Poole-Frenkel law, μ ∞ exp(γ√E), which is commonly viewed as arising from hopping transport among sites with a large degree of energetic disorder. Recent theoretical investigations have focused on long-range correlations that characterize site energies when the dominant mechanism for energetic fluctuations is the interaction of charge carriers with randomly-oriented permanent dipoles of the dopant and host polymer. An exact calculation of the steady-state drift velocity v d for a one-dimensional system with correlated dipolar disorder predicts a Poole-Frenkel law similar to that observed. In order to investigate another feature commonly observed in the high-field measurements, namely, the anomalous dispersion of the current-time transients, we have performed an exact calculation of the field-dependent diffusion constant D for the same dipolar disorder model. In the bulk limit we obtain an expression D = (KT/e)∂v d /∂E that generalizes the normal Einstein relation and predicts a strongly field-dependent diffusion constant.

Driven Tracer Particle and Einstein Relation in One Dimensional Symmetric Simple Exclusion Process

Abstract: We investigate the behavior of a tagged particle under the action of an external constant driving force in an infinite system of particles evolving in a one dimensional lattice according to symmetric random walks with hard core interaction.

The generalized Stokes-Einstein relation for liquid sodium

Abstract: We show that a microscopic generalization of the Stokes - Einstein relation between the diffusion and shear viscosity coefficients, previously tested in simple liquids near melting, has a much wider range of application. The practical validity of the approach is accurately checked by performing extensive computer simulations in liquid sodium at temperatures ranging from 403 K to 1003 K.

On the Einstein Relation in Ultrathin Films of IV-VI Compounds in the Presence of a Parallel Magnetic Field

Abstract: We have studied the Einstein relation for the diffusivity-mobility ratio of carriers in ultrathin films of IV-VI compounds in the presence of a parallel magnetic field at low temperatures on the basis of a new 2D electron dispersion law. It is found, taking ultrathin films of PbS, PbSe, and PbTe as examples, that the diffusivity-mobility ratio increases with increasing electron concentration and decreasing film thickness in various oscillatory manners. The magnetic field and the quantum wire structure enhance the numerical values of the same ratio. We have suggested an experimental method of determining the Einstein relation in degenerate materials having arbitrary dispersion laws from the measurement of thermoelectric power in the presence of a large magnetic field. In addition, the corresponding well-known results for relatively wide-gap quantum confined materials in the absence of the magnetic field have been obtained as special cases of our generalized formulations, under certain limiting conditions.

Monte Carlo study of diffusion phenomena in III - V modulation doped heterostructures

Abstract: This paper presents a Monte Carlo study of diffusion coefficients in two-dimensional electron gas (TDEG) in III - V heterostructures. The model accounts for the quantization of all valleys and for non-parabolicity. The diffusion coefficients are determined by the spreading of a narrow pulse of carriers drifting along the interface. Two kinds of heterostructures have been considered: AlGaAs/InGaAs/AlGaAs and the AlInAs/InGaAs/AlInAs lattice matched on InP. The diffusion coefficient - field characteristics at 77 K temperature have been extensively studied. Large deviations from the Einstein relation have been observed, even at low-fields. The longitudinal diffusion coefficient is shown to be strongly field dependent and may reach high values for fields around . Its evolution is explained by the behaviour of scattering rates, especially for impurity and phonon scattering.

On the Einstein relation for a mechanical system

Abstract: We consider a mechanical model in the plane, consisting of a vertical rod, subject to a constant horizontal force f and to elastic collisions with the particles of a free gas which is “horizontally” in equilibrium at some inverse temperature β. In a previous paper we proved that, in the appropriate space and time scaling, the motion of the rod is described as a drift term plus a diffusion term. In this paper we prove that the drift d(f) and the diffusivity σ 2 (f) are continuous functions of f, and moreover that the Einstein relation holds, i.e., lim f → 0 d(f)f = β2 σ 2 (0) .

Nernst-Einstein Relation and Effective Valence in a Strongly Coupled Tungsten Plasma

Abstract: The Nernst-Einstein relation between electron mobility and diffusion is employed together with very recent measurements of Kloss et al. on strongly coupled tungsten plasma, to throw light on the effective valence of W as the thermodynamic state is varied.

Einstein relation and transport equations in heavily doped silicon

Abstract: This paper proposes two forms of generalized Einstein relation that embed all heavy doping effects in such a way that corresponding transport equations retain their form as in lightly doped silicon. In the first corresponding form of the transport equations all heavy doping effects are added to their diffusion components through so-called effective diffusion coefficients. The second form of the transport equations is based on the effective carrier concentrations and is suitable for application in device simulation programs. Concrete dependencies of relevant physical parameters are given for silicon, heavily doped by phosphorous, while the whole theory is generally applicable for all semiconductors whose bandgap behaviour in terms of its dependence on doping is known.