Search

A dilute suspension of uniform, non-Brownian spheres settles slowly in a viscous solvent. The initially well-mixed system showing Poisson or random occupancy statistics evolves to a system having reduced number fluctuations, but otherwise appearing random. The reduced number fluctuations are consistent with recent measurements of velocity fluctuations in settling suspensions. These experimental results test the assumptions leading to the theoretical predictions by Calflisch and Luke that the velocity fluctuations increase without limit with increasing sample dimension. The theoretical prediction assumes Poisson occupation statistics contrary to our observations.

This paper presents the results of an ongoing experimental investigation of compressive failure of homogeneous and heterogeneous rocks. We used a rapid data acquisition system to monitor the spatio-temporal distribution of acoustic emission (AE) during fault nucleation under conditions of either constant-rate loading or static loading. In order to examine the effect of asperities on faulting, we conducted a series of experiments on mudstone with quartz veins. The bedding planes of the mudstone were oriented at an angle of 30° with the maximum compressive stress and were expected as the fracture plane. The quartz veins are much stronger than the bedding plane of mudstone and thus the samples model faults having strong asperities. Experimental results show that: (1) AE activity initiated close to the peak stress, and almost all AE hypocenters appeared at the intersections of the veins and the fault planes, suggesting that the vein asperities control faulting; (2) the b-value changed with time and shows multiple large and short-term fluctuations; (3) the change of the b-value correlated closely with the spatio-temporal hierarchy of the fracture process; and (4) fault segments along the bedding plane show behavior of slip having large compressive deformation before the peak stress, while the vein asperities show large precursor dilatancy prior to dynamic rupture. These experimental results are helpful for the understanding of seismic precursor phenomena associated with strain localization, dilatancy, change in level of ground water, as well as the b-value.

The balance equations are used to investigate the hot electron magneto-transport in narrow-gap semiconductor InSb at 77 K in crossed weak magnetic field and electric field. In the case of vanishing transverse velocity, the drift mobility and the Hall mobility are calculated and it is shown that the Hall factor in InSb at 77K is less than 1 and decreases with electric field. In the case of vanishing transverse electric field, the longitudinal velocity and the transverse velocity are calculated as a function of the magnetic field and the electric field. The effect of the magnetic field on the longitudinal velocity is different from that on the transverse velocity.

We examine the role of high-lying minibands in superlattice vertical transport using a nonparabolic balance-equation approach. We find that the inclusion of high-lying minibands results in a decrease of the electron temperature, a reduction of the peak drift velocity and a slow-down of the velocity-drop rate in the negative differential mobility (NDM) regime, in comparison with those predicted by a single miniband model. These effects become significant when the strength of the electric field gets close to or falls in the NDM regime.Copyright 1998 Academic Press Limited

A theoretical investigation is carried out to examine the Coulomb interaction effects in linear and nonlinear transport of a coupled electron-hole system which consists of an electron layer and a hole layer separated by a potential barrier. In this we apply the balance equation method to examine the induced current in one charged layer when the other layer is subject to a constant electric field. Nonlinear balance equations are developed for both the electrons and the holes, with Coulomb interactions among them, as well as interactions of the charge carriers with phonons and impurities. Numerically obtained linear results exhibit qualitative agreement with experimental data in regard to dependencies on temperature and carrier densities. Nonlinear effects are shown to be important at low temperatures, in agreement with experimental results. Copyright 1993, 1999 Academic Press

Microwave small-signal mobility of vertical conduction in the presence of a dc bias is investigated for cylindrically confined superlattices having various transverse diameters. At finite dc bias the differential mobility exhibits peculiar frequency-dependence, which stems from a decrease of the average inverse effective mass of carriers in conjunction with an increase of their average longitudinal energy, and is characterized approximately by a momentum and an energy relaxation time. It is found that the effective energy-relaxation time increases significantly as the transverse dimensions of the superlattice increase. PACS: 72.30.+q; 73.50.Mx Copyright 1993, 1999 Academic Press

Miniband electron transport through a laterally-confined superlattice is investigated using the balance equation approach. Up to 15 subbands with intraband and interband impurity and phonon scatterings are included. The nonlinear drift velocity and electron temperature, calculated as functions of the applied electric field for systems of varying degrees of confinement, exhibit significant subband effects, and the cross-over from one-dimensional to three-dimensional behavior. Copyright 1993, 1999 Academic Press

We investigate the full counting statistics of a single quantum dot strongly coupled to a local phonon and weakly tunnel connected to two metallic electrodes. By employing the generalized nonequilibrium Green-function method and the Lang-Firsov transformation, we derive an explicit analytical formula for the cumulant generating function, which makes one able to identify distinctly the elastic and inelastic contributions to the current and zero-frequency shot noise. We find that at zero temperature, the inelastic effect causes upward steps in the current and downward jumps in the noise at the bias voltages corresponding to the opening of the inelastic channels, which are ascribed to the vibration-induced complex dependencies of electronic self-energies on the energy and bias voltage. More interestingly, the Fano factor exhibits oscillatory behavior with increasing bias voltage and its minimum value is observed to be smaller than one-half. [ABSTRACT FROM AUTHOR]