Theory of fluctuations around a nonequilibrium state slowly varying in space and time in two-band semiconductors is developed. The derivation of kinetic equations for fluctuations within the framework of the Keldysh formalism is given. The system is supposed to be under the action of a classic optical and external driving electric field, which can displace the system substantially from equilibrium with a thermal bath, while the carriers can interact also with phonons, with one another via the Coulomb potential, and with the thermal photons causing interband recombination and generation. Matrix of correlation functions for Langevin random forces is obtained. It is shown that in the nonequilibrium state there is an extra correlation contribution of Coulomb and phonon scattering to the correlation functions of the Langevin forces. ͓S0163-1829͑98͒02939-7͔

The nonlocal dynamical response of a ballistic nanobridge to an applied potential oscillating with frequency ω is considered. It is shown that, in addition to the active conductance, there is also a reactive contribution. This contribution turns out to be inductive for relatively small frequencies ω. For bigger frequencies the current response is either inductive or capacitive, depending on the ratio of ωL/v(F), where L is the length of the bridge and v(F) is the Fermi velocity.

The Joule heat generation in a quantum semiconductor nanowire joining two classical reservoirs is considered. We assume that the conductance of the system is determined by the phonon-assisted ballistic resistance. The spatial distribution of the generated heat is analysed. The heat generated within the nanostructure is determined. It is simply related to the phonon-assisted variation ΔG of the conductance. For a fixed voltage, the corresponding variation of the overall heat generation is negative and is determined by ΔG. Due to the same interaction the reduction of the heat generated in each reservoir is the same as the heat generated in the nanostructure. In other words, the presence of collisions within a nanowire does not violate the equality of the heat release in the reservoirs that are connected by the nanowire, although the rates and mechanisms of relaxation there may be different. We come to the conclusion that further investigation of various situations where the nanostructures are involved is needed to gain understanding as to why in some cases different heat generation in the two reservoirs is observed and predicted.

A theory of femtosecond photon echo pulse decay in semiconductors is developed. The echo is due to band-to-band transitions produced by two successive pulses of coherent light.We propose that the decay is a result of the unscreened carrier-carrier interaction that brings about dephasing of the electron-hole wave function. We come to the conclusion that the time of dephasing is proportional to n-lB, where n is the d e r concentration.

We work out a theory of the Coulomb drag current created under the ballistic transport regime in a one-dimensional nanowire by a ballistic non-Ohmic current in a nearby parallel nanowire. As in the Ohmic case, we predict sharp oscillation of the drag current as a function of gate voltage or the chemical potential of electrons. We also study dependence of the drag current on the voltage V across the driving wire. For relatively large values of V the drag current is proportional to V 2 .The purpose of the present paper is to study the Coulomb drag current in the course of ballistic (collisionless) electron transport in a nanowire due to a ballistic driving non-Ohmic current in an adjacent parallel nanowire. The possibility of the Coulomb drag effect in the ballistic regime in quantum wires has been demonstrated by Gurevich, Pevzner and Fenton [1]1 and has been experimentally observed by Debray et al. [2]. If two wires, 1 and 2, are near one another and are parallel, the drag force due to the ballistic current in wire 2 acts as a sort of permanent acceleration on the electrons of wire 1 via the Coulomb interaction.We assume that the largest dimension of the structure is smaller than the electron mean free path in the problem (typically a few µm). Such nanoscale systems are characterized by low electron densities, which may be varied by means of the gate voltage.1 A number of references to early papers on the Coulomb drag is given in [1].