We discuss criteria for discriminating between `ballistic' and `clean' quantum structures created in the 2D electron gas of gated semiconductor heterostructures and show that they are different for impurity potential fluctuations of short and long range. For long-range fluctuations, such as in high-mobility modulation-doped heterostructures, we show that the criterion for the system to be clean - so that impurity scattering is not at all important - can be met. Hence, disorder-induced gaps in the electron spectrum can be ignored while considering dynamical properties in a time-dependent magnetic field even at a relatively low rate of change. The magnetization of a 2D clean quantum dot turns out to be very sensitive to inelastic relaxation processes in the system. In contrast to the usual destructive role played by inelastic scattering in mesoscopic phenomena, here inelastic scattering restores an Aharonov - Bohm type of quantum oscillation in the magnetization. In the absence of such relaxation, strong non-equilibrium behaviour suppresses these oscillations in favour of large diamagnetic moments. We discuss the special type of inelastic backscattering responsible for relaxation in the case of an isolated dot. By monitoring the transient behaviour of the induced magnetic moment as the magnetic field is switched from one value to another we propose to measure the characteristic time of inelastic backscattering estimated to be of the order of seconds.
The ultrasonic attentuation of sound in a superconducting alloy containing non-magnetic impurities has been calculated using Green's function techniques. When the phonon energy is less than the energy gap, the results are identical to those obtained from the usual transport equation. For phonon energies equal to twice the energy gap the attenuation is a function of the ratio of the electron mean free path to the wavelength of sound. 13 refs.
We consider electrons tunneling through a double-barrier resonant-tunneling structure (DBRTS) and interacting with a defect which has internal degrees of freedom. Usually such a defect has two (or more) metastable configurations and can switch between them due to its interaction with a thermal bath. Interaction between the tunneling electron and the dynamic defect creates a noisy environment surrounding the DBRTS, and leads to fluctuations in time of the resonant level. Such fluctuations result in inelastic tunneling and low-frequency noise. This paper is focused on the problem of inelastic resonant tunneling. We have calculated the average transparency for various relations between the switching rate of the dynamic defect, the escape rate of the electron from the resonant level in the well, the coupling strength between the electron and the dynamic defect, and the temperature. The results derived here are entirely different from those found in phonon-assisted resonant tunneling, because phonons obey the Bose statistics but a two-level fluctuator behaves as an effective spin.
The quantum-interference contribution to the phonon-drag component of the thermopower is considered. It is shown that in a typical experimental situation this contribution is suppressed due to dephasing caused by the relatively high frequencies of some phonons. Experiments concerning the quantum-interference contribution to thermoelectric transport are also discussed.
Non-equilibrium current noise of a short quasi–one-dimensional constriction between two superconductors is considered. We derive a general expression for the frequency-dependent current-current correlation function valid for arbitrary temperatures, transparencies, and bias voltages. This formalism is then applied to a single current-carrying quantum mode with perfect transparency, and at zero frequency and temperature. Contrary to a transparent channel separating two normal conductors, a weak link between two superconductors exhibits a finite level of noise. The source of noise is the fractional Andreev scattering of quasi-particles with energies |E| greater than the half-width Δ of the superconducting gap. For high bias voltages, V >> Δ/e, the zero-frequency limit of the noise spectrum, S(0), as well as the excess current Iexc, are twice as large than for a normal-superconductor junction, S(0) = (2/5)|e|Iexc.
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