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We report the first observation of the direct current induced by a surface acoustic wave through a quantum point contact defined in a GaAs-AlGaAs two-dimensional electron gas by means of a split gate. We have observed giant oscillations in the acoustoelectric current as a function of gate voltage, with minima corresponding to the plateaux in quantum point contact conductivity. A theoretical consideration is presented which explains the observed peaks in terms of the matching of sound velocity with electron velocity in the upper one-dimensional subband of the quantum point contact.The interaction of a surface acoustic wave (SAW) with a two-dimensional electron gas (2DEG) in a GaAs-Al x Ga 1−x As heterostructure has recently attracted much attention [1][2][3][4][5][6][7][8][9][10]. Usually, two kinds of effect are studied. The first kind is the attenuation and change in velocity of the sound wave due to interaction with electrons. For small amplitudes, these effects are linear in the acoustic wave amplitude. Analysis of these effects allows us, in principle, to study the linear response of carriers to alternating strain deformation and electric fields at the SAW frequency. An important consideration is that these measurements do not require any contacts to be made to the sample. Very interesting studies of these effects in quantum Hall systems were carried out, in particular, in [1,4].The second class of studies deal with the so-called acoustoelectric effects in 2DEGs. These are due to a drag of the 2D electrons by the SAW [5-10], and for small signals are quadratic in the SAW amplitude. As described, an acoustic wave, while travelling across the sample is attenuated due to interaction with the electrons, and transfers some of its momentum to them. As a result a d.c. current in a closed circuit appears (the acoustoelectric current). In an open circuit, a d.c. voltage is generated. Thus in principle these acoustoelectric effects can be used to study both the d.c. and a.c. response of the carriers.Drag of the electrons in a quantum point contact by non-equilibrium phonons has been considered in [11]. This paper discussed a current flowing through a channel due to a 'phonon wind' in the leads, and predicted its quantization, similar to the conductance quantization. We believe that such a mechanism is not important in our case, because of the strong screening of the interaction outside the QPC.In this letter we present the first experimental and theoretical study of the acoustoelectric current in a quasi-one-dimensional ballistic channel defined in a 2DEG by split-gate-induced depletion. We observed a very specific behaviour of the acoustoelectric current, qualitatively different from the behaviour of the conductance.

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Recent theoretical analysis of spatially-nonuniform modes of the thermomagnetic instability in superconductors 1 is generalized to the case of a thin film in a perpendicular applied field. We solve the thermal diffusion and Maxwell equations taking into account nonlocal electrodynamics in the film and its thermal coupling to the substrate. The instability is found to develop in a nonuniform, fingering pattern if the background electric field, E, is high and the heat transfer coefficient to the substrate, h0, is small. Otherwise, the instability develops in a uniform manner. We find the threshold magnetic field, H fing (E, h0), the characteristic finger width, and the instability build-up time. Thin films are found to be much more unstable than bulk superconductors, and have a stronger tendency for formation of fingering (dendritic) pattern.

A linear analysis of thermal diffusion and Maxwell equations is applied to study the thermomagnetic instability in a type-II superconducting slab. It is shown that the instability can lead to formation of spatially nonuniform distributions of magnetic field and temperature. The distributions acquire a finger structure with fingers perpendicular to the screening current direction. We derive the criterion for the instability, and estimate its build-up time and characteristic finger width. The fingering instability emerges when the background electric field is larger than a threshold field, E > Ec, and the applied magnetic field exceeds a value H fing ∝ 1/ √ E. Numerical simulations support the analytical results, and allow to follow the development of the fingering instability beyond the linear regime. The fingering instability may be responsible for the nucleation of dendritic flux patterns observed in superconducting films using magneto-optical imaging.

The dendritic patterns of magnetic flux motion formed during field penetration into an MgB 2 film were observed using magneto-optic imaging. To investigate the origin of the dendrites, experiments were performed where the sample was partially covered with an thermally conducting foil serving as an efficient heat sink. We observed that the dendrites are formed only in areas lacking the thermal conductor. When dendrites develop in the uncovered part they never invade into the covered region. The results strongly suggest that the dendritic instability is thermal in origin. Ó 2001 Published by Elsevier Science B.V.

Superconducting circuits that incorporate Josephson junctions are of considerable experimental and theoretical interest, particularly in the context of quantum computing. A nanometre-sized superconducting grain (commonly referred to as a Cooper-pair box) connected to a reservoir by a Josephson junction is an important example of such a system. Although the grain contains a large number of electrons, it has been experimentally demonstrated that its states are given by a superposition of only two charge states (differing by 2e, where e is the electronic charge). Coupling between charge transfer and mechanical motion in nanometre-sized structures has also received considerable attention. Here we demonstrate theoretically that a movable Cooper-pair box oscillating periodically between two remote superconducting electrodes can serve as a mediator of Josephson coupling, leading to coherent transfer of Cooper pairs between the electrodes. Both the magnitude and the direction of the resulting Josephson current can be controlled by externally applied electrostatic fields.