We report on an experimental investigation of the direct current induced by transmitting a surface acoustic wave (SAW) with frequency 2.7 GHz through a quasi-one-dimensional (1D) channel defined in a GaAs - AlGaAs heterostructure by a split gate, when the SAW wavelength was approximately equal to the channel length. At low SAW power levels the current reveals oscillatory behaviour as a function of the gate voltage with maxima between the plateaux of quantized 1D conductance. At high SAW power levels, an acoustoelectric current was observed at gate voltages beyond pinch-off. In this region the current displays a step-like behaviour as a function of the gate voltage (or of the SAW power) with the magnitude corresponding to the transfer of one electron per SAW cycle. We interpret this as due to trapping of electrons in the moving SAW-induced potential minima with the number of electrons in each minimum being controlled by the electron - electron interactions. As the number of electrons is reduced, the classical Coulomb charging energy becomes the Mott - Hubbard gap between two electrons and finally the system becomes a sliding Mott insulator with one electron in each well.

We report a detailed experimental study of the quantized acoustoelectric current induced by a surface acoustic wave in a one-dimensional channel defined in a GaAs-Al x Ga 1Ϫx As heterostructure by a split gate. The current measured as a function of the gate voltage demonstrates quantized plateaus in units of Iϭe f where e is the electron charge and f is the surface acoustic wave frequency, the effect first observed by Shilton et al. The quantization is due to trapping of electrons in the moving potential wells induced by the surface acoustic wave, with the number of electrons in each well controlled by electron-electron repulsion. The experimental results demonstrate that acoustic charge transport in a one-dimensional channel may be a viable means of producing a standard of electrical current.

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.

We demonstrate charge pumping in semiconducting carbon nanotubes by a traveling potential wave. From the observation of pumping in the nanotube insulating state we deduce that transport occurs by packets of charge being carried along by the wave. By tuning the potential of a side gate, transport of either electron or hole packets can be realized. Prospects for the realization of nanotube based singleelectron pumps are discussed. DOI: 10.1103/PhysRevLett.95.256802 PACS numbers: 85.35.Kt, 72.50.+b, 73.23.Hk, 73.63.Kv The phenomenon of charge pumping has attracted considerable interest in the last two decades from both fundamental and applied points of view [1][2][3][4][5][6][7][8][9][10]. In pumping, a periodic in time and spatially inhomogeneous external perturbation yields a dc current. If a fixed number n of electrons is transferred during a cycle then the pumping current is quantized in units of ef, where e is the electron charge and f is the perturbation frequency. An important aspect of single-electron pumps is their potential to provide an accurate frequency-current conversion which could close the measurement triangle relating frequency, voltage, and current. Previously, a realization of quantized current I nef has been achieved in two different ways: first, using devices comprising charge islands and controlled by a number of phase-shifted ac signals [3,4,7]; and second, using one-dimensional (1D) channels within a GaAs heterojunction where a surface acoustic wave (SAW) produces traveling potential wells which convey packets of electrons along the channel [5]. In the SAW pumps, transport of charge resembles the pumping of water by an Archimedean screw. When this principle is combined with Coulomb blockade it results in the pumping of a fixed number of electrons n per cycle. For metrological applications, the delivered current should be in the range of 1 nA and at present only the SAW single-electron pumps satisfy this requirement. However, the accuracy of the SAW pumps must be improved significantly for them to find metrological applications.A quantum regime of pumping, in which quantum interference plays a key role, was first described by Thouless [1,2]. In the Thouless mechanism, a traveling periodic perturbation induces minigaps in the spectrum of an electronic system, and when the Fermi level lies in a minigap an integer number of electrons n are transferred during a cycle, resulting in a quantized current flowing without dissipation. From a fundamental physics standpoint, this mechanism represents a new macroscopic quantum phenomenon reminiscent of the quantum Hall effect and of superconductivity. Possible applications of charge pumping are not limited to metrology. For example, the ability of the pumps to control the position of single electrons could be used in various quantum information processing schemes [11,12].Recently it has been pointed out that carbon nanotubes have significant advantages over semiconductor and metallic systems in terms of single-electron pumping [8,9]. The typical Cou...

The coupling of a semimetallic carbon nanotube to a surface acoustic wave (SAW) is proposed as a vehicle to realize quantized adiabatic charge transport. We demonstrate that electron backscattering from a periodic SAW potential can be used to induce a miniband spectrum at energies near the Fermi level. Within the framework of Luttinger liquid theory, electron interaction is shown to enhance minigaps and thereby improve current quantization.