In zero magnetic field, conductance measurements of clean one-dimensional (1D) constrictions defined in GaAs/AlGaAs heterostructures show twenty-six quantized ballistic plateaux, as well as a structure close to $0.7(2e^2/h)$. In an in-plane magnetic field all the 1D subbands show Zeeman splitting and in the wide channel limit the $g$-factor is $\mid g \mid = 0.4$, close to that of bulk GaAs. For the last subband spin-splitting originates from the structure at $0.7(2e^2/h)$, indicating spin polarization at $B=0$. The measured enhancement of the $g$-factor as the subbands are depopulated suggests that the ``0.7 structure'' is induced by electron-electron interactions.Comment: Ten pages with four ps figures. Accepted for publication in Phys. Rev. Lett. (17. June 1996

We have investigated the transport properties of one-dimensional (1D) constrictions defined by split-gates in high quality GaAs/AlGaAs heterostructures. In addition to the usual quantized conductance plateaus, the equilibrium conductance shows a structure close to 0.7(2e 2 /h), and in consolidating our previous work [K. J. Thomas et al., Phys. Rev. Lett. 77, 135 (1996)] this 0.7 structure has been investigated in a wide range of samples as a function of temperature, carrier density, in-plane magnetic field B and source-drain voltage V sd . We show that the 0.7 structure is not due to transmission or resonance effects, nor does it arise from the asymmetry of the heterojunction in the growth direction. All the 1D subbands show Zeeman splitting at high B , and in the wide channel limit the g-factor is | g |≈ 0.4, close to that of bulk GaAs. As the channel is progressively narrowed we measure an exchange-enhanced g-factor. The measurements establish that the 0.7 structure is related to spin, and that electron-electron interactions become important for the last few conducting 1D subbands.

We have detected oscillations of the charge around a potential hill (antidot) in a two-dimensional electron gas as a function of a large magnetic field B. The field confines electrons around the antidot in closed orbits, the areas of which are quantized through the Aharonov-Bohm effect. Increasing B reduces each state's area, pushing electrons closer to the center, until enough charge builds up for an electron to tunnel out. This is a new form of the Coulomb blockade seen in electrostatically confined dots. Addition and excitation spectra in dc bias confirm the Coulomb blockade of tunneling. PACS numbers: 73.23.Hk, 73.40.Gk, 73.40.Hm This paper addresses the fundamental question of whether charging can occur in an open system. Coulomb blockade (CB) of tunneling is generally observed only in electrostatically confined "dots" where there is only partial transmission through the entrance and exit constrictions. It has recently been seen when one constriction is open [1], when both constrictions transmit exactly one onedimensional (1D) channel [2], or when some transmitted channels are decoupled from trapped states [3]. However, an unambiguous demonstration requires a completely open system, such as an antidot, which is a potential hill in a two-dimensional electron gas (2DEG). When a magnetic field B is applied perpendicular to the 2DEG, a set of states, discrete in position and energy, is formed around the antidot, for each Landau level (LL). Aharonov-Bohm (AB) conductance oscillations arising from resonances through such states have been studied extensively in the integer and fractional quantum Hall (QH) regimes [4][5][6][7][8][9]. It has often been assumed that CB does not occur with antidot states because, as charge tries to build up, the system must immediately respond to screen it. However, pairs of AB oscillations from the two spins of the lowest LL were found to lock in antiphase, and this was attributed to charging [4,5]. In a dot system, it was suggested that the charging of edge channels is responsible for a similar regularity of the magnetoconductance peaks [10,11].The aim of the present work was to detect such charge oscillations of an antidot, utilizing a noninvasive voltage probe similar to that employed by Field et al. [12]. They fabricated a 1D constriction as a charge detector next to a dot but in a different circuit separated from it by a narrow gate. When the constriction was nearly pinched off, its resistance was very sensitive to potential variations nearby, and, hence, it could detect charge oscillations in the dot. We have fabricated a similar device with an antidot instead of a dot [see inset of Fig. 1(b)]. A charging signal with the same period as the AB oscillations in the conductance G ad is clearly visible. The line shape and phase show that CB of tunneling through the antidot is occurring. dc-bias measurements are used to measure addition and excitation spectra, confirming this interpretation. The charging energy saturates at high B and the single-particle (SP) energy spacing varies ...

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 have investigated experimentally an open semiconductor system in which electron confinement around an obstacle is obtained using a magnetic field. The magnetic field gives rise to Landau levels, and each associated edge state circulates around the obstacle, forming a set of quantized states. Tunable constrictions are fabricated by using a technique which enables us to control transport in and out of these states, producing Aharonov-Bohm oscillations as the magnetic field is swept. Surprisingly, a strong extra oscillation with the same h /e frequency develops, phase shifted by m. so that the frequency appears to have doubled. We explain these results in terms of charging of isolated circulating edge states.In a metal or semiconductor, the isolation of electrons in a potential well leads to charging efFects because electrons are indivisible. For example, at low applied bias, transport through a cavity via tunnel barriers on either side is blocked when extra energy is required to add an electron, a phenomenon known as Coulomb blockade (CB). For a semiconductor in a magnetic field, it is possible to confine the highest Landau levels (LL's) within the cavity while allowing the lowest ones to extend into the leads, by reducing the height of the tunnel barriers below the Fermi energy EF. The associated edge states can form closed paths and thus give rise to Aharonov-Bohm (AB) oscillations. ' Recently, combined AB and CB oscillations were found, indicating that the confined LL s can charge even when there are also extended states in the cavity.The confinement is usually produced by a physical barrier, such as the edge of a metal sample or the depletion region in a semiconductor structure. In contrast, we have made a completely open (two-dimensional) system, in which confinement around a microscopic obstacle is provided solely by a perpendicular magnetic field B. A11 LL's extend into the bulk, but some edge states (shown schematically as solid lines in the insets, Fig 1) for.m c1osed paths around the obstacle. If the path length is small and the temperature low, these paths are phase coherent. The accumulated phase depends on the circumference, wavelength, and the AB efFect which causes a change of 2n for each increase of h/e in the flux enclosed. Thus, a ladder of allowed single-particle (Sp) states forms. The states are also confined to an LL, which rises in energy as it approaches the edge, so states enclosing less area have higher energy and shorter wavelength. Changing B sweeps the states, each containing one electron, through EF, causing the net charge near the obstacle to osci11ate. In contrast, in electrostaticallyconfined systems the charge is independent of B. It might be expected that such excess charge would not occur, as electrons within the same, u n confine, LL would move to compensate. However, we observe a phenomenon which provides evidence for such charging, showing that edge states encircling the obstacle are unable to move sufFiciently to screen the charge because they consist of a series of quantiz...