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Focusing on the last spin-degenerate one-dimensional subband, conductance measurements of split-gate devices were carried out in the linear and nonlinear regimes. In the linear regime, we observed a structure at 0.7(2e 2/h) in addition to the quantized plateaux. In a strong in-plane magnetic field B this 0.7 structure evolves into the spin-split plateau at e 2/h, suggesting a possible spin polarization for the last subband at B = 0. The 0.7 structure and the e 2/h plateau show similar source—ain bias dependence, suggesting that the 0.7 structure has origin in the lower spin subband.

We have used a quantum antidot (QAD) to detect the oscillation of the Fermi energy in a two-dimensional electron gas (2DEG) as a function of magnetic field. At a magnetic field B 0 = n s h 2 ( e 2 π R 2 ) (where n s is the carrier density and R is the radius of the QAD at the Fermi energy) the Shubnikov-de Haas (SdH) oscillation period becomes greater than the period of the Aharonov-Bohm (AB) oscillations from the QAD. Hence, above this field, the corresponding oscillation of the Fermi energy is superimposed onto the period of the AB oscillations. We have taken measurements of the AB period as a function of magnetic field at a number of different QAD radii. These measurements reveal an average oscillatory period which is consistent with the motion of the Fermi energy through the predicted QAD eigen spectrum.

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.

Recent theoretical studies predict a rich spectrum of edge excitations in a 2DEG in the QH regime. Experimental verification of the theory is hindered by the fact that many of these excitations are uncoupled (neutral) or weakly coupled with an external alternating electric field. We discuss the use of gates for the mutual transformation of coupled and uncoupled excitations with an external field in IQH regime. This process provides a method for the excitation and detection of neutral edge excitations. We report results of the first experimental study of the transmission of edge excitations through a gate-induced potential barrier. A strong oscillatory dependence of the magnitude of transmitted edge magnetoplasmons as a function of the gate voltage was observed. Results of an experiment with two gates in series, where one gate was intended to be the source and the second to be the detector of a neutral edge excitation, give evidence that more than one type of edge excitation can propagate along a 2DEG edge. We discuss our data in terms of existing models of low-frequency edge excitations.

Low temperature measurements of ballistic one-dimensional (1D) wires show an additional structure at 0.7(2e(2)/h) in the conductance characteristics (Phys. Rev. Lett, 77 (1996) 135: Phys. Rev. B 62 (2000) 10950). This 0.7 structure develops into a plateau at e(2)/h, if. either the in-plane magnetic field is increased (Phys. Rev. Lett. 77 ( 1996) 135) or the carrier density is decreased (Phys. Rev. B 61 (2000) 13 365). A suggested explanation for this effect is based on spin polarisation induced by many-body interactions. As temperature increases, the e(2)/h plateau in a magnetic field shifts to 0.6(2e(2)/h). whereas other plateaus remain unchanged. We study the effect of temperature on the evolution of the 0.7 structure with increasing in-plane magnetic field. (C) 2002 Elsevier Science B.V. All rights reserved.

The magnetoresistance of a device fabricated from a bulk InGaAs layer on an InP substrate has been studied. The device exhibited both 2D conduction at the InP-InGaAs interface and 3D conduction in a doped InGaAs layer at the surface. The authors have observed negative magnetoresistance arising from the quenching of weak localization in the device. The effect is observed in both transverse and parallel fields, although that in the parallel field is of smaller magnitude than that in the transverse field. It is proposed that the structure contains a two-dimensional electron gas (2DEG) in parallel with a bulk conducting region. The temperature dependence of the negative magnetoresistance has been investigated and phase breaking times have been calculated by fitting the experimental data to the formula given by Hikami et al. (1980) for the 2D effect. A comparison is made with theoretical electron inelastic scattering times, and conclusions as to the nature of scattering in InGaAs structures are drawn.

We show that small structures can be defined in high mobility two‐dimensional electron gases formed at a depth of 2770 A below the surface in GaAs/Al0.33Ga0.67As heterostructures. The differential conductance of one‐dimensional constrictions defined by split gates in such deep electron gases showed more than 20 quantised plateaus. The absence of resonant structures on the plateaus demonstrates the absence of potential fluctuations in the constrictions. By applying a dc source‐drain bias we have measured the energy spacings of the first 18 subbands, and the effect of a small perpendicular magnetic field on the energy spacings has been investigated.

We have detected oscillations of the charge around a potential hill (antidot) in a two-dimensional electron gas as a function of a perpendicular magnetic field B. The field confines electrons around the antidot in closed orbits, the areas of which are quantised through the Aharonov-Bohm effect. Increasing B reduces each state's area, pushing electrons closer to the centre, 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. We have also studied h/2e oscillations and found evidence for coupling of opposite spin states of the lowest Landau level., 3 pages, 3 Postscript figures, submitted to the proceedings of EP2DS-13

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 quantised through the Aharonov-Bohm effect. Increasing B reduces each state's area, pushing electrons closer to the centre, 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 tunnelling., Comment: 4 pages, 4 Postscript figures

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., Ten pages with four ps figures. Accepted for publication in Phys. Rev. Lett. (17. June 1996)

We report the observation of geometric resonances of electron-cyclotron orbits with the wavelength of a surface acoustic wave (SAW) in a high-mobility two-dimensional electron gas (2DEG). The resonances were observed by measuring the acoustoelectric current induced in the 2DEG by the SAW. The experimental data are compared with calculations based on the theoretical expression for the wave-vector-dependent 2DEG conductivity. The acoustoelectric effects are shown to be a sensitive probe of 2DEG transport properties in the low-magnetic-field regime where the conductivity is high and other SAW methods become insensitive.

Recent theoretical and experimental work demonstrates that a two-dimensional electron gas at Landau-level filling factor $\ensuremath{\nu}=\frac{1}{2}$ can be described in terms of composite fermions (CF's) for which the effective magnetic field ${B}_{\mathrm{eff}}$ vanishes. We have measured the transport properties of CF's in a quantum point contact (QPC) defined by a wide split-gate device. Negative magnetoresistance due to suppression of backscattering in the QPC was observed both around zero magnetic field $B=0$ and ${B}_{\mathrm{eff}}=0$. We have also measured the resistance of a composite fermion QPC at ${B}_{\mathrm{eff}}=0$ as a function of gate voltage, with an applied magnetic field to maintain $\ensuremath{\nu}=\frac{1}{2}$ in the QPC. Using a simple model to compare the results for $B=0$ and $\ensuremath{\nu}=\frac{1}{2}$, we have determined that the channel widths for CF's are narrower than those for electrons.