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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 $B_{\parallel}$ 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 $B_{\parallel}$, and in the wide channel limit the $g$-factor is $\mid g \mid \approx 0.4$, 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., Comment: 8 pages, 7 figures (accepted in Phys. Rev. B)

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 studied the conductance through localised states in a lateral barrier produced by a short (∼ 0.2 μm) gate in GaAs MESFETs. Distinctive peaks in the ohmic conductance as a function of the gate voltage have been associated with hopping and resonant tunneling (RT). In addition to RT through one impurity, a new effect has been observed: RT through two and more impurities. We distinguish between the three mechanisms by the temperature dependence of the conductance, its variation with DC bias along the barrier, and the I–V characteristics of the samples.

Recent theoretical and experimental work demonstrates that at filling factor v = 1/2, a two-dimensional electron gas can be described in terms of composite fermions (CFs), for which the effective magnetic field vanishes. We present low-temperature measurements on a wide split-gate (SG) device investigating CFs in the quantum point contact (QPC) defined by such a structure. Negative magnetoresistance due to suppression of backscattering in the QPC was observed both around zero magnetic field and v = 1/2. We have also measured the resistance of a CF QPC at zero effective magnetic field as a function of gate voltage, with a compensating field to maintain v = 1/2 in the QPC. Using a simple model, we have determined the channel widths for CFs, which are narrower than those for electrons.

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.

We demonstrate that the (311)B surface of GaAs can be used for the fabrication of high mobility (μ ≅ 2.4 × 106 cm2 V−1 s−1) two-dimensional electron gases, in which the mobility is found to be anisotropic with μ[233] > μ[011]. This paper reviews the magneto-transport properties of the (311)B system and sheds light on the nature of the scattering mechanisms determining the electron mobility. These results are of particular relevance to the current discussion of the nature of the {311} surface. It is well known that a similar mobility anisotropy exists in hole gases grown on the (311)A surface, although attempts to interpret such results are complicated by the anisotropic and non-parabolic nature of the valence band structure. For electron gases grown on the (311)B surface we demonstrate experimentally (with ballistic focusing) that the Fermi surface is isotropic, leading to the conclusion that the most likely cause of the mobility anisotropy is anisotropic interface roughness scattering. This is also confirmed by measurements of mobility as a function of carrier density, which can be fitted by a simple interface roughness scattering theory. Further experiments have demonstrated that ballistic quantization can be observed in both [233] and [011] directions, despite the large differences in anisotropic mobility.

We present the results of a detailed experimental study of the electric current and voltage induced in the 2DEG of a GaAs-AlGaAs heterostructure by a surface acoustic wave (SAW). New results are obtained for these acoustoelectric effects at zero and low (

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.

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 \ensuremath{\pi} so that the frequency appears to have doubled. We explain these results in terms of charging of isolated circulating edge states.

We have observed a new phenomenon of charge oscillations in small isolated edge states in the quantum Hall regime. We have developed a unique method of contacting an isolated metal dot independently of the other gates, using a patterned layer of insulator. Using this technique we have made two individually tunable ballistic channels in parallel. In a perpendicular magnetic field edge states form a ring around the dot, with a variable constriction on either side across which they can communicate. Electrons orbiting around the ring interfere with themselves, and a series of single particle (SP) states forms. By fine-tuning the individual constriction widths we can generate huge Aharonov-Bohm (AB) oscillations, and extremely well defined oscillations. In addition, in certain field ranges, we observe a very clear double AB frequency, at times completely losing a distinguishable fundamental frequency. We propose that the discreteness of the SP states gives rise to a charging effect, which modifies the tunnelling of the outermost edge state through the device, and also acts to evenly intersperse the energy levels of all the edge states encircling the dot.

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.

Sub-micron split-gate structures have been fabricated on three pseudomorphic GaAs-InxGa1-xAs-AlGaAs quantum well structures, with indium fraction, x of 0.04, 0.10, and 0.14. The conductance characteristic of the device on the x=0.04 material showed clear plateaux due to the quantization of ballistic conduction in one dimension. Devices fabricated from the x=0.10 and x=0.14 material did not show plateaux, but exhibited a complex structure that varied quantitatively between nominally identical devices. This structure is thought to be due to both the random nature of the InxGa1-xAs alloy, and the roughness of the InxGa1-xAs interfaces. The spatial variation of the potential between the arms of the split gate due to these effects was probed by applying a differential voltage between the two gates.

A series of high quality modulation doped GaAs-AlGaAs heterostructures incorporating a strained 10 nm In x Ga 1-x As layer at the hetero-interface have been grown. The indium mole fraction, x, was varied from 0 (the control) to 0.25 in 0.05 steps. Other growth conditions were unaltered for all wafers. The variation of electronic and structural properties of the wafers was measured as a function of x with a view to optimizing the structure for use in one-dimensional ballistic transport measurements

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 developed a new technique to contact a submicrometer metal dot gate independently of surrounding gates. With this we have made two ballistic channels in parallel on a GaAs‐AlGaAs heterostructure. Independent control over all the gates allows unprecedented flexibility to adjust both the size of the dot and the widths of the channels. In a perpendicular magnetic field we can obtain extremely large Aharonov–Bohm oscillations and a double‐frequency oscillation indicative of edge state charging in an open system. At low magnetic fields we see no evidence for any interaction between the one‐dimensional subbands in the two channels.

Electron transport is studied in GaAs/AlGaAs asymmetric triple‐barrier resonant tunneling structures in a high magnetic field perpendicular to the interfaces. Besides the resonance peaks arising from tunneling between two quantum wells, a series of fine structure is observed in the valley region of the current‐voltage characteristics, which is attributed to electron transitions between well defined totally quantized levels (Landau levels) in the two wells, together with the emission of a longitudinal optical phonon. In the voltage region of the main current peak, any structures which shift with magnetic field are not observed, suggesting that the tunneling in this bias region is mainly determined by Δn=0 elastic transitions.

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

We study the effect of electron-electron interactions on the resonant-tunneling spectroscopy of the localized states in a barrier. Using a simple model of three localized states, we show that, due to the Coulomb interactions, a single state can give rise to two resonant peaks in the conductance as a function of gate voltage, G(Vg). We also demonstrate that an additional higher-order resonance with Vg-position in between these two peaks becomes possibile when interactions are taken into account. The corresponding resonant-tunneling process involves two-electron transitions. We have observed both these effects in GaAs transistor microstructures by studying the time evolution of three adjacent G(Vg) peaks caused by fluctuating occupation of an isolated impurity (modulator). The heights of the two stronger peaks exibit in-phase fluctuations. The phase of fluctuations of the smaller middle peak is opposite. The two stronger peaks have their origin in the same localized state, and the third one corresponds to a co-tunneling process., 9 pages, REVTeX, 4 figures

We report the results of two fundamental transport measurements at a Landau level filling factor $\nu$ of 1/2. The well known ballistic electron transport phenomena of quenching of the Hall effect in a mesoscopic cross-junction and negative magnetoresistance of a constriction are observed close to B~=~0 and $\nu~=~ 1/2$. The experimental results demonstrate semi-classical charge transport by composite fermions, which consist of electrons bound to an even number of flux quanta., Comment: 9 pages TeX 3.1415 C version 6.1, 3 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)

In this paper we present a general picture of the cyclic change in resonant structure seen in conductance across quantum Hall transitions in quantum antidots. The cycle begins with resonant dips below the ${\mathit{ne}}^{2}$/h plateau, which evolve into resonant peaks above it. A sinusoidal oscillation, of shorter period in magnetic field, then appears, which evolves into resonant dips below the (n+1)${\mathit{e}}^{2}$/h plateau. The cycle repeats for successive transitions. We provide an explanation for this behavior in terms of the edge-state model and show by numerical calculation that this picture accounts in detail for the line shapes and dependencies observed experimentally.

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.

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We study effects of Coulomb interactions between localised states in a potential barrier by measuring resonant-tunneling spectra with a small bias applied along the barrier. In the ohmic regime the conductance of 0.2um--gate lateral GaAs microstructures shows distinct peaks associated with individual localised states. However, when an electric field is applied new states start contributing to the current, which becomes a correlated electron flow through two interacting localised states. Several situations of such correlations have been observed, and the conclusions are confirmed by calculations., 10 pages, RevTeX, 4 figures, to be published in Phys. Rev. B, Rapid Communication

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.