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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.

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.

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

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 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