Search

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)

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 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.

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 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.