C. J. B. Ford

Probing Spin-Charge Separation in a Tomonaga-Luttinger Liquid

Electron Breakdown An electron possesses charge and spin. In general, these properties are confined to the electron. However, in strongly interacting many-body electronic systems, such as one-dimensional wires, it has long been theorized that the charge and spin should separate. There have been tantalizing glimpses of this separation experimentally, but questions remain. Jompol et al. (p. 597) looked at the tunneling current between an array of one-dimensional wires and a two-dimensional electron gas and argue that the results reveal a clear signature of spin-charge separation. Electronic spin and charge respond differently during tunneling between low-dimensional electron systems. In a one-dimensional (1D) system of interacting electrons, excitations of spin and charge travel at different speeds, according to the theory of a Tomonaga-Luttinger liquid (TLL) at low energies. However, the clear observation of this spin-charge separation is an ongoing challenge experimentally. We have fabricated an electrostatically gated 1D system in which we observe spin-charge separation and also the predicted power-law suppression of tunneling into the 1D system. The spin-charge separation persists even beyond the low-energy regime where the TLL approximation should hold. TLL effects should therefore also be important in similar, but shorter, electrostatically gated wires, where interaction effects are being studied extensively worldwide.

High-frequency single-electron transport in a quasi-one-dimensional GaAs channel induced by surface acoustic waves

We report on an experimental investigation of the direct current induced by transmitting a surface acoustic wave (SAW) with frequency 2.7 GHz through a quasi-one-dimensional (1D) channel defined in a GaAs - AlGaAs heterostructure by a split gate, when the SAW wavelength was approximately equal to the channel length. At low SAW power levels the current reveals oscillatory behaviour as a function of the gate voltage with maxima between the plateaux of quantized 1D conductance. At high SAW power levels, an acoustoelectric current was observed at gate voltages beyond pinch-off. In this region the current displays a step-like behaviour as a function of the gate voltage (or of the SAW power) with the magnitude corresponding to the transfer of one electron per SAW cycle. We interpret this as due to trapping of electrons in the moving SAW-induced potential minima with the number of electrons in each minimum being controlled by the electron - electron interactions. As the number of electrons is reduced, the classical Coulomb charging energy becomes the Mott - Hubbard gap between two electrons and finally the system becomes a sliding Mott insulator with one electron in each well.

On-demand single-electron transfer between distant quantum dots

Single-electron circuits of the future, consisting of a network of quantum dots, will require a mechanism to transport electrons from one functional part of the circuit to another. For example, in a quantum computer decoherence and circuit complexity can be reduced by separating quantum bit (qubit) manipulation from measurement and by providing a means of transporting electrons between the corresponding parts of the circuit. Highly controlled tunnelling between neighbouring dots has been demonstrated, and our ability to manipulate electrons in single- and double-dot systems is improving rapidly. For distances greater than a few hundred nanometres, neither free propagation nor tunnelling is viable while maintaining confinement of single electrons. Here we show how a single electron may be captured in a surface acoustic wave minimum and transferred from one quantum dot to a second, unoccupied, dot along a long, empty channel. The transfer direction may be reversed and the same electron moved back and forth more than sixty times—a cumulative distance of 0.25 mm—without error. Such on-chip transfer extends communication between quantum dots to a range that may allow the integration of discrete quantum information processing components and devices.

Crosslinked PMMA as a high-resolution negative resist for electron beam lithography and applications for physics of low-dimensional structures

We present a novel technique which employs crosslinked PMMA as a high-resolution negative resist for electron beam lithography. The technique allows the patterning of submicrometre features in an insulating layer, thus simplifying the fabrication process of various multilayer devices. We demonstrate this by reference to specific devices and present simple experimental results which prove the usefulness of the technique.

Electrostatically defined heterojunction rings and the Aharonov–Bohm effect

Micron‐sized loops of high‐mobility two‐dimensional electron gas have been made on GaAs‐AlGaAs heterostructures using a novel split‐gate technique. Aharonov–Bohm oscillations of amplitude up to 20% of the device resistance have been observed at very low temperatures (T<100 mK), together with h/2e oscillations which appear to be due to interference between pairs of time‐reversed paths near B=0. The h/e period is found to vary by ∼25% with magnetic field, possibly as a result of the formation of edge states. In the quantum Hall effect, plateaus in Rxx are seen at high B due to variations in carrier concentration across the ring, which may cause backscattering of some edge states.

Magnetic-field-induced insulator-quantum Hall-insulator transition in a disordered two-dimensional electron gas

We present low-temperature transport measurements on the two-dimensional electron gas in delta -doped GaAs which undergoes an insulator-quantum Hall-insulator transition as the magnetic field is increased. Both low- and high-held transitions are marked by peaks in sigma xx and the temperature-independent critical value of sigma xy of 0.5e2/h per spin. We map out the phase diagram versus disorder and magnetic field and study the temperature dependence of sigma xx throughout. In the quantum Hall region we observe Mott variable range hopping and, around the high-field transitions, scaling via a single parameter: z=(B-B*)T-0.45. The functional dependence on z above this transition is fitted by recent network percolation calculations.

DETECTION OF COULOMB CHARGING AROUND AN ANTIDOT IN THE QUANTUM HALL REGIME

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 states 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.

FLUCTUATIONS AND EVIDENCE FOR CHARGING IN THE QUANTUM HALL EFFECT

We find that mesoscopic conductance fluctuations in the quantum Hall regime in silicon MOSFETs display simple and striking patterns. The fluctuations fall into distinct groups which move along lines parallel to loci of integer filling factor in the gate voltage-magnetic field plane. Also, a relationship appears between the fluctuations on quantum Hall transitions and those found at low densities in zero magnetic field. These phenomena are most naturally attributed to charging effects. We argue that they are the first unambiguous manifestation of interactions in dc transport in the integer quantum Hall effect.

The Aharonov-Bohm effect in electrostatically defined heterojunction rings

Micrometer-sized loops of two-dimensional electron gas have been made on GaAs-AlGaAs heterostructures by electrostatic confinement. A split gate is used to define the loop, allowing the width of the conducting channels to be varied by changing the gate voltage. The magnetoresistance has been measured at low temperatures (T<100 mK) and shows strong Aharonov-Bohm oscillations with amplitudes of up to 7% of the total resistance in the narrowest devices. The oscillations are strong out to B approximately=0.5 T and then die out as B increases to approximately=1 T, with a possible dependence on the channel width. Magnetic depopulation of the ID-sub-bands is also seen.

Energy-dependent tunneling from few-electron dynamic quantum dots.

We measure the electron escape rate from surface-acoustic-wave dynamic quantum dots (QDs) through a tunnel barrier. Rate equations are used to extract the tunneling rates, which change by an order of magnitude with tunnel-barrier-gate voltage. We find that the tunneling rates depend on the number of electrons in each dynamic QD because of Coulomb energy. By comparing this dependence to a saddle-point-potential model, the addition energies of the second and third electron in each dynamic QD are estimated. The scale ( approximately a few meV) is comparable to those in static QDs as expected.