J.G. Williamson

Coherent Electron Focussing in a Two-Dimensional Electron Gas.

The first experimental realization of ballistic point contacts in a two-dimensional electron gas for the study of transverse electron focussing by a magnetic field is reported. Multiple peaks associated with skipping orbits of electrons reflected specularly by the channel boundary are observed. At low temperatures fine structure in the focussing spectra is seen.

Single electron charging effects in semiconductor quantum dots

We have studied charging effects in a lateral split-gate quantum dot defined by metal gates in the two dimensional electron gas (2 DEG) of a GaAs/AlGaAs heterostructure. The gate structure allows an independent control of the conductances of the two tunnel barriers separating the quantum dot from the two 2 DEG leads, and enables us to vary the number of electrons that are localized in the dot. We have measured Coulomb oscillations in the conductance and the Coulomb staircase in current-voltage characteristics and studied their dependence on the conductances of the tunnel barriers. We show experimentally that at zero magnetic field charging effects start to affect the transport properties when both barrier conductances are smaller than the first quantized conductance value of a point contact at 2e2/h. The experiments are described by a simple model in terms of electrochemical potentials, which includes both the discreteness of the electron charge and the quantum energy states due to confinement.

Coulomb-blockade oscillations in a quantum dot

Abstract Periodic oscillations as a function of gate voltage have been observed in the conductance of a quantum dot defined in a two-dimensional electron gas by three pairs of gates. They are interpreted as Coulomb-blockade oscillations. The amplitude of the oscillations can be enhanced up to e 2 / h in the quantum Hall effect regime.

Injection of Ballistic Hot Electrons and Cool Holes in a Two-Dimensional Electron Gas

We have constructed a novel magnetic spectrometer to study the dynamics of hot electrons and cool missing electron states injected by quantum point contacts in the two-dimensional electron gas of a GaAs-AlxGa1-xAs heterostructure. The mean free path of these quasi-particles is found to be longer than recent theoretical estimates. The injection energy of the particles is found to be anomalously low as the point contact approaches pinch-off, and also for high bias voltages.

One-Dimensional Electron Transport in Edge-Channels Studied With Quantum Point Contacts

Electronic transport in a 2-dimensional electron gas (2DEG) has been studied widely, in particular following the discovery of the quantum Hall effect (QHE). Initially, the integer effect (IQHE) was explained as to emerge in the full width of the 2DEG area, i.e. as a bulk effect 1. To explain the width of the quantized plateaux, the existence of localised states residing in the bulk was required.

Lateral electron transport through a quantum dot : Coulomb blockade and quantum transport

We have fabricated a versatile quantum dot device.defined in a twodimensional electron gas by three pairs of gates. In the quantum transport regime, where the entrance and exit barrier conductances were greater than 2e/h, corresponding to two quantum point contacts in series, we have observed conductance plateaux corresponding to the lowest conductance point contact, modulated, at low temperature, by a period similar to that of the conductance plateau Separation for a single quantum point contact. For the case where the conductance of both barriers was less than 2e/h, periodic oscillations have been observed äs a function of gate voltage which we interpret äs coulomb blockade oscillations. These oscillations were observed even when the electrons in the dot were poorly confmed.

Transport in an Electron Interferometer and an Artificial One-Dimensional Crystal

We have studied the electron transport in a one-dimensional electron interferometer. It consists of a quantum dot, defined in a two-dimensional electron gas, to which quantum point contacts are attached. Discrete electronic states are formed due to the constructive interference of electron waves which travel along the circumference of the dot in one-dimensional magnetic edge channels. An artificial one-dimensional crystal has been fabricated, which consists of a sequence of 15 quantum dots, coupled by point contacts. The conductance of this device reveals the formation of a band structure, including the gaps between adjacent bands and the discrete electronic states from which the bands are constructed.