Quantum Electronic Devices

Hole quantum wires: a) Scanning electron micrograph of a one-dimensional hole quantum wire. b) Schematic view of the device. c) The electrical conductance G is quantized in units of 2e2/h: it decreases in discrete steps as the wire is continuously made narrower by applying a positive voltage to the side-gates, at a temperature of 0.1K.

The QED group studies the properties of advanced transistor devices, at nanometer length scales where quantum effects become significant. Research in 2005 concentrated on two main topics: “Does the Coulomb repulsion between electrons in a transistor affect its resistance?” and “Electrical properties of p-type quantum wires”.

Almost all field effect transistors contain thin, two-dimensional sheets of highly mobile electrons that allow current to flow in the device. In most electronics applications we ignore the effects of Coulomb interactions between these electrons. This is because the total momentum of all the electrons is conserved, even if electron-electron collisions redistribute the momentum between them. However, this assumption breaks down in the presence of even trace amounts of impurities, and interaction effects can have a large influence on the resistance of high quality two-dimensional systems.

Recent theoretical advances have shown that if the interaction effects are weak, they increase the device resistance, and ultimately make it an insulator at T=0. On the other hand, if the interactions are strong, they actually decrease the resistance, and may lead to the formation of a new, and as-yet unidentified, state at T=0 which may be metallic or superconducting. So how do we know how strong the interactions are? In 2005 we published the first comparison of two different methods of measuring the interaction parameter , and showed that existing theories work well for weakly interacting electron systems, but there are serious discrepancies for more strongly interacting hole systems.

A second major research area is what happens to the electrical properties of electronic devices as they are made smaller. In particular, we are interested in devices where conduction is by holes rather than electrons, as holes have very different quantum properties than electrons. However, nanoscale hole systems are notoriously difficult to fabricate.

We have developed unique techniques for making hole devices, and in 2005 reported the first observation of the anomalous “0.7 structure” in one-dimensional hole quantum wires (shown above). This work opens up new opportunities for studying spin-orbit coupling (useful for spintronic applications) and interaction effects in nanoscale electronic systems.

Martin Aagesen. Warrick Clarke, Lap-Hang Ho, Oleh Klochan, Romain Danneau, Adam Micolich, Alex Hamilton and Michelle Simmons



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