|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