|Dr. Romain Danneau and Warrick Clarke performing
low temperature measurements of coupled quantum wires fabricated
||Phase breaking rate for 2D holes (time to
lose their coherent wave-like nature) as a function of temperature.
Solid and dashed lines are predictions of Fermi liquid theory;
open triangles are obtained by analysing data using simplest
model while solid symbols are obtained using more complex models
and show far better agreement with theoretical expectations.
The QED group studies the properties of advanced transistor devices,
at nanometer length scales where quantum effects become significant.
A fundamental question for high quality field-effect transistors
is whether, since the electrons are confined to a very thin (two-dimensional)
channel, quantum effects will always make them insulating –
even if they appear to be metallic at finite (room) temperature.
To tell the difference between insulating and metallic ground-states
it is necessary to cool the devices close to the absolute zero of
temperature, and then compare the data with theory to predict whether
the resistance will remain finite as T->0. In high quality devices
this process has been hampered by a large discrepancy between theory
and experiment. In 2004 we published the first comparison of five
different theories with experimental investigations of the quantum
interference correction, and showed that the discrepancy between
theory and experiment can be eliminated by using more sophisticated
models to analyse experimental data.
A second highlight this year was the development of a new type
of extremely low disorder field effect transistor, using custom
grown semiconductor wafers produced at the NTT Basic Research Laboratories
in Japan. Ph.D. student Warrick Clarke devised new processing techniques
to turn these wafers into functional transistors for studying quantum
corrections to classical conduction. Low temperature measurements
revealed extremely large hole mobilities (up to 600,000 cm2V-1s-1),
as well as unexpectedly strong metallic behaviour that appears to
be at odds with many current theories.
A second topic we are studying is how do interactions between very
closely spaced semiconductor devices affect their electronic properties?
This work is being performed in collaboration with the University
of Cambridge (UK) and Boise State University (USA). The devices
we use have two conducting layers separated by an insulating barrier
only 2.5nm thick. At low temperatures and high magnetic fields,
holes in the two layers bind together to form a new quantum phase
– an excitonic superfluid – similar to the atomic Bose-Einstein
Condensates that were the subject of the 2001 Nobel Prize in Physics.
We are now studying interaction effects in bilayer coupled 1D quantum
Martin Aagesen. Warrick Clarke, Alex Collins, Carlin Yasin, Oleh
Klochan Romain Danneau, Adam Micolich, Alex Hamilton and Michelle