School of Physics
Annual Report 2004...

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Quantum electronic devices

 
Dr. Romain Danneau and Warrick Clarke performing low temperature measurements of coupled quantum wires fabricated at UNSW. 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 wires.

Martin Aagesen. Warrick Clarke, Alex Collins, Carlin Yasin, Oleh Klochan Romain Danneau, Adam Micolich, Alex Hamilton and Michelle Simmons.

 

 

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