Serious discussions between Carlin Yasin (left), Alex Hamilton (right) and Michelle Simmons (taking photo) on 2D systems

Field effect transistors are the heart of modern electronic appliances. These transistors use a thin, almost two-dimensional sheet of highly mobile electrons or holes to carry electric current. Despite their technological importance many of the fundamental electronic properties of these 2D systems are still not well understood. Two separate projects have been conducted in the group to address the questions: (i) how do interactions between very closely spaced semiconductor devices affect their electronic properties? and (ii) is there a quantum phase transition to a true metallic state in 2D systems?

Our studies of electron interaction effects focus on high quality devices fabricated at UNSW featuring two 2D conducting channels separated by an insulating barrier only 2.5nm thick. In these bilayer devices two sets of Coulomb interactions exist — intralayer and interlayer. When these interactions become comparable, new many-body quantum states similar to those in the fractional quantum Hall effect are observed. These states bear a striking resemblance to macroscopic quantum coherent states in superconducting and magnetic systems, but in our samples we have a unique ability to tune the strength of the interactions, and produce quantum phase transitions, simply by adjusting a gate voltage.

In 2002, PhD student Warrick Clarke and post-doctoral fellow Adam Micolich spent two months making electrical measurements on a bilayer hole device to map the stability of the bilayer coherent n = 1 quantum Hall state as holes were gradually shifted from one layer into the other. Unexpectedly we found that this coherent state gets stronger with charge imbalance between the layers, but as these interactions are weakened the coherent state becomes less stable with charge imbalance. One of the major challenges in this work is making independent electrical contact to the two layers when they are so close together. Warrick Clarke and Wolfgang Guter, a German exchange student from Freiburg, have developed a processing strategy in the Semiconductor Nanofabrication Facility that overcomes this problem. This strategy will be further developed in 2003 to make new devices for measurement.

A bilayer 2D hole device, consiting of two quantum wells separated by an insulating layer with surface depletion gate allowing selective contact to the lower layer

A fundamental question in 2D transistors is whether the ground state is metallic or is always insulating. Recent experiments have shown evidence for metallic-like behaviour, but it is not known whether this persists to the absolute zero of temperature. In 2002, PhD student Carlin Yasin showed that as the magnetic field is turned off, the metallic-like conduction disappears. Carlin presented this work at the prestigious 26th International Conference on the Physics of Semiconductors in Edinburgh, Scotland, and also won the student poster prize at the AIP Congress in Sydney.

In parallel with Carlin’s work, Sean McPhail, a postdoctoral fellow in the group, undertook complementary studies of high quality 2D systems at milliKelvin temperatures to investigate the effects of quantum interference effects on the anomalous 2D metal. For the first time a comparison was made between all the different theoretical ways in which this data can be analysed, revealing that there is no suppression of the quantum interference effects by the strong interactions, in agreement with Carlin’s measurements. New optical measurements planned for 2003 should shed more light on the anomalous properties of these 2D systems.

Alex Hamilton, Michelle Simmons, AdamMicolich,
Sean McPhail, Warrick Clarke and Carlin Yasin