PhD student Warrick Clarke in the SNF cleanroom fabricating samples for the closely spaced quantum devices project.	Honours student Tom Sobey performing low-temperature measurements of quantum contributions in high-mobility FET devices.

The field effect transistor plays an important role in modern electronic appliances such as computers and mobile phones. 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 not yet fully understood.

Our studies of electronic interactions 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, competition between intralayer and interlayer Coulomb interactions leads to new many-body quantum states similar to those in the fractional quantum Hall effect. In our samples we have the unique ability to tune both the relative strength of these interactions and the balance between the number of electrons in each of the two layers simply by adjusting gate voltages. Our focus in 2003 was the analysis of data obtained from an experiment where we mapped the stability of the bilayer coherent n = 1 quantum Hall state as electrons were gradually shifted from one layer into the other. PhD student Warrick Clarke spent early 2003 correlating our results with detailed theoretical calculations by Prof Charles Hanna from Boise State University in the US to reach a better understanding of the physics in this system. In late 2003 our focus in this project has shifted to investigating interactions between closely spaced 1D quantum wires, and we are currently fabricating devices for these studies.

The search for a metallic ground state in 2D systems naturally leads to the question of what the precise role of electron-electron interactions is in determining the conductivity. In particular, recent theoretical work suggests that the metallic behaviour observed in these systems may be entirely the result of these interactions, and that a signature of this is consistent behaviour in the quantum mechanical contributions to both the longitudinal and Hall conductivity of 2D systems. To test this hypothesis, Tom Sobey (Session II Honours student) and Carlin Yasin (PhD student) performed a careful experiment to compare these quantum contributions to the longitudinal and Hall conductivity of high-mobility FET devices. They found that these corrections are consistent, thereby confirming an important theory and taking us a step closer to understanding the origin of metallic behaviour in 2D systems.

Warrick Clarke, Tom Sobey, Carlin Yasin, Sean McPhail, Romain Danneau, Adam Micolich,
Alex Hamilton and Michelle Simmons