Quantum Electronic Devices

Current research:
Inter-device interactions
in strongly coupled quantum devices

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Researchers within the QED group are investigating the electrical and optical properties of nanometer scale semiconductor devices. At these small length scales the device properties are no longer governed by semi-classical physics, but are instead determined by quantum mechanical effects. The group makes its own quantum semiconductor devices here at UNSW, and uses a variety of electronic and optical probes, at milliKelvin temperatures and in strong magnetic fields, to further the understanding of quantum electronics.

Inter-device interactions in strongly coupled quantum devices

As the dimensions of individual components in a "chip" shrink, and we pack these components ever closer together, it will no longer be possible to ignore interactions between devices. So as well as understanding how electron-electron interactions within a device affect its electrical properties, it is also important to understand the interactions between devices in order to design the next generation of nano-electronic devices.

To probe inter-device interactions our group designs, fabricates, and measures strongly coupled quantum devices in some of Australia's most advanced laboratories. One approach that we are developing is to fabricate bilayer two-dimensional systems, in which two separate 2D transistors are brought to within 2nm of each other. The strong interactions between electrons in one 2D layer and electrons in the other 2D layer gives rise to entirely new quantum behaviour that does not occur when only one layer is present.

Colourmap of the evolution of the sample resistance (white: low resistance due to quantum Shubikov de Haas oscillations) as a function of total carrier density (gate voltage) and magnetic field at T=30mK. For large gate voltages only a single layer is the occupied, with a transition to bilayer occupation at Vg=0.3V. The arrow marks the point at which the carrier density in the two layers is equal and correlated bilayer quantum Hall states form.

The correlated bilayer state forms when the carrier densities in the two layers are equal and the separation between particles in different layers, d, is smaller than the separation between particles within the same layer, a.

One example is a novel quantum state that is formed when the interactions between layers are strong enough that the particles in the two layers act as a single system, sharing a common quantum mechanical wavefunction (much like Cooper pairs in a superconductor). In this bilayer correlated state (see figure on left) the particles in each layer coalesce into a form of Bose-Einstein condensate, despite the fact that the charge carriers remain in separate layers. Like other B-E condensates, a whole range of new transport properties may be possible such as superfluidity and superconductivity. We are interested in accessing and controlling these new transport properties, but more importantly, we are interested in understanding how and why these phenomena exist in the bilayer state at all.

In particular our research group is one of only a handful of groups worldwide that is able to fabricate and study coupled 2D hole systems, in which these correlation effects are particularly strong. Working with our Japanese and UK collaborators the QED Group is continually developing and improving novel processing techniques for fabricating these devices, allowing new experiments with high quality hole devices to examine these strongly correlated systems.

Relevant publications

Evolution of the bilayer n = 1 quantum Hall state under charge imbalance
W. R. Clarke, A. P. Micolich, A. R. Hamilton, M.Y. Simmons, M. Pepper and D.A. Ritchie,
Physical Review B Rapid Communications 71, 081304 (2005).

"Exchange-driven bilayer-to-monolayer charge transfer in an asymmetric double-quantum-well"
"A.R. Hamilton, M.Y. Simmons, C.B. Hanna, J.C. Diaz-Velez, M. Pepper, and D.A. Ritchie",
Physica E, 12 (1-4), 304, (2002)

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Updated: 3-Oct-2002