Current research:
The forbidden metal-insulator transition in two dimensional systems


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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.

Electron-electron interactions and the forbidden metal-insulator transition in two dimensional systems

A schematic phase diagram of the low temperature electronic states of 2D systems.

Most transistors, such as the building block of modern computers, the MOSFET, use a thin (two-dimensional) sheet of highly mobile electrons to carry the electric current. These 2D systems are not only of immense technological significance, but have led to profound new fundamental phenomena, with discoveries such as the Quantum Hall Effect, the Fractional Quantum Hall Effect, and the High Electron Mobility Transistor (HEMT) have each leading to separate Nobel prizes for physics in recent years (1985, 1998, and 2000 respectively). However, despite being created over 40 years ago, there is still no universally accepted theoretical understanding of the fundamental electronic properties of high quality two-dimensional systems. Even the nature of the electronic state is not completely known over the complete range of carrier density and disorder (pictured left).

Data from Simmons et al., Phys. Rev. Lett., 84, 2489 (2000) showing a) metal-insulator transition at a critcal density of approximately 4.6*1010 electrons/cm2 and b) rescaling of the traces onto uninversal metallic and insulating curves

One of the goals of this project is to answer the deceptively simple question: "Can there be a real metallic state in these two-dimensional systems?" For the last 25 years this problem has taxed some of the best minds in condensed matter physics. In 1979 powerful theoretical arguments were put forward that there can be no true metal in two dimensions. Although we know that 2D transistors can switch between metallic "on" states and insulating "off" states at room temperature, it is actually only possible to tell the difference between a bad insulator and a poor metal at very low temperatures (strictly speaking one has to go to T=0 to develop a full quantum mechanical description of these two dimensional systems). However recent experiments (right) on high quality 2D transistors has suggested that strong electron-electron interactions may allow an unconventional metallic state to exist.

Therefore we are conducting detailed experiments at milliKelvin temperatures on custom built ultra-pure two-dimensional systems, investigating the nature of the transition between metallic and insulating behaviour at low temperatures in detail. We are also studying the relationship between the anomalous metallic state that occurs at zero magnetic field and the quantum Hall effect that occurs at large magnetic fields.

Relevant publications:

Interaction Correction to the Longitudinal Conductivity and Hall Resistivity in High Quality Two-Dimensional GaAs Electron and Hole Systems,
C. E. Yasin, T. L. Sobey, A. P. Micolich, A. R. Hamilton, M. Y. Simmons, L. N. Pfeiffer, K. W. West, E. H. Linfield, M. Pepper, D. A. Ritchie,
Physical Review B Rapid Communications 72, 241310 (2005).

"The fate of quantum Hall extended states as B -> 0 and the possibility of a 2D metal"
"C.E. Yasin, M.Y. Simmons, A.R. Hamilton, N.E. Lumpkin, R.G. Clark, L.N. Pfeiffer, and K.W. West",
Physica E
, 12 (1-4), 646, (2002)

"Localisation in strongly interacting 2D GaAs systems"
"M.Y. Simmons, A.R. Hamilton, C.E. Yasin, M. Pepper, E.H. Linfield, D.A. Ritchie, K.W. West, and L.N. Pfeiffer",
Physica Status Solidi B
, 230, 81, (2002)

"Metallic behaviour and localisation in 2D GaAs hole systems"
"M.Y. Simmons, A.R. Hamilton, M. Pepper, E.H. Linfield, P.D. Rose, and D.A. Ritchie",
Physica E
, 11, 161, (2001)

"Metallic behavior in dilute two-dimensional hole systems"
"A.R. Hamilton, M.Y. Simmons, M. Pepper, E.H. Linfield, and D.A. Ritchie",
Phys. Rev. Lett.
, 87 (12), 126802, (2001)

"Localisation and the metal-insulator transition in two dimensions"
"A.R. Hamilton, M.Y. Simmons, M. Pepper, E.H. Linfield and D.A. Ritchie",
Physica B
, 296, 21, (2001)

"Is there a metallic state in two dimensions?"
"M.Y. Simmons, A.R. Hamilton, M. Pepper, E.H. Linfield, P.D. Rose, and D.A. Ritchie",
Aust. J. Phys.
, 53, 513, (2000)

"Influence of inversion symmetry on the metallic behaviour in dilute two- dimensional hole systems"
"A.R. Hamilton, M.Y. Simmons, M. Pepper, and D.A. Ritchie",
Aust. J. Phys.
, 53, 523, (2000)

"Weak localization, hole-hole interactions and the “metal”-insulator transition in two dimensions"
"M.Y. Simmons, A.R. Hamilton, M. Pepper, E.H. Linfield, P.D. Rose and D.A. Ritchie",
Phys. Rev. Lett
, 84, 2489, (2000)

"Real metals - 2D or not 2D? "
M.Y. Simmons and A.R. Hamilton,
, 400, 715, (1999)


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Updated: 3-Oct-2002