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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
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
|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
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).
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)