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Spin-orbit effects and many-body interactions in hole quantum wires


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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.

Hole quantum wires

Optical and electron micrographs of high quality, high stability hole quantum wire fabricated at UNSW.

Although there has been tremendous international interest in the properties of semiconductor nanostructures, almost all of this effort has concentrated on the properties of n-type devices, in which electrons carry the current. We are interested in devices where conduction is by holes rather than electrons, as holes have very different quantum properties. Electrons are spin-1/2 particles, whereas holes can be spin 3/2, and have a very strong coupling between spin and momentum. In principle this would make it possible to control the hole spin by altering its momentum, which can be done with applied electric fields. The ability to manipulate the hole spin could be useful for spintronic (spin-electronic) device applications. However nanoscale hole systems are notoriously difficult to fabricate.

We have developed new techniques for fabricating extremely high quality hole nanostructures, which eliminate the instabilities that have plagued previous experiments for over 10 years.

The hole quantum wire shows conductance quantization in units of 2e²/h at B=0. A magnetic field applied parallel to the wire lifts the spin degeneracy, causing additional plateaus. A perpendicular magnetic field does not.

We have found an extreme anisotropy in the response of these one-dimensional hole quantum wires to an external magnetic field. When the magnetic field is applied along the axis of the wire, it causes a Zeeman splitting of the spin states, but when it is applied perpendicular to the wire it has no effect! This result is completely different to electrons in quantum wires, which show no anisotropy, and is a direct result of the strong coupling between spin and momentum.

An additional feature can be seen below the first subband - the so-called "0.7 feature".

We have also performed the first studies of the anomalous "0.7 feature" that has been the subject of intense interest in one-dimensional systems. An anomalous platueau is seen at 0.7x2e^2/h, which cannot be explained in standard non-interacting theories. Explanations range from a spontaneous spin-splitting at B=0, through to Kondo effect, and formation of a one-dimensional Wigner crystal.

Our studies show that the 0.7 feature, and the related zero-bias anomaly, are both related to spin, and also have an anisotropic response to an applied magnetic field. The observation of the 0.7 feature in strongly interacting, spin 3/2 systems, opens up new potentials for research and places important constraints on theoretical models of this phenomena.

Future work is now focused on fully exploring the effects of strong spin-orbit coupling in hole nanostructures. In addition to their fundamental significance, potential applications may include all-electrical manipulation of spin, and new spintronic devices..

PLENTY OF NOTHING: a hole new quantum spin is a media release for the popular press about our work on hole quantum wires:"Electronic devices are always shrinking in size but it’s hard to imagine anything beating what researchers at the University of New South Wales have created (read more)". Photos are available of some of the group members and the devices, as well as a conceptual web animation of how the devices work.

Relevant publications:

Conductance quantization in induced one-dimensional hole systems,
O. Klochan, W. R. Clarke, R. Danneau, A. P. Micolich, L. H. Ho, A. R. Hamilton, K. Muraki and Y. Hirayama,
American Institute of Physics Conference Proceedings 893, 681 (2007).

Anisotropic Zeeman splitting in ballistic one-dimensional hole systems,
R. Danneau, O. Klochan, W. R. Clarke, L. H. Ho, A. P. Micolich, M. Y. Simmons, A. R. Hamilton, M. Pepper, D. A. Ritchie and U. Zülicke,
American Institute of Physics Conference Proceedings 893, 699 (2007).

Ballistic transport in induced one--dimensional hole systems,
O. Klochan, W.R. Clarke, R. Danneau, A.P. Micolich, L.H. Ho, A.R. Hamilton, K. Muraki and Y. Hirayama,
Applied Physics Letters 89, 092105 (2006).

Zeeman splitting in ballistic hole quantum wires
R. Danneau, O. Klochan, W. R. Clarke, L.H. Ho, A. P. Micolich,M. Y. Simmons, A. R. Hamilton, M. Pepper, D. A. Ritchie and U. Zuelicke,
Physical Review Letters 97, 026403 (2006).
Also appeared in Virtual Journal of Nanoscale Science & Technology 14(4), (2006)

Conductance quantization and the 0.7×2e2/h conductance anomaly in one-dimensional hole systems,
R. Danneau, W. R. Clarke, O. Klochan, A. P. Micolich, A. R. Hamilton, M. Y. Simmons, M. Pepper, and D. A. Ritchie,
Applied Physics Letters 88, 012107 (2006).
Also appeared in Virtual Journal of Nanoscale Science & Technology 13(2), (2006).

Fabrication of induced two-dimensional hole systems on (311)A GaAs,
W.R. Clarke, A.P. Micolich, A.R. Hamilton, M.Y. Simmons, K. Muraki and Y. Hirayama,
Journal of Applied Physics 99, 023707 (2006).

"Closely separated one-dimensional wires: Coupled ballistic conduction and compressibility measurements"
"I.M Castleton, A.G. Davies, A.R. Hamilton, J.E.F. Frost, M.Y. Simmons, D.A. Ritchie, M. Pepper",
Physica B
, 249-251, 157, (1998)

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Updated: 26 June 2007