Fig. 1: Optical and electron micrographs of high quality, high stability hole quantum wire fabricated in the Semiconductor Nanofabrication Facility at UNSW by Warrick Clarke.
Fig. 2: The 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.

The Quantum Electronics Devices group studies the properties of advanced transistor devices, at nanometer length scales where quantum effects become significant.

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. In 2006 we reported two new ways to fabricate extremely high quality hole nanostructures, which eliminated the instabilities that have plagued previous experiments for over 10 years.

Most significantly we discovered 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. In addition to the fundamental significance, potential applications may include all-electrical manipulation of spin, and new spintronic devices.

Members of the ‘hole’ team standing by the dilution refrigerator used to cool devices to 0.1 degrees above absolute zero.