School of Physics - Research Publications
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ItemNonlinear hole transport through a submicron-size channel(AMER INST PHYSICS, 2003-02-10)We investigate hole transport through a submicron-size channel fabricated from a modulation-doped p-type GaAs/(AlGa)As single-quantum-well heterostructure. The intense electric field in the channel accelerates the holes beyond the inflection point of the lowest energy subband dispersion curve. This leads to current saturation and negative differential conduction effects in the current–voltage characteristics.
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ItemModel for the voltage steps in the breakdown of the integer quantum Hall effect(AMERICAN PHYSICAL SOC, 2003-09-19)In samples used to maintain the U.S. resistance standard the breakdown of the dissipationless integer quantum Hall effect occurs as a series of dissipative voltage steps. A mechanism for this type of breakdown is proposed, based on the generation of magnetoexcitons when the quantum Hall fluid flows past an ionized impurity above a critical velocity. The calculated generation rate gives a voltage step height in good agreement with measurements on both electron and hole gases. We also compare this model to a hydrodynamic description of breakdown.
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ItemCreation of solitons and vortices by Bragg reflection of Bose-Einstein condensates in an optical lattice(AMERICAN PHYSICAL SOC, 2003-03-21)We study the dynamics of Bose-Einstein condensates in an optical lattice and harmonic trap. The condensates are set in motion by displacing the trap and initially follow simple semiclassical paths, shaped by the lowest energy band. Above a critical displacement, the condensate undergoes Bragg reflection. For high atom densities, the first Bragg reflection generates a train of solitons and vortices, which destabilize the condensate and trigger explosive expansion. At lower densities, soliton and vortex formation requires multiple Bragg reflections, and damps the center-of-mass motion.
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ItemAn algebraic solution of the multichannel problem applied to low energy nucleon-nucleus scattering( 2003)Compound resonances in nucleon-nucleus scattering are related to the discrete spectrum of the target. Such resonances can be studied in a unified and general framework by a scattering model that uses sturmian expansions of postulated multichannel interactions between the colliding nuclei. Associated with such expanded multichannel interactions are algebraic multichannel scattering matrices. The matrix structure of the inherent Green functions not only facilitates extraction of the sub-threshold (compound nucleus) bound state spin-parity values and energies but also readily gives the energies and widths of resonances in the scattering regime. We exploited also the ability of the sturmian-expansion method to deal with non-local interactions to take into account the strong non-local effects introduced by the Pauli principle. As an example, we have used the collective model (to second order) to define a multichannel potential matrix for low energy neutron-C12 scattering allowing coupling between the 0+ (ground), 2+ (4.4389 MeV), and 0+ (7.64 MeV) states. The algebraic S matrix for this system has been evaluated and the sub-threshold bound states as well as cross sections and polarizations as functions of energy are predicted. The results are reflected in the actual measured data, and are shown to be consistent with expectations as may be based upon a shell model description of the target and of the compound nucleus.
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ItemThree-dimensional imaging of microstructure in Au nanocrystals.(American Physical Society (APS), 2003-05-02)X-ray diffraction using a coherent beam involves the mutual interference among all the extremities of small crystals. The continuous diffraction pattern so produced can be phased because it can be oversampled. We have thus obtained three-dimensional images of the interiors of Au nanocrystals that show 50 nm wide bands of contrast with [111] orientation that probably arise from internal twinning by dynamic recrystallization during their formation at high temperature.
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ItemThe effect of ion implantation on thermally stimulated currents in polycrystalline CVD diamond(ELSEVIER SCIENCE SA, 2003-01-01)
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ItemUnique phase recovery for nonperiodic objects(AMERICAN PHYSICAL SOC, 2003-11-14)It is well known that the loss of phase information at detection means that a diffraction pattern may be consistent with a multitude of physically different structures. This Letter shows that it is possible to perform unique structural determination in the absence of a priori information using x-ray fields with phase curvature. We argue that significant phase curvature is already available using modern x-ray optics and we demonstrate an algorithm that allows the phase to be recovered uniquely and reliably.
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ItemProgress in silicon-based quantum computing(ROYAL SOC, 2003-07-15)We review progress at the Australian Centre for Quantum Computer Technology towards the fabrication and demonstration of spin qubits and charge qubits based on phosphorus donor atoms embedded in intrinsic silicon. Fabrication is being pursued via two complementary pathways: a 'top-down' approach for near-term production of few-qubit demonstration devices and a 'bottom-up' approach for large-scale qubit arrays with sub-nanometre precision. The 'top-down' approach employs a low-energy (keV) ion beam to implant the phosphorus atoms. Single-atom control during implantation is achieved by monitoring on-chip detector electrodes, integrated within the device structure. In contrast, the 'bottom-up' approach uses scanning tunnelling microscope lithography and epitaxial silicon overgrowth to construct devices at an atomic scale. In both cases, surface electrodes control the qubit using voltage pulses, and dual single-electron transistors operating near the quantum limit provide fast read-out with spurious-signal rejection.