Possible evidence of a spontaneous spin-polarization in mesoscopic 2D electron systems

We have experimentally studied the non-equilibrium transport in low-density clean 2D electron systems at mesoscopic length scales. At zero magnetic field (B), a double-peak structure in the non-linear conductance was observed close to the Fermi energy in the localized regime. From the behavior of these peaks at non-zero B, we could associate them to the opposite spin states of the system, indicating a spontaneous spin polarization at B = 0. Detailed temperature and disorder dependence of the structure shows that such a splitting is a ground state property of the low-density 2D systems.Comment: 7 pages, 5 figure

Nuclear spin coherence in a quantum wire

We have observed millisecond-long coherent evolution of nuclear spins in a quantum wire at 1.2 K. Local, all-electrical manipulation of nuclear spins is achieved by dynamic nuclear polarization in the breakdown regime of the Integer Quantum Hall Effect combined with pulsed Nuclear Magnetic Resonance. The excitation thresholds for the breakdown are significantly smaller than what would be expected for our sample and the direction of the nuclear polarization can be controlled by the voltage bias. As a four-level spin system, the device is equivalent to two qubits.Comment: 5 pages, 5 figure

Variation in health and social equity in the spaces where we live: A review of previous literature from the GeoHealth Laboratory

The previous decade has given rise to the importance of Geographic Information Systems (GIS) in explaining inequalities in health outcomes between groups based on their spatial location and social background. The GeoHealth Laboratory, based at the University of Canterbury, is a joint venture with the Health and Disability Intelligence unit within the Ministry of Health (MoH). The aims of this relationship are to add analytical capacity to MoH data collections and increase academic outputs of geospatial health research in New Zealand. GeoHealth research has often been a joint venture between Laboratory staff and students as well as collaboration with local and international researchers. These partnerships along with widely varied research interests have resulted in a large contribution of spatial health research in the field of health geography. This article reports on research undertaken by the GeoHealth Laboratory that has focused on access to neighbourhood determinants of health. An overview of key neighbourhoods and health research areas are outlined within the over-arching themes of indices of access to neighbourhood factors, access to undesireable neighbourhood destinations, health promoting neighbourhood factors, access to and utilisation of health services, and complementary data collection and research groups within New Zealand

Possible effect of collective modes in zero magnetic field transport in an electron-hole bilayer

We report single layer resistivities of 2-dimensional electron and hole gases in an electron-hole bilayer with a 10nm barrier. In a regime where the interlayer interaction is stronger than the intralayer interaction, we find that an insulating state ($d\rho/dT < 0$) emerges at $T\sim1.5{\rm K}$ or lower, when both the layers are simultaneously present. This happens deep in the $"$metallic" regime, even in layers with $k_{F}l>500$, thus making conventional mechanisms of localisation due to disorder improbable. We suggest that this insulating state may be due to a charge density wave phase, as has been expected in electron-hole bilayers from the Singwi-Tosi-Land-Sj\"olander approximation based calculations of L. Liu {\it et al} [{\em Phys. Rev. B}, {\bf 53}, 7923 (1996)]. Our results are also in qualitative agreement with recent Path-Integral-Monte-Carlo simulations of a two component plasma in the low temperature regime [ P. Ludwig {\it et al}. {\em Contrib. Plasma Physics} {\bf 47}, No. 4-5, 335 (2007)]Comment: 5 pages + 3 EPS figures (replaced with published version

Suspended two-dimensional electron gases in In₀.₇₅Ga₀.₂₅As quantum wells

We demonstrate that In0.75Ga0.25As quantum wells can be freely suspended without losing electrical quality when the epitaxial strain-relieving buffer layer is removed. In applied magnetic fields, non-dissipative behavior is observed in the conductivity, and a current induced breakdown of the quantum Hall effect shows a lower critical current in the suspended layers due to efficient thermal isolation compared to the non-suspended-control device. Beyond the critical current, background impurity scattering in the suspended two-dimensional channel regions dominates with stochastic, resonant-like features in the conductivity. This device fabrication scheme offers the potential for thermally isolated devices containing suspension-asymmetry-induced, high spin–orbit coupling strengths with reduced electron–phonon interaction behavior but without introducing high levels of disorder in the processing. This work was funded by EPSRC Grant Nos. EP/K004077/1 and EP/R029075/1, UK. We thank Professor Chris Ford for useful discussions

Surface acoustic wave-induced electroluminescence intensity oscillation in planar light-emitting devices

Electroluminescence emission from surface acoustic wave-driven light-emitting diodes (SAWLEDs) is studied by means of time-resolved techniques. We show that the intensity of the SAW-induced electroluminescence is modulated at the SAW frequency (~1 GHz), demonstrating electron injection into the p-type region synchronous with the SAW wavefronts.Comment: 4 pages, 3 figure

Finite size effects in surface emitting Terahertz quantum cascade lasers

We analyze surface-emitting distributed feedback resonators for Terahertz quantum cascade lasers fabricated from double-metal waveguides. We explain the influence on resonances and surface-emission properties of the finite length and width of the gratings in connection with absorbing boundary conditions, and show that, contrary to the infinite case, the modes on either side of the photonic band-gap have finite surface losses. The lateral design of the resonator is shown to be important to avoid transverse modes of higher order and anti-guiding effects. Experimental findings are indeed in excellent agreement with the simulations. Both modeling and fabrication can easily be applied to arbitrary gratings, of which we discuss here a first interesting example. (c) 2009 Optical Society of Americ

Photovoltage detection of spin excitation of a ferromagnetic stripe and disk at low temperature

Photovoltage spectroscopy is a beneficial technique to investigate the dynamic properties of the spin excitations of ferromagnetic elements fabricated at the surface of a GaAs/Al 0.33Ga 0.67As heterojunction. This method is of particular interest for probing localized spin wave modes. The high sensitivity of the photovoltage technique arises from the high electron mobility of 2D electrons μ = 1.5 × 10 6 cm 2 • V -1 • S -1 which enables efficient rectification of magnetic moment oscillations through the Hall effect. We report on the discrete structure of spin wave eigenmodes as a function of magnetic field orientation, the shape of Co ferromagnets, and the geometry of nanomagnets. We indicated bonding-Antibonding spin waves when the static magnetic field, is parallel to the short side of the stripe at different microwave frequencies at 4 K. We also observed Damon-Eshbach modes when is parallel to the stripe. Micromagnetic simulations confirm the experimental results. We observe the discrete structure of the photovoltage for individual dots. We also investigate the effect of the magnetocrystalline anisotropy field of Co on ferromagnetic resonance. Our results demonstrate that photovoltage measurements in hybrid semiconductor-ferromagnetic structures provide a sensitive and extended tool for probing the spin waves of small magnets with a size of 80 nm. </p

Photovoltage detection of spin excitation of ferromagnetic stripe and disk at low temperature

Photovoltage spectroscopy is a beneficial technique to investigate the dynamic properties of the spin excitations of ferromagnetic elements fabricated at the surface of a GaAs/Al 0.33Ga 0.67As heterojunction. This method is of particular interest for probing localized spin wave modes. The high sensitivity of the photovoltage technique arises from the high electron mobility of 2D electrons μ = 1.5 × 10 6 cm 2 • V -1 • S -1 which enables efficient rectification of magnetic moment oscillations through the Hall effect. We report on the discrete structure of spin wave eigenmodes as a function of magnetic field orientation, the shape of Co ferromagnets, and the geometry of nanomagnets. We indicated bonding-Antibonding spin waves when the static magnetic field, is parallel to the short side of the stripe at different microwave frequencies at 4 K. We also observed Damon-Eshbach modes when is parallel to the stripe. Micromagnetic simulations confirm the experimental results. We observe the discrete structure of the photovoltage for individual dots. We also investigate the effect of the magnetocrystalline anisotropy field of Co on ferromagnetic resonance. Our results demonstrate that photovoltage measurements in hybrid semiconductor-ferromagnetic structures provide a sensitive and extended tool for probing the spin waves of small magnets with a size of 80 nm. </p