Fabrication of Artificial Graphene in a GaAs Quantum Heterostructure

The unusual electronic properties of graphene, which are a direct consequence of its two-dimensional (2D) honeycomb lattice, have attracted a great deal of attention in recent years. Creation of artificial lattices that recreate graphene's honeycomb topology, known as artificial graphene, can facilitate the investigation of graphene-like phenomena, such as the existence of massless Dirac fermions, in a tunable system. In this work, we present the fabrication of artificial graphene in an ultra-high quality GaAs/AlGaAs quantum well, with lattice period as small as 50 nm, the smallest reported so far for this type of system. Electron-beam lithography is used to define an etch mask with honeycomb geometry on the surface of the sample, and different methodologies are compared and discussed. An optimized anisotropic reactive ion etching process is developed to transfer the pattern into the AlGaAs layer and create the artificial graphene. The achievement of such high-resolution artificial graphene should allow the observation for the first time of massless Dirac fermions in an engineered semiconductor.Comment: 13 pages text, 8 figures, plus reference

Surface morphology evolution of m-plane (1(1)over-bar00) GaN during molecular beam epitaxy growth: Impact of Ga/N ratio, miscut direction, and growth temperature

We present a systematic study of morphology evolution of [1 (1) over bar 00] m-plane GaN grown by plasma-assisted molecular beam epitaxy on free-standing m-plane substrates with small miscut angles towards the -c [000 (1) over bar] and +c [0001] directions under various gallium to nitrogen (Ga/N) ratios at substrate temperatures T = 720 degrees C and T = 740 degrees C. The miscut direction, Ga/N ratio, and growth temperature are all shown to have a dramatic impact on morphology. The observed dependence on miscut direction supports the notion of strong anisotropy in the gallium adatom diffusion barrier and growth kinetics. We demonstrate that precise control of Ga/N ratio and substrate temperature yields atomically smooth morphology on substrates oriented towards _c [0001] as well as the more commonly studied -c [000 (1) over bar] miscut substrates. (C) 2013 AIP Publishing LLC

Growth and electrical characterization of Al0.24Ga0.76As/AlxGa1-xAs/Al0.24Ga0.76As modulation-doped quantum wells with extremely low x

We report on the growth and electrical characterization of modulation-doped Al0.24Ga0.76As/AlxGa1-xAs/Al0.24Ga0.76As quantum wells with mole fractions as low as x = 0.00057. Such structures will permit detailed studies of the impact of alloy disorder in the fractional quantum Hall regime. At zero magnetic field, we extract an alloy scattering rate of 24 ns(-1) per% Al. Additionally, we find that for x as low as 0.00057 in the quantum well, alloy scattering becomes the dominant mobility-limiting scattering mechanism in ultra-high purity two-dimensional electron gases typically used to study the fragile nu = 5/2 and nu = 12/5 fractional quantum Hall states. (C) 2013 AIP Publishing LLC

Ultrastrong photon-photon coupling

Correlations between photons are a key feature of nonclassical states of light. Recent studies suggest that the ground state of a cavity quantum electrodynamics system can have light-matter correlations in the form of a squeezed vacuum state in thermal equilibrium when the matter ultrastrongly couples with cavity photons. This raises a question whether different photonic modes can also be correlated in the ground state via ultrastrong light-matter coupling. Here we demonstrate ultrastrong coupling between photonic modes of a multi-mode three-dimensional terahertz photonic-crystal cavity that is mediated by their simultaneous ultrastrong coupling with the cyclotron resonance of a two-dimensional electron gas in GaAs. Terahertz spectroscopy measurements of Landau polaritons showed excellent agreement with our calculations based on a microscopic quantum model. Despite the lack of nonlinearity in the matter system, the model shows significant correlations between the photonic modes in the ground state of the system, which can be controlled by changing the matter and photon frequencies and the spatial overlap of their mode profiles. We propose a detuning-independent figure of merit to quantify all possible couplings in multi-mode systems. These findings pave the way for creating multi-mode nonclassical states and exploring the many-body regime of quantum optics with vacuum fields.Comment: 32 pages, 12 figure

A systematic review of potential long-term effects of sport-related concussion

Systematic review of possible long-term effects of sports-related concussion in retired athletes

Sheep Updates 2005 - Part 7

This session covers eight papers from different authors: POSTERS 1. Sulpher in wool and its implications for fleece weight and sheep health, SM Liu, AC Schlink, JR Williams, CSIRO Livestock Industries Wembley WA, ME Dowling,JCGreef, Department of Agriculture Western Australia. 2. Stubbles for sheep: a reality check, Roy Butler, Keith Croker, Department of Agriculture Western Australia. 3. Genetic benchmarking using artificial insemination, LC Butler, JC Greeff, Department of Agriculture Western Australia. 4. The potential lambing performances of ewes in mixed age flocks, Kieth Croker, Department of Agriculture Western Australia, Rob Davidson, WAMMCO International, formally University of Western Australia, Ken Hart, Department of Agriculture Western Australia,Doug Harrington Cowcher Farms Narrogin, Mario D\u27Antuono, Department of Agriculture Western Australia. 5. National Livestock Identification System (Sheep) in Western Australia, Julian Gardner, Department of Agriculture Western Australia. DISPLAYS - TOOLS 6. To Feed or Not to Feed - I Only Hamlet had the Calculator!, Geoff Duddy, Livestock Officer(Sheep & Wool) Yanco. 7. WormBoss - a national Australian computer-based sheep worm control tool, RG Woodgate, Department of Agriculture Western Australia, A LeFeuvre, Queensland Department of Primary Industries and Fisheries, and Genie Pty Ltd, Warwick Qld, A Bailey, Department of Primary Industries, Water and Environment, Kings Meadow Tas, RB Besier, Department of Agriculture Western Australia, N. Campbell, Department of Primary Industries Victoria, Attwood Vic, I Carmichael, South Australian Research and Development Institute, Glenside SA, S. Love, NSW Department of Primary Industries, Armidale NSW. 8. \u27Eye in the sky\u27 takes guesswork out of farmers pasture decisions, Richard Stovold, Department of Land Informatio

A Connection between Colony Biomass and Death in Caribbean Reef-Building Corals

Increased sea-surface temperatures linked to warming climate threaten coral reef ecosystems globally. To better understand how corals and their endosymbiotic dinoflagellates (Symbiodinium spp.) respond to environmental change, tissue biomass and Symbiodinium density of seven coral species were measured on various reefs approximately every four months for up to thirteen years in the Upper Florida Keys, United States (1994–2007), eleven years in the Exuma Cays, Bahamas (1995–2006), and four years in Puerto Morelos, Mexico (2003–2007). For six out of seven coral species, tissue biomass correlated with Symbiodinium density. Within a particular coral species, tissue biomasses and Symbiodinium densities varied regionally according to the following trends: Mexico≥Florida Keys≥Bahamas. Average tissue biomasses and symbiont cell densities were generally higher in shallow habitats (1–4 m) compared to deeper-dwelling conspecifics (12–15 m). Most colonies that were sampled displayed seasonal fluctuations in biomass and endosymbiont density related to annual temperature variations. During the bleaching episodes of 1998 and 2005, five out of seven species that were exposed to unusually high temperatures exhibited significant decreases in symbiotic algae that, in certain cases, preceded further decreases in tissue biomass. Following bleaching, Montastraea spp. colonies with low relative biomass levels died, whereas colonies with higher biomass levels survived. Bleaching- or disease-associated mortality was also observed in Acropora cervicornis colonies; compared to A. palmata, all A. cervicornis colonies experienced low biomass values. Such patterns suggest that Montastraea spp. and possibly other coral species with relatively low biomass experience increased susceptibility to death following bleaching or other stressors than do conspecifics with higher tissue biomass levels