Orientation of hole quantum Hall nematic phases in an out-of-plane electric field

We present observations of an anisotropic resistance state at Landau level filling factor ν=5/2 in a two-dimensional hole system (2DHS), which occurs for certain values of hole density $p$ and average out-of-plane electric field $E_⊥$. The 2DHS is induced by electric field effect in an undoped GaAs/AlGaAs quantum well, where front and back gates allow independent tuning of $p$ and $E_⊥$, and hence the symmetry of the confining potential. For $p$ ≈ 2 × 10$^{11}$ cm$^{-2}$ and $E_⊥$ ≈ -2 × 10$^5$ V/m, the magnetoresistance along greatly exceeds that along , suggesting the formation of a quantum Hall nematic or "stripe" phase. Reversing the sign of $E_⊥$ rotates the stripes by 90$^{\circ}$. We suggest this behavior may arise from the mixing of the hole Landau levels and a combination of the Rashba and Dresselhaus spin-orbit coupling effects.The Herchel Smith Fund at the University of Cambridge, UK. Trinity College Cambridge, UK Toshiba Research Europe Limite

Experimental progress towards probing the ground state of an electron-hole bilayer by low-temperature transport

Recently, it has been possible to design independently contacted electron-hole bilayers (EHBLs) with carrierdensities  cm2 in each layer and a separation of 10–20 nm in a GaAs/AlGaAs system. In these EHBLs,the interlayer interaction can be stronger than the intralayer interactions. Theoretical works have indicated thepossibility of a very rich phase diagram in EHBLs consisting of excitonic superfluid phases, charge densitywaves, and Wigner crystals. Experiments have revealed that the Coulomb drag on the hole layer shows strongnonmonotonic deviations from a behaviour expected for Fermi-liquids at low temperatures. Simultaneously,an unexpected insulating behaviour in the single-layer resistances (at a highly “metallic” regime with) also appears in both layers despite electron mobilities of above and hole mobilitiesover . Experimental data also indicates that the point of equal densities () is not special.Peer Reviewe

Comparing [C II], H I, and CO dynamics of nearby galaxies

The HI and CO components of the interstellar medium (ISM) are usually used to derive the dynamical mass M-dyn of nearby galaxies. Both components become too faint to be used as a tracer in observations of high-redshift galaxies. In those cases, the 158 mu m line of atomic carbon ([CII]) may be the only way to derive M-dyn. As the distribution and kinematics of the ISM tracer affects the determination of M-dyn, it is important to quantify the relative distributions of HI, CO, and [CII]. HI and CO are well-characterized observationally, however, for [CII] only very few measurements exist. Here we compare observations of CO, HI, and [CII] emission of a sample of nearby galaxies, drawn from the HERACLES, THINGS, and KINGFISH surveys. We find that within R-25, the average [CII] exponential radial profile is slightly shallower than that of the CO, but much steeper than the HI distribution. This is also reflected in the integrated spectrum ("global profile"), where the [CII] spectrum looks more like that of the CO than that of the HI. For one galaxy, a spectrally resolved comparison of integrated spectra was possible; other comparisons were limited by the intrinsic line-widths of the galaxies and the coarse velocity resolution of the [CII] data. Using high-spectral-resolution SOFIA [CII] data of a number of star forming regions in two nearby galaxies, we find that their [CII] linewidths agree better with those of the CO than the HI. As the radial extent of a given ISM tracer is a key input in deriving M-dyn from spatially unresolved data, we conclude that the relevant length-scale to use in determining M-dyn based on [CII] data, is that of the well-characterized CO distribution. This length scale is similar to that of the optical disk

A complete laboratory for transport studies of electron-hole interactions in GaAs/AlGaAs systems

We present GaAs/AlGaAs double quantum well devices that can operate as both electron-hole (e-h) and hole-hole (h-h) bilayers, with separating barriers as narrow as 5 nm or 7.5 nm. With such narrow barriers, in the h-h configuration we observe signs of magnetic-field-induced exciton condensation in the quantum Hall bilayer regime. In the same devices we can study the zero-magnetic-field e-h and h-h bilayer states using Coulomb drag. Very strong e-h Coulomb drag resistivity (up to 10% of the single layer resistivity) is observed at liquid helium temperatures, but no definite signs of exciton condensation are seen in this case. Self-consistent calculations of the electron and hole wavefunctions show this might be because the average interlayer separation is larger in the e-h case than the h-h case.This work was funded by EPSRC EP/H017720/1 and EP/J003417/1 and European Union Grant INDEX 289968. A.F.C. acknowledges funding from Trinity College, Cambridge, and D.T. from St. Catherine's College, Cambridge. I.F. acknowledges funding from Toshiba Research Europe Limited

COMPARING [C ∥], H i, and CO DYNAMICS of NEARBY GALAXIES

The HI and CO components of the interstellar medium (ISM) are usually used to derive the dynamical mass M_dyn of nearby galaxies. Both components become too faint to be used as a tracer in observations of high-redshift galaxies. In those cases, the 158 $\mu$m line of atomic carbon [CII] may be the only way to derive M_dyn. As the distribution and kinematics of the ISM tracer affects the determination of M_dyn, it is important to quantify the relative distributions of HI, CO and [CII]. HI and CO are well-characterised observationally, however, for [CII] only very few measurements exist. Here we compare observations of CO, HI, and [CII] emission of a sample of nearby galaxies, drawn from the HERACLES, THINGS and KINGFISH surveys. We find that within R_25, the average [CII] exponential radial profile is slightly shallower than that of the CO, but much steeper than the HI distribution. This is also reflected in the integrated spectrum ("global profile"), where the [CII] spectrum looks more like that of the CO than that of the HI. For one galaxy, a spectrally resolved comparison of integrated spectra was possible; other comparisons were limited by the intrinsic line-widths of the galaxies and the coarse velocity resolution of the [CII] data. Using high-spectral-resolution SOFIA [CII] data of a number of star forming regions in two nearby galaxies, we find that their [CII] linewidths agree better with those of the CO than the HI. As the radial extent of a given ISM tracer is a key input in deriving M_dyn from spatially unresolved data, we conclude that the relevant length-scale to use in determining M_dyn based on [CII] data, is that of the well-characterised CO distribution. This length scale is similar to that of the optical disk.This is the accepted manuscript. It is currently embargoed pending publication

Resolving the far-IR line deficit : photoelectric heating and far-IR line cooling in NGC 1097 and NGC 4559

The physical state of interstellar gas and dust is dependent on the processes which heat and cool this medium. To probe heating and cooling of the interstellar medium over a large range of infrared surface brightness, on sub-kiloparsec scales, we employ line maps of [C II] 158 mu m, [O I] 63 mu m, and [N II] 122 mu m in NGC 1097 and NGC 4559, obtained with the Photodetector Array Camera & Spectrometer on board Herschel. We matched new observations to existing Spitzer Infrared Spectrograph data that trace the total emission of polycyclic aromatic hydrocarbons (PAHs). We confirm at small scales in these galaxies that the canonical measure of photoelectric heating efficiency, ([C II] + [O I])/TIR, decreases as the far-infrared (far-IR) color, nu f(nu)(70 mu m) nu f(nu)(100 mu m), increases. In contrast, the ratio of far-IR cooling to total PAH emission, ([C II] + [O I])/PAH, is a near constant similar to 6% over a wide range of far-IR color, 0.5 , derived from models of the IR spectral energy distribution. Emission from regions that exhibit a line deficit is characterized by an intense radiation field, indicating that small grains are susceptible to ionization effects. We note that there is a shift in the 7.7/11.3 mu m PAH ratio in regions that exhibit a deficit in ([C II] + [O I])/PAH, suggesting that small grains are ionized in these environments

Effects of Hydrographic Variability on the Spatial, Seasonal and Diel Diving Patterns of Southern Elephant Seals in the Eastern Weddell Sea

Weddell Sea hydrography and circulation is driven by influx of Circumpolar Deep Water (CDW) from the Antarctic Circumpolar Current (ACC) at its eastern margin. Entrainment and upwelling of this high-nutrient, oxygen-depleted water mass within the Weddell Gyre also supports the mesopelagic ecosystem within the gyre and the rich benthic community along the Antarctic shelf. We used Conductivity-Temperature-Depth Satellite Relay Data Loggers (CTD-SRDLs) to examine the importance of hydrographic variability, ice cover and season on the movements and diving behavior of southern elephant seals in the eastern Weddell Sea region during their overwinter feeding trips from Bouvetøya. We developed a model describing diving depth as a function of local time of day to account for diel variation in diving behavior. Seals feeding in pelagic ice-free waters during the summer months displayed clear diel variation, with daytime dives reaching 500-1500 m and night-time targeting of the subsurface temperature and salinity maxima characteristic of CDW around 150–300 meters. This pattern was especially clear in the Weddell Cold and Warm Regimes within the gyre, occurred in the ACC, but was absent at the Dronning Maud Land shelf region where seals fed benthically. Diel variation was almost absent in pelagic feeding areas covered by winter sea ice, where seals targeted deep layers around 500–700 meters. Thus, elephant seals appear to switch between feeding strategies when moving between oceanic regimes or in response to seasonal environmental conditions. While they are on the shelf, they exploit the locally-rich benthic ecosystem, while diel patterns in pelagic waters in summer are probably a response to strong vertical migration patterns within the copepod-based pelagic food web. Behavioral flexibility that permits such switching between different feeding strategies may have important consequences regarding the potential for southern elephant seals to adapt to variability or systematic changes in their environment resulting from climate change

Biology and ecology of the world’s largest invertebrate, the colossal squid (Mesonychoteuthis hamiltoni): a short review

The colossal squid Mesonychoteuthis hamiltoni (Robson 1925) is the largest (heaviest) living invertebrate and although it is preyed upon by many top predators, its basic biology and ecology remain one of the ocean’s great mysteries. The present study aims to review the current biological knowledge on this squid. It is considered to be endemic in the Southern Ocean (SO) with a circumpolar distribution spreading from the Antarctic continent up to the Sub-Antarctic Front. Small juveniles (<40 mm mantle length) are mainly found from the surface to 500 m, and the late juvenile stages are assumed to undergo ontogenetic descent to depths reaching 2000 m. Thus, this giant spends most of its life in the meso- and bathypelagic realms, where it can reach a total length of 6 m. The maximum weight recorded so far was 495 kg. M. hamiltoni is presently reported from the diets of 17 different predator species, comprising penguins and other seabirds, fishes and marine mammals, and may feed on various prey types, including myctophids, Patagonian toothfish, sleeper sharks and other squid. Stable isotopic analysis places the colossal squid as one of the top predators in the SO. It is assumed that this squid is not capable of high-speed predator–prey interactions, but it is rather an ambush predator. Its eyes, the largest on the planet, seem to have evolved to detect very large predators (e.g., sperm whales) rather than to detect prey at long distances. The study of this unique invertebrate giant constitutes a valuable source of insight into the biophysical principles behind body-size evolution

Demonstration and characterization of an ambipolar high mobility transistor in an undoped GaAs/AlGaAs quantum well

We report an ambipolar device fabricated in undoped GaAs/AlGaAs quantum wells (widths 10 and 25 nm) with front and backgates that allow almost two orders of magnitude in density to be accessed in the same device (7 x 10(9) cm(-2) to 5 x 10(11) cm(-2)). By changing the well width, the relative electron and hole mobilities can be tuned, approaching similar velocities. We describe an approach to fully characterize the quantum well, including the impurity backgrounds and both the upper and lower interfaces, making use of the ability to control the carrier density and the position of the wavefunction independently over a wide range. (C) 2013 American Institute of Physics. [http://dx.doi.org/10.1063/1.4793658