Heat-mode excitation in a proximity superconductor

Mesoscopic superconductivity deals with various quasiparticle excitation modes, only one of them -- the charge-mode -- being directly accessible for conductance measurements due to the imbalance in populations of quasi-electron and quasi-hole excitation branches. Other modes carrying heat or even spin, valley etc. currents populate the branches equally and are charge-neutral that makes them much harder to control. This noticeable gap in the experimental studies of mesoscopic non-equilibrium superconductivity can be filled by going beyond the conventional DC transport measurements and exploiting spontaneous current fluctuations. Here, we perform the first experiment of this kind and investigate the transport of heat in an open hybrid device based on a superconductor proximitized InAs nanowire. Using shot noise measurements we observe a novel effect of sub-gap Andreev heat guiding along the superconducting interface and fully characterize it in terms of the thermal conductance on the order of $G_\mathrm{th}\sim e^2/h$, tunable by a back gate voltage. Understanding of the heat-mode also uncovers its implicit signatures in the non-local charge transport. Our experiments open a direct pathway to probe generic neutral excitations in superconducting hybrids.Comment: revised, 20 pages with supplementa

The visibility of IQHE at sharp edges: Experimental proposals based on interactions and edge electrostatics

The influence of the incompressible strips on the integer quantized Hall effect (IQHE) is investigated, considering a cleaved-edge overgrown (CEO) sample as an experimentally realizable sharp edge system. We propose a set of experiments to clarify the distinction between the large-sample limit when bulk disorder defines the IQHE plateau width and the small-sample limit smaller than the disorder correlation length, when self-consistent edge electrostatics define the IQHE plateau width. The large-sample or bulk QH regime is described by the usual localization picture, whereas the small-sample or edge regime is discussed within the compressible/incompressible strips picture, known as the screening theory of QH edges. Utilizing the unusually sharp edge profiles of the CEO samples, a Hall bar design is proposed to manipulate the edge potential profile from smooth to extremely sharp. By making use of a side-gate perpendicular to the two dimensional electron system, it is shown that the plateau widths can be changed or even eliminated altogether. Hence, the visibility of IQHE is strongly influenced when adjusting the edge potential profile and/or changing the dc current direction under high currents in the non-linear transport regime. As a second investigation, we consider two different types of ohmic contacts, namely highly transmitting (ideal) and highly reflecting (non-ideal) contacts. We show that if the injection contacts are non-ideal, however still ohmic, it is possible to measure directly the non-quantized transport taking place at the bulk of the CEO samples. The results of the experiments we propose will clarify the influence of the edge potential profile and the quality of the contacts, under quantized Hall conditions.Comment: Substantially revised version of manuscript arXiv:0906.3796v1, including new figures et

Reduction in Predator Defense in the Presence of Neighbors in a Colonial Fish

Predation pressure has long been considered a leading explanation of colonies, where close neighbors may reduce predation via dilution, alarming or group predator attacks. Attacking predators may be costly in terms of energy and survival, leading to the question of how neighbors contribute to predator deterrence in relationship to each other. Two hypotheses explaining the relative efforts made by neighbors are byproduct-mutualism, which occurs when breeders inadvertently attack predators by defending their nests, and reciprocity, which occurs when breeders deliberately exchange predator defense efforts with neighbors. Most studies investigating group nest defense have been performed with birds. However, colonial fish may constitute a more practical model system for an experimental approach because of the greater ability of researchers to manipulate their environment. We investigated in the colonial fish, Neolamprologus caudopunctatus, whether prospecting pairs preferred to breed near conspecifics or solitarily, and how breeders invested in anti-predator defense in relation to neighbors. In a simple choice test, prospecting pairs selected breeding sites close to neighbors versus a solitary site. Predators were then sequentially presented to the newly established test pairs, the previously established stimulus pairs or in between the two pairs. Test pairs attacked the predator eight times more frequently when they were presented on their non-neighbor side compared to between the two breeding sites, where stimulus pairs maintained high attack rates. Thus, by joining an established pair, test pairs were able to reduce their anti-predator efforts near neighbors, at no apparent cost to the stimulus pairs. These findings are unlikely to be explained by reciprocity or byproduct-mutualism. Our results instead suggest a commensal relationship in which new pairs exploit the high anti-predator efforts of established pairs, which invest similarly with or without neighbors. Further studies are needed to determine the scope of commensalism as an anti-predator strategy in colonial animals

Phenotypic divergence but not genetic distance predicts assortative mating among species of a cichlid fish radiation.

The hypothesis of ecological divergence giving rise to premating isolation in the face of gene flow is controversial. However, this may be an important mechanism to explain the rapid multiplication of species during adaptive radiation following the colonization of a new environment when geographical barriers to gene flow are largely absent but underutilized niche space is abundant. Using cichlid fish, we tested the prediction of ecological speciation that the strength of premating isolation among species is predicted by phenotypic rather than genetic distance. We conducted mate choice experiments between three closely related, sympatric species of a recent radiation in Lake Mweru (Zambia/DRC) that differ in habitat use and phenotype, and a distantly related population from Lake Bangweulu that resembles one of the species in Lake Mweru. We found significant assortative mating among all closely related, sympatric species that differed phenotypically, but none between the distantly related allopatric populations of more similar phenotype. Phenotypic distance between species was a good predictor of the strength of premating isolation, suggesting that assortative mating can evolve rapidly in association with ecological divergence during adaptive radiation. Our data also reveals that distantly related allopatric populations that have not diverged phenotypically, may hybridize when coming into secondary contact, e.g. upon river capture because of diversion of drainage systems

2.5 lambda microcavity InGaN light-emitting diodes fabricated by a selective dry-etch thinning process

The authors report on InGaN microcavity light-emitting diodes with an effective thickness of similar to 450 nm at the emission wavelength of similar to 415 nm. The starting material for the flip-chip laser lift-off device is a structure with an active region embedding six InGaN/GaN quantum wells, similar to 60-nm-thick AlGaN, and a GaN template grown on a c-plane sapphire substrate. High-precision control of the final microcavity thickness was facilitated by SF6-based selective inductively coupled plasma etching on the flipped material with an etch rate of >= 5:1 for GaN:AlxGa1-xN, where x >= 0.15. Pronounced microcavity effects were observed by angular measurements, in agreement with the theoretical cavity-mode dispersion characteristics. (c) 2007 American Institute of Physics

Surface, bulk, and interface electronic properties of nonpolar InN

The electronic properties of a-plane and m-plane InN have been investigated by x-ray photoemission spectroscopy, infrared reflectivity, and surface space-charge calculations. Electron accumulation has been observed at the surface of nonpolar InN and the surface Fermi level has been found to be lower than previously observed on InN samples. A high electron density in the InN close to the interface with GaN was found in each nonpolar InN sample. (c) 2010 American Institute of Physics. [doi:10.1063/1.3488821]</p

Influence of growth conditions and polarity on interface-related electron density in InN

Electron accumulation at the oxidized surface of In- and N-polarity indium nitride is shown to exhibit no dependence on the growth conditions (varied from In- to N-rich), revealing the surface Fermi level to be pinned 1.4 +/- 0.1 eV above the valence band maximum for all cases. This is in contrast to the interpretation of recent multiple-field Hall effect measurements, which suggested almost an order of magnitude increase in the sheet density of the accumulation layer upon moving from In-rich to N-rich growth conditions, and sample thickness dependent single-field Hall effect measurements which suggested different surface sheet densities for In- and N-polarity samples. However, an increase in the electron density approaching the InN/GaN (buffer layer) interface was not considered in the analysis of these Hall effect measurements, and this is invoked here to reconcile the constant surface Fermi level with the variations in "excess" sheet density observed in the previous Hall effect studies.</p

Temperature-dependence and microscopic origin of low frequency 1/f noise in GaN/AlGaN high electron mobility transistors

We have performed low frequency 1/f noise measurements from 85 K to 450 K to investigate the energy distribution of defects in GaN/AlGaN high electron mobility transistors fabricated in three different processes. The noise is well described by the model of P. Dutta and P. M. Horn, Rev. Mod. Phys. 53, 497 (1981). A peak in the defect energy distribution is observed at ∼0.2 eV for all device types investigated, which we attribute to the reconfiguration of an oxygen DX-like center in AlGaN. An additional peak at an energy 1 eV is observed for devices grown under nitrogen-rich conditions, which we attribute to the reconfiguration energy of negatively charged nitrogen antisites. © 2011 American Institute of Physics