Proximity Effect, Andreev Reflections, and Charge Transport in Mesoscopic Superconducting-Semiconducting Heterostructures

In the quasi-twodimensional (Q2D) electron gas of an InAs channel between an AlSb substrate and superconducting Niobium layers the proximity effect induces a pair potential so that a Q2D mesoscopic superconducting-normal-superconducting (SNS) junction forms in the channel. The pair potential is calculated with quasiclassical Green's functions in the clean limit. For such a junction alternating Josephson currents and current-voltage characteristics (CVCs) are computed, using the non-equilibrium quasiparticle wavefunctions which solve the time-dependent Bogoliubov-de Gennes Equations. The CVCs exhibit features found experimentally by the Kroemer group: A steep rise of the current at small voltages ("foot") changes at a "corner current" to a much slower increase of current with higher voltages, and the zero-bias differential resistance increases with temperature. Phase-coherent multiple Andreev reflections and the associated Cooper pair transfers are the physical mechanisms responsible for the oscillating Josephson currents and the CVCs. Additional experimental findings not reproduced by the theory require model improvements, especially a consideration of the external current leads which should give rise to hybrid quasiparticle/collective mode excitations.Comment: 8 pages, 4 figures (consisting of 5 .ps-files), added referenc

The Hubble Legacy Archive ACS Grism Data

A public release of slitless spectra, obtained with ACS/WFC and the G800L grism, is presented. Spectra were automatically extracted in a uniform way from 153 archival fields (or "associations") distributed across the two Galactic caps, covering all observations to 2008. The ACS G800L grism provides a wavelength range of 0.55-1.00 \mu$m, with a dispersion of$40 \ \AA / pixel$and a resolution of$\sim 80\ \AA$for point-like sources. The ACS G800L images and matched direct images were reduced with an automatic pipeline that handles all steps from archive retrieval, alignment and astrometric calibration, direct image combination, catalogue generation, spectral extraction and collection of metadata. The large number of extracted spectra (73,581) demanded automatic methods for quality control and an automated classification algorithm was trained on the visual inspection of several thousand spectra. The final sample of quality controlled spectra includes 47,919 datasets (65of extracted spectra) for$32,149$unique objects, with a median$i_{\rm AB}$-band magnitude of 23.7, reaching 26.5 AB for the faintest objects. Each released dataset contains science-ready 1D and 2D spectra, as well as multi-band image cutouts of corresponding sources and a useful preview page summarising the direct and slitless data, astrometric and photometric parameters. In order to characterize the slitless spectra, emission-line flux and equivalent width sensitivity of the ACS data were compared with public ground-based spectra in the GOODS-South field. An example list of emission line galaxies with two or more identified lines is also included, covering the redshift range$0.2-4.6$.Comment: Accepted for publication in Astronomy and Astrophysics; 29 pages, 16 Figures, 4 Tables in text and 3Tables in Appendi

Simple iterative construction of the optimized effective potential for orbital functionals, including exact exchange

For exchange-correlation functionals that depend explicitly on the Kohn-Sham orbitals, the potential V_{\mathrm{xc}\sigma}(\re) must be obtained as the solution of the optimized effective potential (OEP) integral equation. This is very demanding and has limited the use of orbital functionals like exact exchange. We demonstrate that the OEP can be obtained iteratively by solving a system of partial differential equations instead of an integral equation. This amounts to calculating the orbital shifts that exactify the Krieger-Li-Iafrate (KLI) approximation. Unoccupied orbitals do not need to be calculated. Accuracy and efficiency of the method are shown for atoms and clusters using the exact exchange energy. Counter-intuitive asymptotic limits of the exact OEP, not accessible from previous constructions, are presented.Comment: Physical Review Letters, accepted for publication. 4 pages, 1 figur

The Hubble Legacy Archive NICMOS Grism Data

The Hubble Legacy Archive (HLA) aims to create calibrated science data from the Hubble Space Telescope archive and make them accessible via user-friendly and Virtual Observatory (VO) compatible interfaces. It is a collaboration between the Space Telescope Science Institute (STScI), the Canadian Astronomy Data Centre (CADC) and the Space Telescope - European Coordinating Facility (ST-ECF). Data produced by the Hubble Space Telescope (HST) instruments with slitless spectroscopy modes are among the most difficult to extract and exploit. As part of the HLA project, the ST-ECF aims to provide calibrated spectra for objects observed with these HST slitless modes. In this paper, we present the HLA NICMOS G141 grism spectra. We describe in detail the calibration, data reduction and spectrum extraction methods used to produce the extracted spectra. The quality of the extracted spectra and associated direct images is demonstrated through comparison with near-IR imaging catalogues and existing near-IR spectroscopy. The output data products and their associated metadata are publicly available through a web form at http://hla.stecf.org and via VO interfaces. In total, 2470 spectra of 1923 unique targets are included in the current release.Comment: 18 pages, 21 figures, accepted for publication in Astronomy & Astrophysic

Dynamics of conversion of supercurrents into normal currents, and vice versa

The generation and destruction of the supercurrent in a superconductor (S) between two resistive normal (N) current leads connected to a current source is computed from the source equation for the supercurrent density. This equation relates the gradient of the pair potential's phase to electron and hole wavepackets that create and destroy Cooper pairs in the N/S interfaces. Total Andreev reflection and supercurrent transmission of electrons and holes are coupled together by the phase rigidity of the non-bosonic Cooper-pair condensate. The calculations are illustrated by snapshots from a computer film.Comment: 8 pages, 1 figure, accepted by Phys. Rev.

Active Brownian Motion Tunable by Light

Active Brownian particles are capable of taking up energy from their environment and converting it into directed motion; examples range from chemotactic cells and bacteria to artificial micro-swimmers. We have recently demonstrated that Janus particles, i.e. gold-capped colloidal spheres, suspended in a critical binary liquid mixture perform active Brownian motion when illuminated by light. In this article, we investigate in some more details their swimming mechanism leading to active Brownian motion. We show that the illumination-borne heating induces a local asymmetric demixing of the binary mixture generating a spatial chemical concentration gradient, which is responsible for the particle's self-diffusiophoretic motion. We study this effect as a function of the functionalization of the gold cap, the particle size and the illumination intensity: the functionalization determines what component of the binary mixture is preferentially adsorbed at the cap and the swimming direction (towards or away from the cap); the particle size determines the rotational diffusion and, therefore, the random reorientation of the particle; and the intensity tunes the strength of the heating and, therefore, of the motion. Finally, we harness this dependence of the swimming strength on the illumination intensity to investigate the behaviour of a micro-swimmer in a spatial light gradient, where its swimming properties are space-dependent

Description of hysteretic current-voltage characteristics of SNS junctions

Simplified model for current-voltage characteristics of weak links is suggested. It is based on an approach considering the multiple Andreev reflection in metallic Josephson junction. The model allows to calculate current-voltage characteristics of the superconductor - normal metal - superconductor junctions with different thicknesses of normal layer at different temperatures. A hysteretic peculiarity of $V(I)$ dependence is described as result of the negative differential resistance. The current-voltage characteristic of high-$T_c$ composite YBCO +BaPbO${_3}$ were computed.Comment: 9 pages, 5 figures, submited to Supercond. Sci. Technol, replased Fig.5 for more correct comparison with experimen

Ground state correlations and mean-field in 16^{16}O

We use the coupled cluster expansion ($\exp(S)$ method) to generate the complete ground state correlations due to the NN interaction. Part of this procedure is the calculation of the two-body G matrix inside the nucleus in which it is being used. This formalism is being applied to $^{16}O$ in a configuration space of 50 $\hbar\omega$. The resulting ground state wave function is used to calculate the binding energy and one- and two-body densities for the ground state of $^{16}O$.Comment: 9 pages, 9 figures, LaTe

Dirac Hartree-Fock for Finite Nuclei Employing realistic Forces

We discuss two different approximation schemes for the self-consistent solution of the {\it relativistic} Brueckner-Hartree-Fock equation for finite nuclei. In the first scheme, the Dirac effects are deduced from corresponding nuclear matter calculations, whereas in the second approach the local-density approximation is used to account for the effects of correlations. The results obtained by the two methods are very similar. Employing a realistic one-boson-exchange potential (Bonn~A), the predictions for energies and radii of $^{16}$O and $^{40}$Ca come out in substantially better agreement with experiment as compared to non-relativistic approaches. As a by-product of our study, it turns out that the Fock exchange-terms, ignored in a previous investigation, are not negligible.Comment: