Coherent control of injection currents in high-quality films of Bi2Se3

Films of the topological insulator Bi2Se3 are grown by molecular beam epitaxy with in-situ reflection high-energy electron diffraction. The films are shown to be high-quality by X-ray reflectivity and diffraction and atomic-force microscopy. Quantum interference control of photocurrents is observed by excitation with harmonically related pulses and detected by terahertz radiation. The injection current obeys the expected excitation irradiance dependence, showing linear dependence on the fundamental pulse irradiance and square-root irradiance dependence of the frequency-doubled optical pulses. The injection current also follows a sinusoidal relative-phase dependence between the two excitation pulses. These results confirm the third-order nonlinear optical origins of the coherently controlled injection current. Experiments are compared to a tight-binding band structure to illustrate the possible optical transitions that occur in creating the injection current.Comment: 11 pages, 3 figure, journal articl

Where are the Baryons?

New, high resolution, large-scale, cosmological hydrodynamic galaxy formation simulations of a standard cold dark matter model (with a cosmological constant) are utilized to predict the distribution of baryons at the present and at moderate redshift. It is found that the average temperature of baryons is an increasing function of time, with most of the baryons at the present time having a temperature in the range 10^{5-7} K. Thus, not only is the universe dominated by dark matter, but more than one half of the normal matter is yet to be detected. Detection of this warm/hot gas poses an observational challenge, requiring sensitive EUV and X-ray satellites. Signatures include a soft, cosmic X-ray background, apparent warm components in hot clusters due to both intrinsic warm intra-cluster gas and warm inter-cluster gas projected onto clusters along the line of sight, absorption lines in X-ray and UV quasar spectra [e.g., O VI (1032,1038)A lines, OVII 574 eV line], strong emission lines (e.g., O VIII 653 eV line) and low redshift, broad, low column density \lya absorption lines. We estimate that approximately 1/4 of the extragalactic soft X-ray background (SXRB) (at 0.7 keV) arises from the warm/hot gas, half of it coming from $z<0.65$ and three-quarters from $z<1.00$, so the source regions should be identifiable on deep optical images.Comment: ApJ in press, revised (fig 3 is in jpg). Whole paper including fig3.ps can be obtained at "http://astro.princeton.edu/~cen/PAPERS_TO_APPEAR/64

Hubble Space Telescope Counts of Elliptical Galaxies: Constraints on Cosmological Models ?

The interpretation of galaxy number counts in terms of cosmological models is fraught with difficulty due to uncertainties in the overall galaxy population (mix of morphological types, luminosity functions etc.) and in the observations (loss of low surface brightness images, image blending etc.). Many of these can be overcome if we use deep high resolution imaging of a single class of high surface brightness galaxies, whose evolution is thought to be fairly well understood. This is now possible by selecting elliptical and S0 galaxies using Hubble Space Telescope images from the Medium Deep Survey and other ultradeep WFPC2 images. In the present paper, we examine whether such data can be used to discriminate between open and closed universes, or between conventional cosmological models and those dominated by a cosmological constant. We find, based on the currently available data, that unless elliptical galaxies undergo very strong merging since $z \sim 1$ (and/or very large errors exist in the morphological classifications), then flat models dominated by a cosmological constant are ruled out. However, both an Einstein-de Sitter ($\Omega_{0}=1$) model with standard passive stellar evolution and an open ($\Omega_{0}=0.05$) model with no net evolution ({\it i.e.} cancelling stellar and dynamical evolution) predict virtually identical elliptical and S0 galaxy counts. Based on these findings and the recent reportings of $H_{o} \simeq 80$ km/s Mpc/s, we find that the maximum acceptable age of the universe is 13.3 Gyrs and a value of $\leq 9$ Gyrs favored. A flat---$\Lambda \neq 0$---universe is therefore {\it not} a viable solution to the $H_{o}$/globular cluster age problem.Comment: Accepted for publication in the Astrophysical Journal (April, 1996). 34 pages (including 4 figures) of gzip compressed and uuencoded PS. Also available at http://www.phys.unsw.edu.au/~spd/bib.htm

Clinical spirocercosis in a dog in the UK

A 2-year-old female neutered crossbreed dog was presented for evaluation of a 3-day history of haematemesis, melaena and hyporexia. The dog had been imported from a rescue centre in Hungary 4 months prior to presentation. Abdominal CT revealed the presence of a 3 cm×3 cm×4 cm diameter heterogenous intraluminal mass lesion in the gastric cardia, surrounding the ostium of the cardia and continuous with the distal oesophagus. The mass lesion was subsequently surgically resected. Histopathology of the gastric mass lesion was consistent with a Spirocerca lupi granuloma. The patient unfortunately developed a pyothorax and suffered cardiopulmonary arrest resulting in death 2 days postoperatively. To the authors’ knowledge, this is the first reported case of clinical spirocercosis reported in a dog in the UK

Surgical valvulotomy for tricuspid valve stenosis in a dog

A 2 year 4 month old female neutered Labrador retriever was presented for evaluation right sided congestive heart failure. Echocardiographic examination revealed tricuspid valve dysplasia with only two small orifices in the valve resulting in severe tricuspid stenosis. The dog underwent a right fifth lateral intercostal thoracotomy and surgical tricuspid valvulotomy, under cardiopulmonary bypass. The stenosis was relieved by dividing the valve leaflets between the two orifices with continuation to the commissures, creating a ‘bi-leaflet’ valve. The dog made a good recovery initially with echocardiography at 48 hours after surgery showing a reduction in tricuspid valve E and A wave velocities and pressure half time (from 230 ms to to 65 ms). She was discharged five days after surgery with spironolactone, benazepril, pimobendan and clopidogrel. The dog was re-presented two days later having collapsed, with pyrexia, facial swelling and pitting edema on the ventral neck and intermandibular region. Investigations did not reveal an underlying cause and the clinical signs resolved with supportive therapy. Two years after surgery the dog was free of clinical signs with normal exercise tolerance and only mild tricuspid regurgitation on echocardiography, with discontinuation of all medications

Quenched nematic criticality separating two superconducting domes in an iron-based superconductor under pressure

The nematic electronic state and its associated nematic critical fluctuations have emerged as potential candidates for superconducting pairing in various unconventional superconductors. However, in most materials their coexistence with other magnetically-ordered phases poses significant challenges in establishing their importance. Here, by combining chemical and hydrostatic physical pressure in FeSe$_{0.89}$S$_{0.11}$, we provide a unique access to a clean nematic quantum phase transition in the absence of a long-range magnetic order. We find that in the proximity of the nematic phase transition, there is an unusual non-Fermi liquid behavior in resistivity at high temperatures that evolves into a Fermi liquid behaviour at the lowest temperatures. From quantum oscillations in high magnetic fields, we trace the evolution of the Fermi surface and electronic correlations as a function of applied pressure. We detect experimentally a Lifshitz transition that separates two distinct superconducting regions: one emerging from the nematic electronic phase with a small Fermi surface and strong electronic correlations and the other one with a large Fermi surface and weak correlations that promotes nesting and stabilization of a magnetically-ordered phase at high pressures. The lack of mass divergence suggests that the nematic critical fluctuations are quenched by the strong coupling to the lattice. This establishes that superconductivity is not enhanced at the nematic quantum phase transition in the absence of magnetic order.Comment: 4 figures, 9 page

Ternary organic photodetectors based on pseudo–binaries nonfullerene–based acceptors

The addition of a third component to a donor:acceptor blend is a powerful tool to enhance the power conversion efficiency of organic solar cells. Featuring a similar operating mechanism, organic photodetectors are also expected to benefit from this approach. Here, we fabricated ternary organic photodetectors, based on a polymer donor and two nonfullerene acceptors, resulting in a low dark current of 0.42 nA cm−2 at −2 V and a broadband specific detectivity of 1012 Jones. We found that exciton recombination in the binary blend is reduced in ternary devices due to the formation of a pseudo-binary microstructure with mixed donor–acceptor phases. With this approach a wide range of intermediate open-circuit voltages is accessible, without sacrificing light-to-current conversion. This results in ternary organic photodetector (TOPD) with improved Responsivity values in the near-infrared. Moreover, morphology analyses reveal that TOPD devices showed improved microstructure ordering and consequentially higher charge carrier mobilities compared to the reference devices