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Kinematic analysis and membership status of TWA22ABR  [PDF]
R. Teixeira,C. Ducourant,G. Chauvin,A. Krone-Martins,M. Bonnefoy,I. Song
Physics , 2009, DOI: 10.1051/0004-6361/200912173
Abstract: TWA22 was initially regarded as a member of the TW Hydrae association (TWA). In addition to being one of the youngest (~8Myr) and nearest (~20pc) stars to Earth, TWA22 has proven to be very interesting after being resolved as a tight, very low-mass binary. This binary can serve as a very useful dynamical calibrator for pre-main sequence evolutionary models. However, its membership in the TWA has been recently questioned despite due to the lack of accurate kinematic measurements. Based on proper motion, radial velocity, and trigonometric parallax measurements, we aim here to re-analyze the membership of TWA22 to young, nearby associations. Using the ESO NTT/SUSI2 telescope, we observed TWA22AB during 5 different observing runs over 1.2 years to measure its trigonometric parallax and proper motion. This is a part of a larger project measuring trigonometric parallaxes and proper motions of most known TWA members at a sub-milliarcsec level. HARPS at the ESO 3.6m telescope was also used to measure the system's radial velocity over 2 years. We report an absolute trigonometric parallax of TWA22AB, 57.0mas, corresponding to a distance 17.5pc from Earth. Measured proper motions of TWA22AB are -175.8mas/yr in right ascension and -21.3mas/yr in declination. Finally, from HARPS measurements, we obtain a radial velocity 14.8km/s.
Tight-binding theory of spin-orbit coupling in graphynes  [PDF]
Guido van Miert,Vladimir Juricic,Cristiane Morais Smith
Physics , 2014, DOI: 10.1103/PhysRevB.90.195414
Abstract: We investigate the effects of Rashba and intrinsic spin-orbit couplings in graphynes. First, we develop a general method to address spin-orbit couplings within the tight-binding theory. Then, we apply this method to $\alpha$, $\beta$, and $\gamma$-graphyne, and determine the SOC parameters in terms of the microscopic hopping and on-site energies. We find that for $\alpha$-graphyne, as in graphene, the intrinsic SOC opens a non-trivial gap, whereas the Rashba SOC splits each Dirac cone into four. In $\beta$ and $\gamma$ graphyne, the Rashba SOC can lead to a Lifshitz phase transition, thus transforming the zero-gap semiconductor into a gapped system or vice versa, when pairs of Dirac cones annihilate or emerge. The existence of internal (within the benzene ring) and external SOC in these compounds allow us to explore a myriad of phases not available in graphene.
Spin-Orbit Coupling in an f-electron Tight-Binding Model  [PDF]
M. D. Jones,R. C. Albers
Physics , 2008, DOI: 10.1103/PhysRevB.79.045107
Abstract: We extend a tight-binding method to include the effects of spin-orbit coupling, and apply it to the study of the electronic properties of the actinide elements Th, U, and Pu. These tight-binding parameters are determined for the fcc crystal structure using the equivalent equilibrium volumes. In terms of the single particle energies and the electronic density of states, the overall quality of the tight-binding representation is excellent and of the same quality as without spin-orbit coupling. The values of the optimized tight-binding spin-orbit coupling parameters are comparable to those determined from purely atomic calculations.
30 kV coaxial vacuum-tight feedthrough for operation at cryogenic temperatures  [PDF]
I. Kreslo,I. Badhrees,S. Delaquis,A. Ereditato,S. Janos,M. Messina,U. Moser,B. Rossi,M. Zeller
Physics , 2010, DOI: 10.1088/1748-0221/5/11/T11002
Abstract: In this paper we describe the technology of building a vacuum-tight high voltage feedthrough which is able to operate at voltages up to 30 kV. The feedthrough has a coaxial structure with a grounded sheath which makes it capable to lead high voltage potentials into cryogenic liquids, without risk of surface discharges in the gas phase above the liquid level. The feedthrough is designed to be used in ionization detectors, based on liquefied noble gases, such as Argon or Xenon.
Tight-binding electronic spectra on graphs with spherical topology. II. The effect of spin-orbit interaction  [PDF]
Y. Avishai,J. M. Luck
Statistics , 2008, DOI: 10.1088/1742-5468/2008/06/P06008
Abstract: This is the second of two papers devoted to tight-binding electronic spectra on graphs with the topology of the sphere. We investigate the problem of an electron subject to a spin-orbit interaction generated by the radial electric field of a static point charge sitting at the center of the sphere. The tight-binding Hamiltonian considered is a discretization on polyhedral graphs of the familiar form ${\bm L}\cdot{\bm S}$ of the spin-orbit Hamiltonian. It involves SU(2) hopping matrices of the form $\exp({\rm i}\mu{\bm n}\cdot{\bm\sigma})$ living on the oriented links of the graph. For a given structure, the dimensionless coupling constant $\mu$ is the only parameter of the model. An analysis of the energy spectrum is carried out for the five Platonic solids (tetrahedron, cube, octahedron, dodecahedron and icosahedron) and the C$_{60}$ fullerene. Except for the latter, the $\mu$-dependence of all the energy levels is obtained analytically in closed form. Rather unexpectedly, the spectra are symmetric under the exchange $\mu\leftrightarrow\Theta-\mu$, where $\Theta$ is the common arc length of the links. For the symmetric point $\mu=\Theta/2$, the problem can be exactly mapped onto a tight-binding model in the presence of the magnetic field generated by a Dirac monopole, studied recently. The dependence of the total energy at half filling on $\mu$ is investigated in all examples.
Spin-Orbit Coupled Bose Gases at Finite Temperatures  [PDF]
Renyuan Liao,Oleksandr Fialko
Physics , 2014, DOI: 10.1103/PhysRevA.89.063614
Abstract: Spin-orbit coupling is predicted to have dramatic effects on thermal properties of a two-component atomic Bose gas. We show that in three spatial dimensions it lowers the critical temperature of condensation and enhances thermal depletion of the condensate fraction. In two dimensions we show that spin-orbit coupling destroys superfluidity at any finite temperature, modifying dramatically the cerebrated Berezinskii-Kosterlitz-Thouless scenario. We explain this by the increase of the number of low energy states induced by spin-orbit coupling, enhancing the role of quantum fluctuations.
Spin-orbit coupled ultracold gases in optical lattices: High-band physics and insufficiency of tight-binding models  [PDF]
Lihong Zhou,Xiaoling Cui
Physics , 2015, DOI: 10.1103/PhysRevB.92.140502
Abstract: We study the interplay effect of spin-orbit coupling(SOC) and optical lattice to the single-particle physics and superfluid-insulator transition in ultracold Fermi gases. We consider the type of SOC that has been realized in cold atoms experiments via two-photon Raman processes. Our analyses are based on the knowledge of full single-particle spectrum in lattices, without relying on any tightbinding approximation.We evaluate existing tight-binding models and point out their limitations in predicting the correct single-particle physics due to the missed high-band contributions. Moreover, we show that the Raman field (creating SOC) can induce band-gap closing in a two-dimensional optical lattice, leading to the intriguing phenomenon of superfluidity-reentrance for interacting fermions at integer filling. We present the superfluid-insulator phase diagram in a wide parameter regime of chemical potentials and Raman fields. All these results are far beyond any tight-binding model can predict, and can be directly probed in current cold atoms experiments.
The LOFAR long baseline snapshot calibrator survey  [PDF]
J. Moldón,A. T. Deller,O. Wucknitz,N. Jackson,A. Drabent,T. Carozzi,J. Conway,A. D. Kapińska,P. McKean,L. Morabito,E. Varenius,P. Zarka,J. Anderson,A. Asgekar,I. M. Avruch,M. E. Bell,M. J. Bentum,G. Bernardi,P. Best,L. B?rzan,J. Bregman,F. Breitling,J. W. Broderick,M. Brüggen,H. R. Butcher,D. Carbone,B. Ciardi,F. de Gasperin,E. de Geus,S. Duscha,J. Eisl?ffel,D. Engels,H. Falcke,R. A. Fallows,R. Fender,C. Ferrari,W. Frieswijk,M. A. Garrett,J. Grie?meier,A. W. Gunst,J. P. Hamaker,T. E. Hassall,G. Heald,M. Hoeft,E. Juette,A. Karastergiou,V. I. Kondratiev,M. Kramer,M. Kuniyoshi,G. Kuper,P. Maat,G. Mann,S. Markoff,R. McFadden,D. McKay-Bukowski,R. Morganti,H. Munk,M. J. Norden,A. R. Offringa,E. Orru,H. Paas,M. Pandey-Pommier,R. Pizzo,A. G. Polatidis,W. Reich,H. R?ttgering,A. Rowlinson,A. M. M. Scaife,D. Schwarz,J. Sluman,O. Smirnov,B. W. Stappers,M. Steinmetz,M. Tagger,Y. Tang,C. Tasse,S. Thoudam,M. C. Toribio,R. Vermeulen,C. Vocks,R. J. van Weeren,S. White,M. W. Wise,S. Yatawatta,A. Zensus
Physics , 2014, DOI: 10.1051/0004-6361/201425042
Abstract: Aims. An efficient means of locating calibrator sources for International LOFAR is developed and used to determine the average density of usable calibrator sources on the sky for subarcsecond observations at 140 MHz. Methods. We used the multi-beaming capability of LOFAR to conduct a fast and computationally inexpensive survey with the full International LOFAR array. Sources were pre-selected on the basis of 325 MHz arcminute-scale flux density using existing catalogues. By observing 30 different sources in each of the 12 sets of pointings per hour, we were able to inspect 630 sources in two hours to determine if they possess a sufficiently bright compact component to be usable as LOFAR delay calibrators. Results. Over 40% of the observed sources are detected on multiple baselines between international stations and 86 are classified as satisfactory calibrators. We show that a flat low-frequency spectrum (from 74 to 325 MHz) is the best predictor of compactness at 140 MHz. We extrapolate from our sample to show that the density of calibrators on the sky that are sufficiently bright to calibrate dispersive and non-dispersive delays for the International LOFAR using existing methods is 1.0 per square degree. Conclusions. The observed density of satisfactory delay calibrator sources means that observations with International LOFAR should be possible at virtually any point in the sky, provided that a fast and efficient search using the methodology described here is conducted prior to the observation to identify the best calibrator.
Calibrator Design for the COBE Far Infrared Absolute Spectrophotometer (FIRAS)  [PDF]
J. C. Mather,D. J. Fixsen,R. A. Shafer,C. Mosier,D. T. Wilkinson
Physics , 1998, DOI: 10.1086/306805
Abstract: The photometric errors of the external calibrator for the FIRAS instrument on the COBE are smaller than the measurement errors on the cosmic microwave background (CMBR) spectrum (typically 0.02 MJy/sr, 1 sigma), and smaller than 0.01% of the peak brightness of the CMBR. The calibrator is a re-entrant cone, shaped like a trumpet mute, made of Eccosorb iron-loaded epoxy. It fills the entire beam of the instrument and is the source of its accuracy. Its known errors are caused by reflections, temperature gradients, and leakage through the material and around the edge. Estimates and limits are given for all known error sources. Improvements in understanding the temperature measurements of the calibrator allow an improved CMBR temperature determination of 2.725 +/- 0.002 K.
The VLBA Calibrator Survey - VCS1  [PDF]
A. J. Beasley,D. Gordon,A. B. Peck,L. Petrov,D. S. MacMillan,E. B. Fomalont,C. Ma
Physics , 2002, DOI: 10.1086/339806
Abstract: A catalog containing milliarcsecond--accurate positions of 1332 extragalactic radio sources distributed over the northern sky is presented - the Very Long Baseline Array Calibrator Survey (VCS1). The positions have been derived from astrometric analysis of dual-frequency 2.3 and 8.4 GHz VLBA snapshot observations; in a majority of cases, images of the sources are also available. These radio sources are suitable for use in geodetic and astrometric experiments, and as phase-reference calibrators in high-sensitivity astronomical imaging. The VCS1 is the largest high-resolution radio survey ever undertaken, and triples the number of sources available to the radio astronomy community for VLBI applications. In addition to the astrometric role, this survey can be used in active galactic nuclei, Galactic, gravitational lens and cosmological studies. The VCS1 catalog will soon be available at www.nrao.edu/vlba/VCS1 .
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