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61.
The local atomic order around very dilute indium impurities in c-axis-oriented YBa2Cu3O7 and YBa2Cu3O6.25 films at room temperature and in YBa2Cu3Ox bulk powders held in air or oxygen over a wide temperature range were investigated using 111In/Cd \gamma--\gamma perturbed angular correlation (PAC) spectroscopy. The probe atoms occupy a single substitutional lattice position in YBa2Cu3Ox, and evidence reported here strongly supports previous claims that this is the yttrium position. In YBa2Cu3Ox powders, the local atomic order changes continuously with temperature. At room temperature the electric field gradient measured by PAC in bulk YBa2Cu3Ox powder is indistinguishable from that in the fully oxygenated YBa2Cu3O7 film. Near the decomposition temperature, the bulk YBa2Cu3Ox powder data are semiquantitatively similar to data in the room temperature YBa2Cu3O6.25 film, with quantitative differences that we attribute to temperature averaging in the former. Other sites usually found in PAC spectra of YBa2Cu3Ox bulk samples arise from 111In dissolved in Y2BaCuO5 and Y2Cu2O5 impurity phases. The latter phase apparently arises because of significant barium loss during processing; the barium deficiency is clearly demonstrated by comparison of PAC data with the alloy phase diagram above the decomposition temperature. PAC data on pure Y2Cu2O5 are reported here also. This revised version was published online in July 2006 with corrections to the Cover Date.  相似文献   
62.
The density of states for the NiCu alloys system has been calculated as a function of concentration by the supercell method. This method takes into account inter-site interactions in contrast with current effective medium or single site theories. We show that the method is practical for the study of disordered systems. Comparison is made with previous work as well as with photoemission data.  相似文献   
63.
The Kerr solution describes, in Einstein's theory, the gravitational field of a rotating black hole. The axial symmetry and stationarity of the solution are shown here to arise in a simple way from properties of the curvature tensor.A portion of this work was completed, in part, during the summer of 1972 at the University of Texas at Dallas, Division of Mathematics and Mathematical Physics, and at the Black Hole session of the Ecole d'été de Physique Théorique in Les Houches; supported, in part, by the National Science Foundation, Grants GP-8868, GP-3463 9X, GP-20023, and GU-1598; the Air Force Office of Scientific Research, Grant 903-67; the National Aeronautics and Space Administration, Grant 44-004-001; the Westinghouse Corparation; the Clark Foundation; and the Rhodes Trust at Oxford.  相似文献   
64.
A first principles computation of the Hubbard intra-atomic electronic Coulomb interaction parameter U is performed for the VO2 molecular cluster. The results confirm earlier suggestions that the metal-semiconductor transition in VO2 is driven by intra-atomic electron correlations of the Heitler-London type.  相似文献   
65.
Quasi-phase-matched (QPM) materials allow the generation of spectroscopically useful infrared radiation in an efficient and broadly tunable format. Here, we describe several applications of QPM-based light sources to remote and local chemical sensing. The remote systems are gas imagers that employ a fiber-pumped continuous-wave optical parametric oscillator or a microlaser-pumped, diode-seeded optical parametric amplifier as the illumination source. Technology described for local sensing includes a cavity ring down spectrometer that employs a novel optical parametric generator–amplifier to achieve ≥350 cm-1 of contiguous tuning and a long-wave infrared light source based on QPM GaAs. In each case the use of QPM materials in conjunction with effective pump sources instills simplicity and ruggedness into the sensing systems. Received: 15 April 2002 / Revised version: 6 June 2002 / Published online: 12 September 2002 RID="*" ID="*"Corresponding author. Fax: +1-925/294-2595, E-mail: tjkulp@sandia.gov RID="**" ID="**"Present address: Corning Inc., Corning, NY 14831, USA RID="*" ID="*"Present address: Corning Inc., Corning, NY 14831, USA RID="**" ID="**"Present address: Blue Leaf Networks, Sunnyvale, CA 94086, USA RID="***" ID="***"Present address: Sandia National Laboratories, Albuquerque, NM 87185, USA  相似文献   
66.
Time differential perturbed angular correlation spectra of181Ta produced by beta decay of181Hf in monoclinic ZrO2 have been measured over a temperature range 10K to 1300K. If times near t=0 are excluded, G2(t) is well fitted by a standard single site model for nuclei subject to an interaction with a static electric field gradient. For high purity samples, the effective anisotropy A2 is equal to its expected value above 200°C but decreases abruptly by approximately a factor of two for lower temperatures. This unusual decrease, which does not occur in natural crystals or in Nb-doped powders, is attributed to an aftereffect of the beta emission that populates with approximately 50% probability an electron trap located about one eV below the conduction band. At low temperatures, this trapped electron causes the Ta nucleus to relax rapidly and contributes to G2(t) only near t=0. At high temperatures or in doped samples the electron escapes quickly enough to have negligible effect on G2(t). Supported in part by USDOE contract DE-FG06-85ER45191  相似文献   
67.
Measurements of the heat capacity by quasi-adiabatic, intermittent energy increments from 5 to 350 K show small high heat-capacity anomalies near 7 and 10 K which are attributed to superconducting transitions seen by magnetic measurements on the same carefully synthesized and well-characterized sample of (Hf0.934Zr0.057)(N0.97). Although no previous heat capacity measurements over the cryogenic region are known, the estimated 298.15 K standard entropy values (S/R) vary in the literature from about 200 per cent higher to 5 per cent lower than our measured value of (5.28±0.01)R–1 when the formula is represented as above. A simple scheme to represent and predict values based on both molar volumes and atomic masses for related materials is presented which seems more reliable on a limited sample than do others despite the intrusion of lanthanide contraction.This revised version was published online in November 2005 with corrections to the Cover Date.  相似文献   
68.
The low temperature properties of the averages over solutions of the TAP equations are discussed. For the white average the entropy goes to + ∞ indicating a thermodynamical instability. For the microcanonical average the entropy goes to the unphysical limit ? ∞. Some properties of an acceptable solution are discussed.  相似文献   
69.
A new solution of the Sherrington-Kirkpatrick spin glass model is discussed which is valid below the temperature where the replica method becomes unstable. The free energy is lowered, the entropy has no unphysical properties forT 0. The specific heat approaches zero exponentially and has a second maximum near a lower critical temperature for given external field. Near the critical point the difference of the free energy to that of Sherrington and Kirkpatrick is extremely small. Thus the Sherrington-Kirkpatrick phase may be undercooled.  相似文献   
70.
The spin glass model of Edwards and Anderson is solved for Ising spins starting from a renormalized diagrammatic expansion. One gets two qualitatively distinct phases for arbitrary external field. The high temperature phase is identical with the solution of Sherrington and Kirkpatrick. The low temperature phase does not have unphysical properties forT0, in contrast to previous investigations.  相似文献   
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