340 research outputs found
How the hydrogen bond in NHF is revealed with Compton scattering
In order to probe electron wave functions involved in the bonding of NHF,
we have performed Compton scattering experiments in an oriented single crystal
and in a powder. Ab initio calculations of the Compton profiles for NHF and
NHCl are used to enlighten the nature of the bonds in the NHF crystal.
As a consequence, we are able to show significant charge transfer in the
ammonium ion which is not observable using other methods. Our study provides a
compelling proof for hydrogen bond formation in NHF.Comment: 4 pages, 5 figures, accepted for publication as a Regular Article in
Physical Review
Pressure-induced enhancement of superconductivity and superconducting-superconducting transition in CaC
We measured the electrical resistivity, , of superconducting
CaC at ambient and high pressure up to 16 GPa. For 8 GPa, we found
a large increase of with pressure from 11.5 up to 15.1 K. At 8 GPa,
drops and levels off at 5 K above 10 GPa. Correspondingly, the residual
increases by 200 times and the behavior
becomes flat. The recovery of the pristine behavior after depressurization is
suggestive of a phase transition at 8 GPa between two superconducting phases
with good and bad metallic properties, the latter with a lower and more
static disorder
Complete conditions for legitimate Wigner distributions
Given a real-valued phase-space function, it is a nontrivial task to
determine whether it corresponds to a Wigner distribution for a physically
acceptable quantum state. This topic has been of fundamental interest for long,
and in a modern application, it can be related to the problem of entanglement
detection for multi-mode cases. In this paper, we present a hierarchy of
complete conditions for a physically realizable Wigner distribution. Our
derivation is based on the normally-ordered expansion, in terms of annihilation
andcreation operators, of the quasi-density operator corresponding to the
phase-space function in question. As a by-product, it is shown that the
phase-space distributions with elliptical symmetry can be readily diagonalized
in our representation, facilitating the test of physical realizability. We also
illustrate how the current formulation can be connected to the detection of
bipartite entanglement for continuous variables.Comment: 6 pages, published version with improved presentatio
Scanning Tunneling Spectroscopy on the novel superconductor CaC6
We present scanning tunneling microscopy and spectroscopy of the newly
discovered superconductor CaC. The tunneling conductance spectra, measured
between 3 K and 15 K, show a clear superconducting gap in the quasiparticle
density of states. The gap function extracted from the spectra is in good
agreement with the conventional BCS theory with = 1.6 0.2
meV. The possibility of gap anisotropy and two-gap superconductivity is also
discussed. In a magnetic field, direct imaging of the vortices allows to deduce
a coherence length in the ab plane 33 nm
Electron localization and possible phase separation in the absence of a charge density wave in single-phase 1T-VS
We report on a systematic study of the structural, magnetic and transport
properties of high-purity 1T-VS powder samples prepared under high
pressure. The results differ notably from those previously obtained by
de-intercalating Li from LiVS. First, no Charge Density Wave (CDW) is found
by transmission electron microscopy down to 94 K. Though, \textit{ab initio}
phonon calculations unveil a latent CDW instability driven by an acoustic
phonon softening at the wave vector (0.21,0.21,0)
previously reported in de-intercalated samples. A further indication of latent
lattice instability is given by an anomalous expansion of the V-S bond distance
at low temperature. Second, infrared optical absorption and electrical
resistivity measurements give evidence of non metallic properties, consistent
with the observation of no CDW phase. On the other hand, magnetic
susceptibility and NMR data suggest the coexistence of localized moments with
metallic carriers, in agreement with \textit{ab initio} band structure
calculations. This discrepancy is reconciled by a picture of electron
localization induced by disorder or electronic correlations leading to a phase
separation of metallic and non-metallic domains in the nm scale. We conclude
that 1T-VS is at the verge of a CDW transition and suggest that residual
electronic doping in Li de-intercalated samples stabilizes a uniform CDW phase
with metallic properties.Comment: 22 pages, 10 Figures. Full resolution pictures available at
http://journals.aps.org/prb/abstract/10.1103/PhysRevB.89.23512
Imprints of the Quantum World in Classical Mechanics
The imprints left by quantum mechanics in classical (Hamiltonian) mechanics
are much more numerous than is usually believed. We show Using no physical
hypotheses) that the Schroedinger equation for a nonrelativistic system of
spinless particles is a classical equation which is equivalent to Hamilton's
equations.Comment: Paper submitted to Foundations of Physic
\u3cem\u3eIn situ\u3c/em\u3e pressure study of Rb\u3csub\u3e4\u3c/sub\u3eC\u3csub\u3e60\u3c/sub\u3e insulator to metal transition by Compton scattering
Compton scattering has been shown to be a powerful tool for studying the ground state electronic density in real materials. Using synchrotron radiation, we have studied pressure effects on Rb4C60 by measuring the Compton profiles below and above the insulator to metal transition at 0.8 GPa. The experimental results are compared with the corresponding calculated results, obtained from new ab initio energy band structure calculations. These results allow us to quantitatively evaluate contributions to the Compton profiles resulting from the contraction of the unit cell as well as from the contraction of the C60 molecule itself. In this paper, we point out an unexpected contraction of the volume of the C60 molecule, leading to a major effect on the electronic density of the Rb4C60 compound
Superconductivity of bulk CaC6
We have obtained bulk samples of the graphite intercalation compound, CaC6,
by a novel method of synthesis from highly oriented pyrolytic graphite. The
crystal structure has been completely determined showing that it is the only
member of the MC6, metal-graphite compounds, which has rhombohedral symmetry.
We have clearly shown the occurrence of superconductivity in the bulk sample at
11.5K, using magnetization measurements.Comment: 8 pages of text + 4 figures = 12 page
Why are Prices Sticky? Evidence from Business Survey Data
This paper offers new insights on the price setting behaviour of German retail firms using a novel dataset that
consists of a large panel of monthly business surveys from 1991-2006. The firm-level data allows matching changes
in firms' prices to several other firm-characteristics. Moreover, information on price expectations allow analyzing
the determinants of price updating. Using univariate and bivariate ordered probit specifications, empirical menu
cost models are estimated relating the probability of price adjustment and price updating, respectively, to both
time- and state- dependent variables. First, results suggest an important role for state-dependence; changes in
the macroeconomic and institutional environment as well as firm-specific factors are significantly related to the
timing of price adjustment. These findings imply that price setting models should endogenize the timing of price
adjustment in order to generate realistic predictions concerning the transmission of monetary policy. Second, an
analysis of price expectations yields similar results providing evidence in favour of state-dependent sticky plan
models. Third, intermediate input cost changes are among the most important determinants of price adjustment
suggesting that pricing models should explicitly incorporate price setting at different production stages. However, the results show that adjustment to input cost changes takes time indicating "additional stickiness" at the last stage of processing
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