7,932 research outputs found
Studies of superconductivity and structure for CaC6 to pressures above 15 GPa
The dependence of the superconducting transition temperature Tc of CaC6 has
been determined as a function of hydrostatic pressure in both helium-loaded gas
and diamond-anvil cells to 0.6 and 32 GPa, respectively. Following an initial
increase at the rate +0.39(1) K/GPa, Tc drops abruptly from 15 K to 4 K at 10
GPa. Synchrotron x-ray measurements to 15 GPa point to a structural transition
near 10 GPa from a rhombohedral to a higher symmetry phase
Anomalous He-Gas High-Pressure Studies on Superconducting LaO1-xFxFeAs
AC susceptibility measurements have been carried out on superconducting
LaO1-xFxFeAs for x=0.07 and 0.14 under He-gas pressures to about 0.8 GPa. Not
only do the measured values of dTc/dP differ substantially from those obtained
in previous studies using other pressure media, but the Tc(P) dependences
observed depend on the detailed pressure/temperature history of the sample. A
sizeable sensitivity of Tc(P) to shear stresses provides a possible
explanation
The year in cardiology: arrhythmias and pacing.
During this last year, there has been much progress with regard to anticoagulant and ablation therapy for atrial fibrillation (AF). Apart from recently issued European Society of Cardiology Guidelines for the management of patients with supraventricular arrhythmias, there has been little progress in research in this field. Ventricular arrhythmias and device therapy have seen modest progress
Microscopic theory of glassy dynamics and glass transition for molecular crystals
We derive a microscopic equation of motion for the dynamical orientational
correlators of molecular crystals. Our approach is based upon mode coupling
theory. Compared to liquids we find four main differences: (i) the memory
kernel contains Umklapp processes, (ii) besides the static two-molecule
orientational correlators one also needs the static one-molecule orientational
density as an input, where the latter is nontrivial, (iii) the static
orientational current density correlator does contribute an anisotropic,
inertia-independent part to the memory kernel, (iv) if the molecules are
assumed to be fixed on a rigid lattice, the tensorial orientational correlators
and the memory kernel have vanishing l,l'=0 components. The resulting mode
coupling equations are solved for hard ellipsoids of revolution on a rigid
sc-lattice. Using the static orientational correlators from Percus-Yevick
theory we find an ideal glass transition generated due to precursors of
orientational order which depend on X and p, the aspect ratio and packing
fraction of the ellipsoids. The glass formation of oblate ellipsoids is
enhanced compared to that for prolate ones. For oblate ellipsoids with X <~ 0.7
and prolate ellipsoids with X >~ 4, the critical diagonal nonergodicity
parameters in reciprocal space exhibit more or less sharp maxima at the zone
center with very small values elsewhere, while for prolate ellipsoids with 2 <~
X <~ 2.5 we have maxima at the zone edge. The off-diagonal nonergodicity
parameters are not restricted to positive values and show similar behavior. For
0.7 <~ X <~ 2, no glass transition is found. In the glass phase, the
nonergodicity parameters show a pronounced q-dependence.Comment: 17 pages, 12 figures, accepted at Phys. Rev. E. v4 is almost
identical to the final paper version. It includes, compared to former
versions v2/v3, no new physical content, but only some corrected formulas in
the appendices and corrected typos in text. In comparison to version v1, in
v2-v4 some new results have been included and text has been change
Precursor-mediated crystallization process in suspensions of hard spheres
We report on a large scale computer simulation study of crystal nucleation in
hard spheres. Through a combined analysis of real and reciprocal space data, a
picture of a two-step crystallization process is supported: First dense,
amorphous clusters form which then act as precursors for the nucleation of
well-ordered crystallites. This kind of crystallization process has been
previously observed in systems that interact via potentials that have an
attractive as well as a repulsive part, most prominently in protein solutions.
In this context the effect has been attributed to the presence of metastable
fluid-fluid demixing. Our simulations, however, show that a purely repulsive
system (that has no metastable fluid-fluid coexistence) crystallizes via the
same mechanism.Comment: 4 figure
On the "generalized Generalized Langevin Equation"
In molecular dynamics simulations and single molecule experiments,
observables are usually measured along dynamic trajectories and then averaged
over an ensemble ("bundle") of trajectories. Under stationary conditions, the
time-evolution of such averages is described by the generalized Langevin
equation. In contrast, if the dynamics is not stationary, it is not a priori
clear which form the equation of motion for an averaged observable has. We
employ the formalism of time-dependent projection operator techniques to derive
the equation of motion for a non-equilibrium trajectory-averaged observable as
well as for its non-stationary auto-correlation function. The equation is
similar in structure to the generalized Langevin equation, but exhibits a
time-dependent memory kernel as well as a fluctuating force that implicitly
depends on the initial conditions of the process. We also derive a relation
between this memory kernel and the autocorrelation function of the fluctuating
force that has a structure similar to a fluctuation-dissipation relation. In
addition, we show how the choice of the projection operator allows to relate
the Taylor expansion of the memory kernel to data that is accessible in MD
simulations and experiments, thus allowing to construct the equation of motion.
As a numerical example, the procedure is applied to Brownian motion initialized
in non-equilibrium conditions, and is shown to be consistent with direct
measurements from simulations
Enhancement of the Critical Current Density of YBa2Cu3Ox Superconductors under Hydrostatic Pressure
The dependence of the critical current density Jc on hydrostatic pressure to
0.6 GPa is determined for a single 25-degree [001]-tilt grain boundary in a
bicrystalline ring of nearly optimally doped melt-textured YBa2Cu3Ox. Jc is
found to increase rapidly under pressure at +20 %/GPa. A new diagnostic method
is introduced (pressure-induced Jc relaxation) which reveals a sizeable
concentration of vacant oxygen sites in the grain boundary region. Completely
filling such sites with oxygen anions should lead to significant enhancements
in Jc.Comment: revised manuscript, graphic errors in figures correcte
Dispersion strenghthening of metals Progress report, Sep. 1965 - Feb. 1966
Apparatus constructed for controlled oxidation of iron-beryllium alloy powder
Improved Upsilon Spectrum with Dynamical Wilson Fermions
We present results for the b \bar b spectrum obtained using an
O(M_bv^6)-correct non-relativistic lattice QCD action, where M_b denotes the
bare b-quark mass and v^2 is the mean squared quark velocity. Propagators are
evaluated on SESAM's three sets of dynamical gauge configurations generated
with two flavours of Wilson fermions at beta = 5.6. These results, the first of
their kind obtained with dynamical Wilson fermions, are compared to a quenched
analysis at equivalent lattice spacing, beta = 6.0. Using our three sea-quark
values we perform the ``chiral'' extrapolation to m_eff = m_s/3, where m_s
denotes the strange quark mass. The light quark mass dependence is found to be
small in relation to the statistical errors. Comparing the full QCD result to
our quenched simulation we find better agreement of our dynamical data with
experimental results in the spin-independent sector but observe no unquenching
effects in hyperfine-splittings. To pin down the systematic errors we have also
compared quenched results in different ``tadpole'' schemes as well as using a
lower order action. We find that spin-splittings with an O(M_bv^4) action are
O(10%) higher compared to O(M_bv^6) results. Relative to the results obtained
with the plaquette method the Landau gauge mean link tadpole scheme raises the
spin splittings by about the same margin so that our two improvements are
opposite in effect.Comment: 24 pages (latex file, Phys Rev D style file, uses epsf-style
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