5,338 research outputs found
QED radiative correction to spin-density matrix elements in exclusive vector meson production
QED radiative effects are considered in the case of measurement of
spin-density matrix elements of diffractive -meson electroproduction.
Large radiative correction for is found in the kinematics of
collider experiments at HERA.Comment: 7 pages, 5 figure
Superconductivity at 17 K in Yttrium Metal under Nearly Hydrostatic Pressures to 89 GPa
In an experiment in a diamond anvil cell utilizing helium pressure medium,
yttrium metal displays a superconducting transition temperature which increases
monotonically from Tc ? 3.5 K at 30 GPa to 17 K at 89.3 GPa, one of the highest
transition temperatures for any elemental superconductor. The pressure
dependence of Tc differs substantially from that observed in previous studies
under quasihydrostatic pressure to 30 GPa. Remarkably, the dependence of Tc on
relative volume V/Vo is linear over the entire pressure range above 33 GPa,
implying that higher values of Tc are likely at higher pressures. For the
trivalent metals Sc, Y, La, Lu there appears to be some correlation between Tc
and the ratio of the Wigner-Seitz radius to the ion core radius.Comment: submitted for publicatio
More efficient time integration for Fourier pseudo-spectral DNS of incompressible turbulence
Time integration of Fourier pseudo-spectral DNS is usually performed using
the classical fourth-order accurate Runge--Kutta method, or other methods of
second or third order, with a fixed step size. We investigate the use of
higher-order Runge-Kutta pairs and automatic step size control based on local
error estimation. We find that the fifth-order accurate Runge--Kutta pair of
Bogacki \& Shampine gives much greater accuracy at a significantly reduced
computational cost. Specifically, we demonstrate speedups of 2x-10x for the
same accuracy. Numerical tests (including the Taylor-Green vortex,
Rayleigh-Taylor instability, and homogeneous isotropic turbulence) confirm the
reliability and efficiency of the method. We also show that adaptive time
stepping provides a significant computational advantage for some problems (like
the development of a Rayleigh-Taylor instability) without compromising
accuracy
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
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
A crystal theoretic method for finding rigged configurations from paths
The Kerov--Kirillov--Reshetikhin (KKR) bijection gives one to one
correspondences between the set of highest paths and the set of rigged
configurations. In this paper, we give a crystal theoretic reformulation of the
KKR map from the paths to rigged configurations, using the combinatorial R and
energy functions. This formalism provides tool for analysis of the periodic
box-ball systems.Comment: 24 pages, version for publicatio
The effects of disorder in dimerized quantum magnets in mean field approximations
We study theoretically the effects of disorder on Bose-Einstein condensates
(BEC) of bosonic triplon quasiparticles in doped dimerized quantum magnets. The
condensation occurs in a strong enough magnetic field Hc, where the
concentration of bosons in the random potential is sufficient to form the
condensate. The effect of doping is partly modeled by delta - correlated
disorder potential, which (i) leads to the uniform renormalization of the
system parameters and (ii) produces disorder in the system with renormalized
parameters. These approaches can explain qualitatively the available
magnetization data in the Tl_(1-x)K_(x)CuCl_3 compound taken as an example. In
addition to the magnetization, we found that the speed of the Bogoliubov mode
has a peak as a function of doping parameter, x. No evidence of the pure Bose
glass phase has been obtained in the BEC regime.Comment: Includes 19 pages, 5 figure
Effect of mixing and spatial dimension on the glass transition
We study the influence of composition changes on the glass transition of
binary hard disc and hard sphere mixtures in the framework of mode coupling
theory. We derive a general expression for the slope of a glass transition
line. Applied to the binary mixture in the low concentration limits, this new
method allows a fast prediction of some properties of the glass transition
lines. The glass transition diagram we find for binary hard discs strongly
resembles the random close packing diagram. Compared to 3D from previous
studies, the extension of the glass regime due to mixing is much more
pronounced in 2D where plasticization only sets in at larger size disparities.
For small size disparities we find a stabilization of the glass phase quadratic
in the deviation of the size disparity from unity.Comment: 13 pages, 8 figures, Phys. Rev. E (in print
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