3,336 research outputs found
Spectral functions at small energies and the electrical conductivity in hot, quenched lattice QCD
In lattice QCD, the Maximum Entropy Method can be used to reconstruct
spectral functions from euclidean correlators obtained in numerical
simulations. We show that at finite temperature the most commonly used
algorithm, employing Bryan's method, is inherently unstable at small energies
and give a modification that avoids this. We demonstrate this approach using
the vector current-current correlator obtained in quenched QCD at finite
temperature. Our first results indicate a small electrical conductivity above
the deconfinement transition.Comment: 4 pages, v2: minor changes, footnote corrected, to appear in PR
Interoperating Context Discovery Mechanisms
Context-Aware applications adapt their behaviour to the current situation of the user. This information, for instance user location and user availability, is called context information. Context is delivered by distributed context sources that need to be discovered before they can be used to retrieve context. Currently, multiple context discovery mechanisms exist, exhibiting heterogeneous capabilities (e.g. communication mechanisms, and data formats), which can be available to context-aware applications at arbitrary moments during the ap-plication’s lifespan. In this paper, we discuss a middleware mechanism that en-ables a (mobile) context-aware application to interoperate transparently with different context discovery mechanisms available at run-time. The goal of the proposed mechanism is to hide the heterogeneity and availability of context discovery mechanisms for context-aware applications, thereby facilitating their development
Charmonium at high temperature in two-flavor QCD
We compute charmonium spectral functions in 2-flavor QCD on anisotropic
lattices using the maximum entropy method. Our results suggest that the S-waves
(J/psi and eta_c) survive up to temperatures close to 2Tc, while the P-waves
(chi_c0 and chi_c1) melt away below 1.2Tc.Comment: 11 pages, 19 figures. v2: expanded discussion and modified
conclusions. One figure changed. To appear in PR
Inhomogeneous superconductivity induced in a weak ferromagnet
Under certain conditions, the order parameter induced by a superconductor (S)
in a ferromagnet (F) can be inhomogeneous and oscillating, which results e.g.
in the so-called pi-coupling in S/F/S junctions. In principle, the
inhomogeneous state can be induced at T_c as function of the F-layer thickness
d_F in S/F bilayers and multilayers, which should result in a dip-like
characteristic of T_c(d_F). We show the results of measurements on the S/F
system Nb/Cu_{1-x}Ni_x, for Ni-concentrations in the range x = 0.5-0.7, where
such effects might be expected. We find that the critical thickness for the
occurrence of superconductivity is still relatively high, even for these weak
ferromagnets. The resulting dip then is intrinsically shallow and difficult to
observe, which explains the lack of a clear signature in the T_c(d_F) data.Comment: 4 pages, 4 figures. To be publishedin Physica C (proceedings of the
Second Euroconference on Vortex Matter in Superconductors, Crete, 2001
Phase diagram for a mixture of colloids and polymers with equal size
We present the phase diagram of a colloid-polymer mixture in which the radius a of the colloidal spheres is approximately the same as the radius R of a polymer coil (q=R/a1). A three-phase coexistence region is experimentally observed, previously only reported for colloid-polymer mixtures with smaller polymer chains (q0.6). A recently developed generalized free-volume theory (GFVT) for mixtures of hard spheres and non-adsorbing excluded-volume polymer chains gives a quantitative description of the phase diagram. Monte Carlo simulations also agree well with experimen
The 2PI finite temperature effective potential of the O(N) linear sigma model in 1+1 dimensions, at next-to-leading order in 1/N
We study the O(N) linear sigma model in 1+1 dimensions. We use the 2PI
formalism of Cornwall, Jackiw and Tomboulis in order to evaluate the effective
potential at finite temperature. At next-to-leading order in a 1/N expansion
one has to include the sums over "necklace" and generalized "sunset" diagrams.
We find that - in contrast to the Hartree approximation - there is no
spontaneous symmetry breaking in this approximation, as to be expected for the
exact theory. The effective potential becomes convex throughout for all
parameter sets which include N=4,10,100, couplings lambda=0.1 and 0.5, and
temperatures between 0.2 and 1. The Green's functions obtained by solving the
Schwinger-Dyson equations are enhanced in the infrared region. We also compare
the effective potential as function of the external field phi with those
obtained in various other approximations.Comment: 19 pages, 9 figures; v2: references added, some changes in the tex
The equilibrium intrinsic crystal-liquid interface of colloids
We use confocal microscopy to study an equilibrated crystal-liquid interface
in a colloidal suspension. Capillary waves roughen the surface, but locally the
intrinsic interface is sharply defined. We use local measurements of the
structure and dynamics to characterize the intrinsic interface, and different
measurements find slightly different widths of this interface. In terms of the
particle diameter , this width is either (based on structural
information) or (based on dynamics), both not much larger than the
particle size. This work is the first direct experimental visualization of an
equilibrated crystal-liquid interface.Comment: 6 pages; revised version, submitted to PNA
Critical voltage of a mesoscopic superconductor
We study the role of the quasiparticle distribution function f on the
properties of a superconducting nanowire. We employ a numerical calculation
based upon the Usadel equation. Going beyond linear response, we find a
non-thermal distribution for f caused by an applied bias voltage. We
demonstrate that the even part of f (the energy mode f_L) drives a first order
transition from the superconducting state to the normal state irrespective of
the current
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