5,050 research outputs found
The Hartree ensemble approximation revisited: The "symmetric phase"
The Hartree ensemble approximation is studied in the ``symmetric phase'' of
1+1 dimensional lambda phi^4 theory. In comparison with the ``broken phase''
studied previously, it is shown that the dynamical evolution of observables
such as the particle distribution, energy exchange and auto-correlation
functions, is substantially slower. Approximate thermalization is found only
for relatively large energy densities and couplings.Comment: 17 pages RevTeX, 16 figures, 3 tables, uses amsmath and feynmp.
Extended some sections, reordered Sec.IV, added 3 refs, numerical typo
corrected, published versio
Ward identity and electrical conductivity in hot QED
We study the Ward identity for the effective photon-electron vertex summing
the ladder diagrams contributing to the electrical conductivity in hot QED at
leading logarithmic order. It is shown that the Ward identity requires the
inclusion of a new diagram in the integral equation for the vertex that has not
been considered before. The real part of this diagram is subleading and
therefore the final expressions for the electrical conductivity at leading
logarithmic order are not affected.Comment: 25 pages with 5 eps figures, discussion in section 3 improved; to
appear in JHE
Transport coefficients and quantum fields
Various aspects of transport coefficients in quantum field theory are
reviewed. We describe recent progress in the calculation of transport
coefficients in hot gauge theories using Kubo formulas, paying attention to the
fulfillment of Ward identities. We comment on why the color conductivity in hot
QCD is much simpler to compute than the electrical conductivity. The
nonperturbative extraction of transport coefficients from lattice QCD
calculations is briefly discussed.Comment: 15 pages with 9 eps figures. Combined invited talk by G.A. and poster
by J.M.M.R. presented at Strong and Electroweak Matter (SEWM 2002),
Heidelberg, Germany, 2-5 Oct 200
Transport coefficients and the 2PI effective action in the large N limit
We discuss the computation of transport coefficients in large N_f QCD and the
O(N) model for massive particles. The calculation is organized using the 1/N
expansion of the 2PI effective action to next-to-leading order. For the gauge
theory, we verify gauge fixing independence and consistency with the Ward
identity. In the gauge theory, we find a nontrivial dependence on the fermion
mass.Comment: 10 pages, based on presentations at Strong and Electroweak Matter
(SEWM04), Helsinki, Finland, June 16-19 2004, and the Workshop on QCD in
Extreme Environments, Argonne National Laboratory, USA, June 29-July 3 200
Continuum and lattice meson spectral functions at nonzero momentum and high temperature
We analyse discretization effects in the calculation of high-temperature
meson spectral functions at nonzero momentum and fermion mass on the lattice.
We do so by comparing continuum and lattice spectral functions in the infinite
temperature limit. Complete analytical results for the spectral densities in
the continuum are presented, along with simple expressions for spectral
functions obtained with Wilson and staggered fermions on anisotropic lattices.
We comment on the use of local and point split currents.Comment: 20 pages, 7 eps figure
Transport coefficients from the lattice?
The prospects of extracting transport coefficients from euclidean lattice
simulations are discussed. Some general comments on the reconstruction of
spectral functions using the Maximal Entropy Method are given as well.Comment: Lattice2002(nonzerot), 3 pages with 2 eps figure
Transport properties of microstructured ultrathin films of La0.67Ca0.33MnO3 on SrTiO3
We have investigated the electrical transport properties of 8 nm thick
La0.67Ca0.33MnO3 films, sputter-deposited on SrTiO3 (STO), and etched into 5
micrometer-wide bridges by Ar-ion etching. We find that even slight overetching
of the film leads to conductance of the STO substrate, and asymmetric and
non-linear current-voltage (I-V) characteristics. However, a brief oxygen
plasma etch allows full recovery of the insulating character of the substrate.
The I-V characteristics of the bridges are then fully linear over a large range
of current densities. We find colossal magnetoresistance properties typical for
strained LCMO on STO but no signature of non-linear effects (so-called
electroresistance) connected to electronic inhomogeneites. In the metallic
state below 150 K, the highest current densities lead to heating effects and
non-linear I-V characteristics.Comment: 3 pages, 5 figure
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
Nonequilibrium time evolution of the spectral function in quantum field theory
Transport or kinetic equations are often derived assuming a quasi-particle
(on-shell) representation of the spectral function. We investigate this
assumption using a three-loop approximation of the 2PI effective action in real
time, without a gradient expansion or on-shell approximation. For a scalar
field in 1+1 dimensions the nonlinear evolution, including the integration over
memory kernels, can be solved numerically. We find that a spectral function
approximately described by a nonzero width emerges dynamically. During the
nonequilibrium time evolution the Wigner transformed spectral function is
slowly varying, even in presence of strong qualitative changes in the effective
particle distribution. These results may be used to make further analytical
progress towards a quantum Boltzmann equation including off-shell effects and a
nonzero width.Comment: 20 pages with 6 eps figures, explanation and references added; to
appear in Phys.Rev.
- …