6,457 research outputs found
Closed time path approach to the Casimir energy in real media
The closed time path formalism is applied, in the framework of open quantum
systems, to study the time evolution of the expectation value of the
energy-momentum tensor of a scalar field in the presence of real materials. We
analyze quantum fluctuations in a fully non-equilibrium scenario, when the
scalar field is interacting with the polarization degrees of freedom of matter,
described as quantum Brownian particles. A generalized analysis was done for
two types of couplings between the field and the material. On the one hand, we
considered a bilinear coupling, and on the other hand, a (more realistic)
current-type coupling as in the case of the electromagnetic field interacting
with matter. We considered the high temperature limit for the field, keeping
arbitrary temperatures for each part of the volume elements of the material. We
obtained a closed form for the Hadamard propagator, which let us study the
dynamical evolution of the expectations values of the energy-momentum tensor
components from the initial time. We showed that two contributions always take
place in the transient evolution: one of these is associated to the material
and the other one is only associated to the field. Transient features were
studied and the long-time limit was derived in several cases. We proved that in
the steady situation of a field in n + 1 dimensions, the material always
contribute unless is non-dissipative. Conversely, the proper field contribution
vanishes unless the material is non-dissipative or, moreover, at least for the
1 + 1 case, if there are regions without material. We conclude that any steady
quantization scheme in 1 + 1 dimensions must consider both contributions and we
argue why these results are physically expected from a dynamical point of view,
and also could be valid for higher dimensions based on the expected continuity
between the non-dissipative and real material cases.Comment: 28 pages, no figures. Version to appear in Phys. Rev.
Evidence for a conformal phase in SU(N) gauge theories
We discuss the existence of a conformal phase in SU(N) gauge theories in four
dimensions. In this lattice study we explore the model in the bare parameter
space, varying the lattice coupling and bare mass. Simulations are carried out
with three colors and twelve flavors of dynamical staggered fermions in the
fundamental representation. The analysis of the chiral order parameter and the
mass spectrum of the theory indicates the restoration of chiral symmetry at
zero temperature and the presence of a Coulomb-like phase, depicting a scenario
compatible with the existence of an infrared stable fixed point at nonzero
coupling. Our analysis supports the conclusion that the onset of the conformal
window for QCD-like theories is smaller than Nf=12, before the loss of
asymptotic freedom at sixteen and a half flavors. We discuss open questions and
future directions.Comment: 11 pages, 11 figures; extended analysis, conclusions unchanged.
(version to appear in PRD
QCD in One Dimension at Nonzero Chemical Potential
Using an integration formula recently derived by Conrey, Farmer and
Zirnbauer, we calculate the expectation value of the phase factor of the
fermion determinant for the staggered lattice QCD action in one dimension. We
show that the chemical potential can be absorbed into the quark masses; the
theory is in the same chiral symmetry class as QCD in three dimensions at zero
chemical potential. In the limit of a large number of colors and fixed number
of lattice points, chiral symmetry is broken spontaneously, and our results are
in agreement with expressions based on a chiral Lagrangian. In this limit, the
eigenvalues of the Dirac operator are correlated according to random matrix
theory for QCD in three dimensions. The discontinuity of the chiral condensate
is due to an alternative to the Banks-Casher formula recently discovered for
QCD in four dimensions at nonzero chemical potential. The effect of temperature
on the average phase factor is discussed in a schematic random matrix model.Comment: Latex, 23 pages and 5 figures; Added two references and corrected
several typo
A Monte Carlo study of temperature-programmed desorption spectra with attractive lateral interactions
We present results of a Monte Carlo study of temperature-programmed
desorption in a model system with attractive lateral interactions. It is shown
that even for weak interactions there are large shifts of the peak maximum
temperatures with initial coverage. The system has a transition temperature
below which the desorption has a negative order. An analytical expression for
this temperature is derived. The relation between the model and real systems is
discussed.Comment: Accepted for publication in Phys.Rev.B15, 10 pages (REVTeX), 2
figures (PostScript); discussion about Xe/Pt(111) adde
Nuclear Pairing in the T=0 channel revisited
Recent published data on the isoscalar gap in symmetric nuclear matter using
the Paris force and the corresponding BHF single particle dispersion are
corrected leading to an extremely high proton-neutron gap of
MeV at . Arguments whether this value can be reduced due
to screening effects are discussed. A density dependent delta interaction with
cut off is adjusted so as to approximately reproduce the nuclear matter values
with the Paris force.Comment: 4 pages, 4 figure
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