40 research outputs found
Super-Radiance and the Unstable Photon Oscillator
If the damping of a simple harmonic oscillator from a thermally random force
is sufficiently strong, then the oscillator may become unstable. For a photon
oscillator (radiatively damped by electric dipole moments), the instability
leads to a low temperature Hepp-Lieb-Preparata super-radiant phase transition.
The stable oscillator regime is described by the free energy of the
conventional Casimir effect. The unstable (strongly damped) oscillator has a
free energy corresponding to Dicke super-radiance.Comment: 6 pages ReVTeX 2 figures *.ep
Origin of second-harmonic generation in the incommensurate phase of K2SeO4
We show that a ferroelectric phase transition takes place in the
incommensurate phase of the K2SeO4 crystal. The ferroelectric character of the
IC phase explains the second-harmonic generation observed in the corresponding
temperature range.Comment: 5 pages, 1 figur
Disclination Asymmetry in Two-Dimensional Nematic Liquid Crystals with Unequal Frank Constants
The behavior of a thin film of nematic liquid crystal with unequal Frank
constants is discussed. Distinct Frank constants are found to imply unequal
core energies for and disclinations. Even so, a topological
constraint is shown to ensure that the bulk densities of the two types of
disclinations are the same. For a system with free boundary conditions, such as
a liquid membrane, unequal core energies simply renormalize the Gaussian
rigidity and line tension.Comment: RevTex forma
An operator representation for Matsubara sums
In the context of the imaginary-time formalism for a scalar thermal field
theory, it is shown that the result of performing the sums over Matsubara
frequencies associated with loop Feynman diagrams can be written, for some
classes of diagrams, in terms of the action of a simple linear operator on the
corresponding energy integrals of the Euclidean theory at T=0. In its simplest
form the referred operator depends only on the number of internal propagators
of the graph.
More precisely, it is shown explicitly that this \emph{thermal operator
representation} holds for two generic classes of diagrams, namely, the
two-vertex diagram with an arbitrary number of internal propagators, and the
one-loop diagram with an arbitrary number of vertices.
The validity of the thermal operator representation for diagrams of more
complicated topologies remains an open problem. Its correctness is shown to be
equivalent to the correctness of some diagrammatic rules proposed a few years
ago.Comment: 4 figures; references added, minor changes in notation, final version
accepted for publicatio
Kinetic vs. Thermal-Field-Theory Approach to Cosmological Perturbations
A closed set of equations for the evolution of linear perturbations of
homogeneous, isotropic cosmological models can be obtained in various ways. The
simplest approach is to assume a macroscopic equation of state, e.g.\ that of a
perfect fluid. For a more refined description of the early universe, a
microscopic treatment is required. The purpose of this paper is to compare the
approach based on classical kinetic theory to the more recent
thermal-field-theory approach. It is shown that in the high-temperature limit
the latter describes cosmological perturbations supported by collisionless,
massless matter, wherein it is equivalent to the kinetic theory approach. The
dependence of the perturbations in a system of a collisionless gas and a
perfect fluid on the initial data is discussed in some detail. All singular and
regular solutions are found analytically.Comment: 31 pages, 10 figures (uu encoded ps-file appended), REVTEX 3.0, DESY
94-040 / TUW-93-2
d-wave superconductivity near charge instabilities
We investigate the symmetry of the superconducting order parameter in the
proximity of a phase-separation or of an incommensurate charge-density-wave
instability. The attractive effective interaction at small or intermediate
transferred momenta is singular near the instability. This strongly
-dependent interaction, together with a residual local repulsion between the
quasiparticles and an enhanced density of states for band structures
appropriate for the high temperature superconducting oxides, strongly favors
the formation of -wave superconductivity. The relative stability with
respect to superconductivity in the -wave channel is discussed in detail,
finding this latter hardly realized in the above conditions. The
superconducting temperature is mostly determined by the closeness to the
quantum critical point associated to the charge instability and displays a
stronger dependence on doping with respect to the simple proximity to a Van
Hove singularity. The relevance of this scenario and the generic agreement of
the resulting phase diagram with the properties displayed by high temperature
superconducting oxides is discussed.Comment: 1 revtex file and 12 postscript figure