88 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
3-dimensional Rules for Finite-Temperature Loops
We present simple diagrammatic rules to write down Euclidean n-point
functions at finite temperature directly in terms of 3-dimensional momentum
integrals, without ever performing a single Matsubara sum. The rules can be
understood as describing the interaction of the external particles with those
of the thermal bath.Comment: 12 pages, 4 figures, to appear in Physics Letters
Low-energy excitations in a one-dimensional orthogonal dimer model with the Dzyaloshinski-Moriya interaction
Effects of the Dzyaloshinski-Moriya (DM) interaction on low-energy
excitations in a one-dimensional orthogonal-dimer model are studied by using
the perturbation expansions and the numerical diagonalization method. In the
absence of the DM interaction, the triplet excitations show two flat spectra
with three-fold degeneracy, which are labeled by magnetization .
These spectra split into two branches with M=0 and with by
switching-on of the DM interaction and besides the curvature appears in the
triplet excitations with more strongly than those of M=0.Comment: 4 pages, 2 figures, Proceeding for The 9th ISSP International
Symposium (ISSP-9) on Quantum Condensed System (Nov. 2004
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
Spectral sum rules for the Tomonaga-Luttinger model
In connection with recent publications we discuss spectral sum rules for the
Tomonaga-Luttinger model without using the explicit result for the one-electron
Green's function. They are usefull in the interpretation of recent high
resolution photoemission spectra of quasi-one-dimensional conductors. It is
shown that the limit of infinite frequency and band cut\-off do not commute.
Our result for arbitrary shape of the interaction potential generalizes an
earlier discussion by Suzumura. A general analytical expression for the
spectral function for wave vectors far from the Fermi wave vector is
presented. Numerical spectra are shown to illustrate the sum rules.Comment: 9 pages, REVTEX 3.0, 2 figures added as postscript file
The thermal operator representation for Matsubara sums
We prove in full generality the thermal operator representation for Matsubara
sums in a relativistic field theory of scalar and fermionic particles. It
states that the full result of performing the Matsubara sum associated to any
given Feynman graph, in the imaginary-time formalism of finite-temperature
field theory, can be directly obtained from its corresponding zero-temperature
energy integral, by means of a simple linear operator, which is independent of
the external Euclidean energies and whose form depends solely on the topology
of the graph.Comment: 9 pages, 1 figure, RevTe
Upper Critical Field in a Spin-Charge Separated Superconductor
It is demonstrated that the spatial decay of the pair propagator in a
Luttinger liquid with spin charge separation contains a logarithmic correction
relative to the free fermi gas result in a finite interval between the spin and
charge thermal lengths. It is argued that similar effects can be expected in
higher dimensional systems with spin charge separation and that the temperature
dependence of the upper critical field curve is a probe of this
effect.Comment: 3 pages, postscript file (compressed and uuencoded
Radiative Phase Transitions and Casmir Effect Instabilities
Molecular quantum electrodynamics leads to photon frequency shifts and thus
to changes in condensed matter free energies often called the Casimir effect.
Strong quantum electrodynamic coupling between radiation and molecular motions
can lead to an instability beyond which one or more photon oscillators undergo
a displacement phase transition. The phase boundary of the transition can be
located by a Casimir free energy instability.Comment: ReVTeX4 format 1 *.eps figur
A Uniform Approach to Antiferromagnetic Heisenberg Spins on Low Dimensional Lattices
Using group theoretical methods we show for both the triangular and square
lattices that in the continuum limit the antiferromagnetic order parameter
lives on SO3 without respect of the initial lattice. For the antiferromagnetic
chain we recover the Haldane decomposition. This order parameter interacts with
a local gauge field rather than with a global one as implicitly suggested in
the literature which in our approach appears in a rather natural manner. In
fact this merely corresponds to a novel extension of the spin group by a local
gauge field. This analysis based on the real division algebras applies to low
dimensional lattices.Comment: 5 pages; REVTeX
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
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