31 research outputs found
Coherence properties of the two-dimensional Bose-Einstein condensate
We present a detailed finite-temperature Hartree-Fock-Bogoliubov (HFB)
treatment of the two-dimensional trapped Bose gas. We highlight the numerical
methods required to obtain solutions to the HFB equations within the Popov
approximation, the derivation of which we outline. This method has previously
been applied successfully to the three-dimensional case and we focus on the
unique features of the system which are due to its reduced dimensionality.
These can be found in the spectrum of low-lying excitations and in the
coherence properties. We calculate the Bragg response and the coherence length
within the condensate in analogy with experiments performed in the
quasi-one-dimensional regime [Richard et al., Phys. Rev. Lett. 91, 010405
(2003)] and compare to results calculated for the one-dimensional case. We then
make predictions for the experimental observation of the quasicondensate phase
via Bragg spectroscopy in the quasi-two-dimensional regime.Comment: 9 pages, 9 figure
Superconducting Instability in the Periodic Anderson Model
Employing a quantum Monte Carlo simulation we find a pairing instability in
the normal state of the infinite dimensional periodic Anderson model.
Superconductivity arises from a normal state in which the screening is
protracted and which is clearly not a Fermi liquid. The phase diagram is
reentrant reflecting competition between superconductivity and Fermi liquid
formation. The estimated superconducting order parameter is even, but has nodes
as a function of frequency. This opens the possibility of a temporal node and
an effective order parameter composed of charge pairs and spin excitations.Comment: one postscript file, 6 pages including 6 figures. To appear in Phil.
Mag.
M Theory from World-Sheet Defects in Liouville String
We have argued previously that black holes may be represented in a D-brane
approach by monopole and vortex defects in a sine-Gordon field theory model of
Liouville dynamics on the world sheet. Supersymmetrizing this sine-Gordon
system, we find critical behaviour in 11 dimensions, due to defect condensation
that is the world-sheet analogue of D-brane condensation around an extra
space-time dimension in M theory. This supersymmetric description of Liouville
dynamics has a natural embedding within a 12-dimensional framework suggestive
of F theory.Comment: 17 pages LATEX, 1 epsf figure include
Quantum spin pumping at fractionally quantized magnetization state for a system with competing exchange interactions
We study the quantum spin pumping of an antiferromagnetic spin-1/2 chain with
competing exchange interactions. We show that spatially periodic potential
modulated in space and time acts as a quantum spin pump. In our model system,
an applied electric field causes a spin gap to its critical ground state by
introducing bond-alternation exchange interactions. We study quantum spin
pumping at different quantized magnetization states and also explain physically
the presence and absence of quantum spin pumping at different fractionally
quantized magnetization states
Failure of Mean Field Theory at Large N
We study strongly coupled lattice QCD with colors of staggered fermions
in 3+1 dimensions. While mean field theory describes the low temperature
behavior of this theory at large , it fails in the scaling region close to
the finite temperature second order chiral phase transition. The universal
critical region close to the phase transition belongs to the 3d XY universality
class even when becomes large. This is in contrast to Gross-Neveu models
where the critical region shrinks as (the number of flavors) increases and
mean field theory is expected to describe the phase transition exactly in the
limit of infinite . Our work demonstrates that close to second order phase
transitions infrared fluctuations can sometimes be important even when is
strictly infinite.Comment: 4 pages, 3 figure
An asymptotic formula for marginal running coupling constants and universality of loglog corrections
Given a two-loop beta function for multiple marginal coupling constants, we
derive an asymptotic formula for the running coupling constants driven to an
infrared fixed point. It can play an important role in universal loglog
corrections to physical quantities.Comment: 16 pages; typos fixed, one appendix removed for quick access to the
main result; to be published in J. Phys.
Confinement in Gauge Theories from the Condensation of World-Sheet Defects in Liouville String
We present a Liouville-string approach to confinement in four-dimensional
gauge theories, which extends previous approaches to include non-conformal
theories. We consider Liouville field theory on world sheets whose boundaries
are the Wilson loops of gauge theory, which exhibit vortex and spike defects.
We show that world-sheet vortex condensation occurs when the Wilson loop is
embedded in four target space-time dimensions, and show that this corresponds
to the condensation of gauge magnetic monopoles in target space. We also show
that vortex condensation generates a effective string tension corresponding to
the confinement of electric degrees of freedom. The tension is independent of
the string length in a gauge theory whose electric coupling varies
logarithmically with the length scale. The Liouville field is naturally
interpreted as an extra target dimension, with an anti-de-Sitter (AdS)
structure induced by recoil effects on the gauge monopoles, interpreted as D
branes of the effective string theory. Black holes in the bulk AdS space
correspond to world-sheet defects, so that phases of the bulk gravitational
system correspond to the different world-sheet phases, and hence to different
phases of the four-dimensional gauge theory. Deconfinement is associated with a
Berezinskii-Kosterlitz-Thouless transition of vortices on the Wilson-loop world
sheet, corresponding in turn to a phase transition of the black holes in the
bulk AdS space.Comment: 29 pages LATEX, three eps figures incorporate
Quantum creep and quantum creep transitions in 1D sine-Gordan chains
Discrete sine-Gordon (SG) chains are studied with path-integral molecular
dynamics. Chains commensurate with the substrate show the transition from
collective quantum creep to pinning at bead masses slightly larger than those
predicted from the continuous SG model. Within the creep regime, a field-driven
transition from creep to complete depinning is identified. The effects of
disorder in the external potential on the chain's dynamics depend on the
potential's roughness exponent , i.e., quantum and classical fluctuations
affect the current self-correlation functions differently for .Comment: 4 pages, 3 figure
Magnetization Plateau of an S=1 Frustrated Spin Ladder
We study the magnetization plateau at 1/4 of the saturation magnetization of
the S=1 antiferromagnetic spin ladder both analytically and numerically, with
the aim of explaining recent experimental results on BIP-TENO by Goto et al. We
propose two mechanisms for the plateau formation and clarify the plateau phase
diagram on the plane of the coupling constants between spins
Phase Diagram of the Two-Channel Kondo Lattice
The phase diagram of the two-channel Kondo lattice model is examined with a
Quantum Monte Carlo simulation in the limit of infinite dimensions.
Commensurate (and incommensurate) antiferromagnetic and superconducting states
are found. The antiferromagnetic transition is very weak and continuous;
whereas the superconducting transition is discontinuous to an odd-frequency
channel-singlet and spin-singlet pairing state.Comment: 5 pages, LaTeX and 4 PS figures (see also cond-mat/9609146 and
cond-mat/9605109