791 research outputs found
Dimensional reduction and a Z(3) symmetric model
We present first results from a numerical investigation of a Z(3) symmetric
model based on dimensional reduction.Comment: Talk presented at XXI International Symposium on Lattice Field Theory
lattice2003(Non-zero temperature and density
N=1 Supersymmetric Spin-Charge Separation in effective gauge theories of planar magnetic superconductors
We present a N=1 Supersymmetric extension of a spin-charge separated
effective `particle-hole' gauge theory of excitations
about the nodes of the gap of a d-wave planar magnetic superconductor. The
supersymmetry is achieved without introducing extra degrees of freedom, as
compared to the non-supersymmetric models. The only exception, the introduction
of gaugino fieds, finds a natural physical interpretation as describing
interlayer coupling in the statistical model. The low-energy continuum theory
is described by a relativistic (2+1)-dimensional supersymmetric
-model with Gross-Neveu-Thirring-type four-fermion interactions. We
emphasize the crucial r\^ole of the constraint in inducing a non-trivial
dynamical mass generation for fermions (and thus superconductivity), in a way
compatible with manifest N=1 supersymmetry. We also give a preliminary
discussion of non-perturbative effects. We argue that supersymmetry suppresses
the dangerous for superconductivity instanton contributions to the mass of the
perturbatively massless gauge boson of the unbroken U(1) subgroup of SU(2).
Finally, we point out the possibility of applying these ideas to effective
gauge models of spin-charge separation in one-space dimensional superconducting
chains of holons, which, for example, have recently been claimed to be
important in the stripe phase of underdoped cuprates.Comment: 19 pages LATEX (minor misprints in formula (43) corrected
Application of a multi-site mean-field theory to the disordered Bose-Hubbard model
We present a multi-site formulation of mean-field theory applied to the
disordered Bose-Hubbard model. In this approach the lattice is partitioned into
clusters, each isolated cluster being treated exactly, with inter-cluster
hopping being treated approximately. The theory allows for the possibility of a
different superfluid order parameter at every site in the lattice, such as what
has been used in previously published site-decoupled mean-field theories, but a
multi-site formulation also allows for the inclusion of spatial correlations
allowing us, e.g., to calculate the correlation length (over the length scale
of each cluster). We present our numerical results for a two-dimensional
system. This theory is shown to produce a phase diagram in which the stability
of the Mott insulator phase is larger than that predicted by site-decoupled
single-site mean-field theory. Two different methods are given for the
identification of the Bose glass-to-superfluid transition, one an approximation
based on the behaviour of the condensate fraction, and one of which relies on
obtaining the spatial variation of the order parameter correlation. The
relation of our results to a recent proposal that both transitions are non
self-averaging is discussed.Comment: Accepted for publication in Physical Review
Effective Theory of Wilson Lines and Deconfinement
To study the deconfining phase transition at nonzero temperature, I outline
the perturbative construction of an effective theory for straight, thermal
Wilson lines. Certain large, time dependent gauge transformations play a
central role. They imply the existence of interfaces, which can be used to
determine the form of the effective theory as a gauged, nonlinear sigma model
of adjoint matrices. Especially near the transition, the Wilson line may
undergo a Higgs effect. As an adjoint field, this can generate eigenvalue
repulsion in the effective theory.Comment: 6 pages, LaTeX. Final, published version. Refs. 7, 39, and 40 added.
In Ref. 37, there is an expanded discussion of a "fuzzy" bag mode
Multi-site mean-field theory for cold bosonic atoms in optical lattices
We present a detailed derivation of a multi-site mean-field theory (MSMFT)
used to describe the Mott-insulator to superfluid transition of bosonic atoms
in optical lattices. The approach is based on partitioning the lattice into
small clusters which are decoupled by means of a mean field approximation. This
approximation invokes local superfluid order parameters defined for each of the
boundary sites of the cluster. The resulting MSMFT grand potential has a
non-trivial topology as a function of the various order parameters. An
understanding of this topology provides two different criteria for the
determination of the Mott insulator superfluid phase boundaries. We apply this
formalism to -dimensional hypercubic lattices in one, two and three
dimensions, and demonstrate the improvement in the estimation of the phase
boundaries when MSMFT is utilized for increasingly larger clusters, with the
best quantitative agreement found for . The MSMFT is then used to examine
a linear dimer chain in which the on-site energies within the dimer have an
energy separation of . This system has a complicated phase diagram
within the parameter space of the model, with many distinct Mott phases
separated by superfluid regions.Comment: 30 pages, 23 figures, accepted for publication in Phys. Rev.
Dynamical Gauge Symmetry Breaking and Superconductivity in three-dimensional systems
We discuss dynamical breaking of non-abelian gauge groups in three
dimensional (lattice) gauge systems via the formation of fermion condensates. A
physically relevant example, motivated by condensed-matter physics, is that of
a fermionic gauge theory with group . In
the strong U_S(1) region, the SU(2) symmetry breaks down to a U(1), due to the
formation of a parity-invariant fermion condensate. We conjecture a phase
diagram for the theory involving a critical line, which separates the regions
of broken SU(2) symmetry from those where the symmetry is restored. In the
broken phase, the effective Abelian gauge theory is closely related to an
earlier model of two-dimensional parity-invariant superconductivity in doped
antiferromagnets. The superconductivity in the model occurs in the
Kosterlitz-Thouless mode, since strong phase fluctuations prevent the existence
of a local order parameter. Some physical consequences of the phase diagram for the (doping-dependent) parameter space of this
condensed-matter model are briefly discussed.Comment: 17 pages Latex, 1 macro, three figures (included) (minor typo on page
14 concerning the critical coupling of SU(2) corrected
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