91 research outputs found
Nature of the quantum phase transitions in the two-dimensional hardcore boson model
We use two Quantum Monte Carlo algorithms to map out the phase diagram of the
two-dimensional hardcore boson Hubbard model with near () and next near
() neighbor repulsion. At half filling we find three phases: Superfluid
(SF), checkerboard solid and striped solid depending on the relative values of
, and the kinetic energy. Doping away from half filling, the
checkerboard solid undergoes phase separation: The superfluid and solid phases
co-exist but not as a single thermodynamic phase. As a function of doping, the
transition from the checkerboard solid is therefore first order. In contrast,
doping the striped solid away from half filling instead produces a striped
supersolid phase: Co-existence of density order with superfluidity as a single
phase. One surprising result is that the entire line of transitions between the
SF and checkerboard solid phases at half filling appears to exhibit dynamical
O(3) symmetry restoration. The transitions appear to be in the same
universality class as the special Heisenberg point even though this symmetry is
explicitly broken by the interaction.Comment: 10 pages, 14 eps figures, include
Current Distribution in the Three-Dimensional Random Resistor Network at the Percolation Threshold
We study the multifractal properties of the current distribution of the
three-dimensional random resistor network at the percolation threshold. For
lattices ranging in size from to we measure the second, fourth and
sixth moments of the current distribution, finding {\it e.g.\/} that
where is the conductivity exponent and is the
correlation length exponent.Comment: 10 pages, latex, 8 figures in separate uuencoded fil
Phase Transitions in One-Dimensional Truncated Bosonic Hubbard Model and Its Spin-1 Analog
We study one-dimensional truncated (no more than 2 particles on a site)
bosonic Hubbard model in both repulsive and attractive regimes by exact
diagonalization and exact worldline Monte Carlo simulation. In the commensurate
case (one particle per site) we demonstrate that the point of Mott-insulator --
superfluid transition, , is remarkably far from that of
the full model. In the attractive region we observe the phase transition from
one-particle superfluid to two-particle one. The paring gap demonstrates a
linear behavior in the vicinity of the critical point. The critical state
features marginal response to the gauge phase. We argue that the two-particle
superfluid is a macroscopic analog of a peculiar phase observed earlier in a
spin-1 model with axial anisotropy.Comment: Revtex, 5 pages, 9 figures. Submitted to Phys. Rev.
Destruction of diagonal and off-diagonal long range order by disorder in two-dimensional hard core boson systems
We use quantum Monte Carlo simulations to study the effect of disorder, in
the form of a disordered chemical potential, on the phase diagram of the hard
core bosonic Hubbard model in two dimensions. We find numerical evidence that
in two dimensions, no matter how weak the disorder, it will always destroy the
long range density wave order (checkerboard solid) present at half filling and
strong nearest neighbor repulsion and replace it with a bose glass phase. We
study the properties of this glassy phase including the superfluid density,
energy gaps and the full Green's function. We also study the possibility of
other localized phases at weak nearest neighbor repulsion, i.e. Anderson
localization. We find that such a phase does not truly exist: The disorder must
exceed a threshold before the bosons (at weak nn repulsion) are localized. The
phase diagram for hard core bosons with disorder cannot be obtained easily from
the soft core phase diagram discussed in the literature.Comment: 7 pages, 10 eps figures include
Numerical Study of the Spin-Flop Transition in Anisotropic Spin-1/2 Antiferromagnets
Magnetization processes of the spin-1/2 antiferromagnetic model in two
and three spatial dimensions are studied using quantum Monte Carlo method based
on stochastic series expansions. Recently developed operator-loop algorithm
enables us to show a clear evidence of the first-order phase transition in the
presence of an external magnetic field. Phase diagrams of closely related
systems, hard core bosons with nearest-neighbor repulsions, are also discussed
focusing on possibilities of phase-separated and supersolid phases.Comment: 4 pages, Revtex version 4, with 4 figures embedded, To appear in
Phys. Rev.
Phase Diagram of Bosonic Atoms in Two-Color Superlattices
We investigate the zero temperature phase diagram of a gas of bosonic atoms
in one- and two-color standing-wave lattices in the framework of the
Bose-Hubbard model. We first introduce some relevant physical quantities;
superfluid fraction, condensate fraction, quasimomentum distribution, and
matter-wave interference pattern. We then discuss the relationships between
them on the formal level and show that the superfluid fraction, which is the
relevant order parameter for the superfluid to Mott-insulator transition,
cannot be probed directly via the matter wave interference patterns. The formal
considerations are supported by exact numerical solutions of the Bose-Hubbard
model for uniform one-dimensional systems. We then map out the phase diagram of
bosons in non-uniform lattices. The emphasis is on optical two-color
superlattices which exhibit a sinusoidal modulation of the well depth and can
be easily realized experimentally. From the study of the superfluid fraction,
the energy gap, and other quantities we identify new zero-temperature phases,
including a localized and a quasi Bose-glass phase, and discuss prospects for
their experimental observation.Comment: 18 pages, 17 figures, using REVTEX
3-d Lattice QCD Free Energy to Four Loops
We compute the expansion of the 3-d Lattice QCD free energy to four loop
order by means of Numerical Stochastic Perturbation Theory. The first and
second order are already known and are correctly reproduced. The third and
fourth order coefficients are new results. The known logarithmic divergence in
the fourth order is correctly identified. We comment on the relevance of our
computation in the context of dimensionally reduced finite temperature QCD.Comment: 8 pages, 3 figures, latex typeset with JHEP3.cl
Simultaneous Diagonal and Off Diagonal Order in the Bose--Hubbard Hamiltonian
The Bose-Hubbard model exhibits a rich phase diagram consisting both of
insulating regimes where diagonal long range (solid) order dominates as well as
conducting regimes where off diagonal long range order (superfluidity) is
present. In this paper we describe the results of Quantum Monte Carlo
calculations of the phase diagram, both for the hard and soft core cases, with
a particular focus on the possibility of simultaneous superfluid and solid
order. We also discuss the appearance of phase separation in the model. The
simulations are compared with analytic calculations of the phase diagram and
spin wave dispersion.Comment: 28 pages plus 24 figures, uuencoded Revtex+postscript file
Fourier Acceleration of Langevin Molecular Dynamics
Fourier acceleration has been successfully applied to the simulation of
lattice field theories for more than a decade. In this paper, we extend the
method to the dynamics of discrete particles moving in continuum. Although our
method is based on a mapping of the particles' dynamics to a regular grid so
that discrete Fourier transforms may be taken, it should be emphasized that the
introduction of the grid is a purely algorithmic device and that no smoothing,
coarse-graining or mean-field approximations are made. The method thus can be
applied to the equations of motion of molecular dynamics (MD), or its Langevin
or Brownian variants. For example, in Langevin MD simulations our acceleration
technique permits a straightforward spectral decomposition of forces so that
the long-wavelength modes are integrated with a longer time step, thereby
reducing the time required to reach equilibrium or to decorrelate the system in
equilibrium. Speedup factors of up to 30 are observed relative to pure
(unaccelerated) Langevin MD. As with acceleration of critical lattice models,
even further gains relative to the unaccelerated method are expected for larger
systems. Preliminary results for Fourier-accelerated molecular dynamics are
presented in order to illustrate the basic concepts. Possible extensions of the
method and further lines of research are discussed.Comment: 11 pages, two illustrations included using graphic
Disordered Bosons: Condensate and Excitations
The disordered Bose Hubbard model is studied numerically within the
Bogoliubov approximation. First, the spatially varying condensate wavefunction
in the presence of disorder is found by solving a nonlinear Schrodinger
equation. Using the Bogoliubov approximation to find the excitations above this
condensate, we calculate the condensate fraction, superfluid density, and
density of states for a two-dimensional disordered system. These results are
compared with experiments done with adsorbed in porous media.Comment: RevTeX, 26 pages and 10 postscript figures appended (Figure 9 has
three separate plots, so 12 postcript files altogether
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