3,639 research outputs found
Crossover from Two- to Three-Dimensional Behavior in Superfluids
We have studied the superfluid density on various size-lattices in
the geometry by numerical simulation of the model
using the Cluster Monte Carlo method. Applying the Kosterlitz-Thouless-Nelson
renormalization group equations for the superfluid density we have been able to
extrapolate to the limit for a given value of . In the
superfluid phase we find that the superfluid density faithfully obeys the
expected scaling law with , using the experimental value for the critical
exponent . For the sizes of film thickness studied here the
critical temperature and the coefficient entering the equation
are in agreement with the
expected -dependence deduced from general scaling ideas.Comment: 16 pages, postscript file, 264 kbyte
Monolayer charged quantum films: A quantum simulation study
We use path-integral Monte Carlo (PIMC) to study the effects of adding a
long-range repulsive Coulomb interaction to the usual Van der Waals interaction
between two atoms of a submonolayer quantum film such as helium on graphite or
a pure two-dimensional superfluid. Such interactions frustrate or compete with
the natural tendency of the system for phase separation namely to form a
macroscopic liquid or solid phase. We find that as a function of the relative
strength of the long-range repulsion, surface coverage and temperature, the
system undergoes a series of transformations, including a triangular
Wigner-like crystal of clusters, a charge stripe-ordered phase and a fluid
phase. The goal of these studies is to understand the role of quantum
fluctuations when such competing interactions appear together with formation of
preexisting electron pairs as might be the case in cuprate superconductors.Comment: 10 pages, 7 figures to be published in International Journal of
Modern Physics
Universal Magnetic Properties of at Intermediate Temperatures
We present the theory of two-dimensional, clean quantum antiferromagnets with
a small, positive, zero temperature () stiffness , but with the
ratio arbitrary. Universal scaling forms for the uniform
susceptibility (), correlation length(), and NMR relaxation rate
() are proposed and computed in a expansion and by Mont\'{e}-Carlo
simulations. For large , and asymptote
to universal values, while is nearly -independent. We find good
quantitative agreement with experiments and some numerical studies on
.Comment: 14 pages, REVTEX, 1 postscript figure appende
Signatures of the superfluid to Mott insulator transition in equilibrium and in dynamical ramps
We investigate the equilibrium and dynamical properties of the Bose-Hubbard
model and the related particle-hole symmetric spin-1 model in the vicinity of
the superfluid to Mott insulator quantum phase transition. We employ the
following methods: exact-diagonalization, mean field (Gutzwiller), cluster
mean-field, and mean-field plus Gaussian fluctuations. In the first part of the
paper we benchmark the four methods by analyzing the equilibrium problem and
give numerical estimates for observables such as the density of double
occupancies and their correlation function. In the second part, we study
parametric ramps from the superfluid to the Mott insulator and map out the
crossover from the regime of fast ramps, which is dominated by local physics,
to the regime of slow ramps with a characteristic universal power law scaling,
which is dominated by long wavelength excitations. We calculate values of
several relevant physical observables, characteristic time scales, and an
optimal protocol needed for observing universal scaling.Comment: 23 pages, 13 figure
Nearly frozen Coulomb Liquids
We show that very long range repulsive interactions of a generalized
Coulomb-like form , with (-dimensionality),
typically introduce very strong frustration, resulting in extreme fragility of
the charge-ordered state. An \textquotedbl{}almost frozen\textquotedbl{} liquid
then survives in a broad dynamical range above the (very low) melting
temperature which is proportional to . This
\textquotedbl{}pseudogap\textquotedbl{} phase is characterized by unusual
insulating-like, but very weakly temperature dependent transport, similar to
experimental findings in certain low carrier density systems.Comment: 5 pages,4 figure
Oscillons and oscillating kinks in the Abelian-Higgs model
We study the classical dynamics of the Abelian Higgs model employing an
asymptotic multiscale expansion method, which uses the ratio of the Higgs to
the gauge field amplitudes as a small parameter. We derive an effective
nonlinear Schr\"{o}dinger equation for the gauge field, and a linear equation
for the scalar field containing the gauge field as a nonlinear source. This
equation is used to predict the existence of oscillons and oscillating kinks
for certain regimes of the ratio of the Higgs to the gauge field masses.
Results of numerical simulations are found to be in very good agreement with
the analytical findings, and show that the oscillons are robust, while kinks
are unstable. It is also demonstrated that oscillons emerge spontaneously as a
result of the onset of the modulational instability of plane wave solutions of
the model. Connections of the obtained solutions with the phenomenology of
superconductors is discussed.Comment: arXiv admin note: substantial text overlap with arXiv:1306.386
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