1,321 research outputs found
Monte Carlo Study of Correlations in Quantum Spin Chains at Non-Zero Temperature
Antiferromagnetic Heisenberg spin chains with various spin values
() are studied numerically with the quantum Monte Carlo
method. Effective spin chains are realized by ferromagnetically coupling
antiferromagnetic spin chains with . The temperature dependence
of the uniform susceptibility, the staggered susceptibility, and the static
structure factor peak intensity are computed down to very low temperatures,
. The correlation length at each temperature is deduced from
numerical measurements of the instantaneous spin-spin correlation function. At
high temperatures, very good agreement with exact results for the classical
spin chain is obtained independent of the value of . For =2 chains which
have a gap , the correlation length and the uniform susceptibility in
the temperature range are well predicted by a semi-classical
theory due to Damle and Sachdev.Comment: LaTeX EPJ macr
Effects of Nonmagnetic Impurity Doping on Spin Ladder System
Effects of nonmagnetic impurity doping on an AF spin-1/2 Heisenberg ladder
system are studied by the QMC method. A single nonmagnetic impurity induces a
localized spin-1/2 moment accompanied by "static" and enhanced AF correlations
around it. Small and finite concentration of impurities induces a remarkable
change of magnetic and thermodynamic properties with gapless excitations. It
also shows rather sharp but continuous crossover around the concentration of
about 4%. Above the crossover concentration, all the spins are strongly coupled
participating in the enhanced and rather uniform power-law decay of the
antiferromagnetic correlation. Below the crossover, each impurity forms an
antiferromagnetic cluster only weakly coupled each other. For random
distribution of impurities, large Curie-like susceptibility accompanied with
small residual entropy is obtained at low temperatures in agreement with recent
experimental observation in Zn-doped . Temperature dependence of
AF susceptibility shows power-law-like but weaker divergence than the single
chain AFH in the temperature range studied.Comment: 4 pages, LaTeX+epsf.sty, submitted to J.Phys.Soc.Jpn. New results of
AF susceptibility are adde
Phase transitions of a tethered surface model with a deficit angle term
Nambu-Goto model is investigated by using the canonical Monte Carlo
simulations on fixed connectivity surfaces of spherical topology. Three
distinct phases are found: crumpled, tubular, and smooth. The crumpled and the
tubular phases are smoothly connected, and the tubular and the smooth phases
are connected by a discontinuous transition. The surface in the tubular phase
forms an oblong and one-dimensional object similar to a one-dimensional linear
subspace in the Euclidean three-dimensional space R^3. This indicates that the
rotational symmetry inherent in the model is spontaneously broken in the
tubular phase, and it is restored in the smooth and the crumpled phases.Comment: 6 pages with 6 figure
Freezing Transition in Decaying Burgers Turbulence and Random Matrix Dualities
We reveal a phase transition with decreasing viscosity at \nu=\nu_c>0
in one-dimensional decaying Burgers turbulence with a power-law correlated
random profile of Gaussian-distributed initial velocities
\sim|x-x'|^{-2}. The low-viscosity phase exhibits non-Gaussian
one-point probability density of velocities, continuously dependent on \nu,
reflecting a spontaneous one step replica symmetry breaking (RSB) in the
associated statistical mechanics problem. We obtain the low orders cumulants
analytically. Our results, which are checked numerically, are based on
combining insights in the mechanism of the freezing transition in random
logarithmic potentials with an extension of duality relations discovered
recently in Random Matrix Theory. They are essentially non mean-field in nature
as also demonstrated by the shock size distribution computed numerically and
different from the short range correlated Kida model, itself well described by
a mean field one step RSB ansatz. We also provide some insights for the finite
viscosity behaviour of velocities in the latter model.Comment: Published version, essentially restructured & misprints corrected. 6
pages, 5 figure
Shock statistics in higher-dimensional Burgers turbulence
We conjecture the exact shock statistics in the inviscid decaying Burgers
equation in D>1 dimensions, with a special class of correlated initial
velocities, which reduce to Brownian for D=1. The prediction is based on a
field-theory argument, and receives support from our numerical calculations. We
find that, along any given direction, shocks sizes and locations are
uncorrelated.Comment: 4 pages, 8 figure
Quantum vs. Geometric Disorder in a Two-Dimensional Heisenberg Antiferromagnet
We present a numerical study of the spin-1/2 bilayer Heisenberg
antiferromagnet with random interlayer dimer dilution. From the temperature
dependence of the uniform susceptibility and a scaling analysis of the spin
correlation length we deduce the ground state phase diagram as a function of
nonmagnetic impurity concentration p and bilayer coupling g. At the site
percolation threshold, there exists a multicritical point at small but nonzero
bilayer coupling g_m = 0.15(3). The magnetic properties of the single-layer
material La_2Cu_{1-p}(Zn,Mg)_pO_4 near the percolation threshold appear to be
controlled by the proximity to this new quantum critical point.Comment: minor changes, updated figure
QCD as a Quantum Link Model
QCD is constructed as a lattice gauge theory in which the elements of the
link matrices are represented by non-commuting operators acting in a Hilbert
space. The resulting quantum link model for QCD is formulated with a fifth
Euclidean dimension, whose extent resembles the inverse gauge coupling of the
resulting four-dimensional theory after dimensional reduction. The inclusion of
quarks is natural in Shamir's variant of Kaplan's fermion method, which does
not require fine-tuning to approach the chiral limit. A rishon representation
in terms of fermionic constituents of the gluons is derived and the quantum
link Hamiltonian for QCD with a U(N) gauge symmetry is expressed in terms of
glueball, meson and constituent quark operators. The new formulation of QCD is
promising both from an analytic and from a computational point of view.Comment: 27 pages, including three figures. ordinary LaTeX; Submitted to Nucl.
Phys.
Interference in disordered systems: A particle in a complex random landscape
We consider a particle in one dimension submitted to amplitude and phase
disorder. It can be mapped onto the complex Burgers equation, and provides a
toy model for problems with interplay of interferences and disorder, such as
the NSS model of hopping conductivity in disordered insulators and the
Chalker-Coddington model for the (spin) quantum Hall effect. The model has
three distinct phases: (I) a {\em high-temperature} or weak disorder phase,
(II) a {\em pinned} phase for strong amplitude disorder, and (III) a {\em
diffusive} phase for strong phase disorder, but weak amplitude disorder. We
compute analytically the renormalized disorder correlator, equivalent to the
Burgers velocity-velocity correlator at long times. In phase III, it assumes a
universal form. For strong phase disorder, interference leads to a logarithmic
singularity, related to zeroes of the partition sum, or poles of the complex
Burgers velocity field. These results are valuable in the search for the
adequate field theory for higher-dimensional systems.Comment: 16 pages, 7 figure
Quantum Monte Carlo Loop Algorithm for the t-J Model
We propose a generalization of the Quantum Monte Carlo loop algorithm to the
t-J model by a mapping to three coupled six-vertex models. The autocorrelation
times are reduced by orders of magnitude compared to the conventional local
algorithms. The method is completely ergodic and can be formulated directly in
continuous time. We introduce improved estimators for simulations with a local
sign problem. Some first results of finite temperature simulations are
presented for a t-J chain, a frustrated Heisenberg chain, and t-J ladder
models.Comment: 22 pages, including 12 figures. RevTex v3.0, uses psf.te
First-order phase transition in the tethered surface model on a sphere
We show that the tethered surface model of Helfrich and Polyakov-Kleinert
undergoes a first-order phase transition separating the smooth phase from the
crumpled one. The model is investigated by the canonical Monte Carlo
simulations on spherical and fixed connectivity surfaces of size up to N=15212.
The first-order transition is observed when N>7000, which is larger than those
in previous numerical studies, and a continuous transition can also be observed
on small-sized surfaces. Our results are, therefore, consistent with those
obtained in previous studies on the phase structure of the model.Comment: 6 pages with 7 figure
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