176 research outputs found
Evidence for a Critical Behavior in Pure Compact QED
We present evidence about a critical behavior of compact QED (CQED) pure
gauge theory. Regularizing the theory on lattices homotopic to a sphere, we
present evidence for a critical, i.e. second order like behavior at the
deconfinement phase transition for certain values of the coupling parameter
.Comment: 3 pages, 3 figures, POSTSCRIPT file (127KB uuencoded
A quantum Monte Carlo algorithm realizing an intrinsic relaxation
We propose a new quantum Monte Carlo algorithm which realizes a relaxation
intrinsic to the original quantum system. The Monte Carlo dynamics satisfies
the dynamic scaling relation and is independent of the Trotter
number. Finiteness of the Trotter number just appears as the finite-size
effect. An infinite Trotter number version of the algorithm is also formulated,
which enables us to observe a true relaxation of the original system. The
strategy of the algorithm is a compromise between the conventional worldline
local flip and the modern cluster loop flip. It is a local flip in the
real-space direction and is a cluster flip in the Trotter direction. The new
algorithm is tested by the transverse-field Ising model in two dimensions. An
accurate phase diagram is obtained.Comment: 9 pages, 4 figure
Quantum Monte Carlo Simulation of the Trellis Lattice Heisenberg Model for SrCuO and CaVO
We study the spin-1/2 trellis lattice Heisenberg model, a coupled spin ladder
system, both by perturbation around the dimer limit and by quantum Monte Carlo
simulations. We discuss the influence of the inter-ladder coupling on the spin
gap and the dispersion, and present results for the temperature dependence of
the uniform susceptibility. The latter was found to be parameterized well by a
mean-field type scaling ansatz. Finally we discuss fits of experimental
measurements on SrCuO and CaVO to our results.Comment: 7 pages, 8 figure
Interaction effects between impurities in low dimensional spin-1/2 antiferromagnets
We are considering the interplay between several non-magnetic impurities in
the spin-1/2 Heisenberg antiferromagnet in chains, ladders and planes by
introducing static vacancies in numerical quantum Monte Carlo simulations. The
effective potential between two and more impurities is accurately determined,
which gives a direct measure of the quantum correlations in the systems. Large
effective interaction potentials are an indication of strong quantum
correlations in the system and reflect the detailed nature of the valence bond
ground states. In two-dimensions (2D) the interactions are smaller, but can
still be analyzed in terms of valence bonds.Comment: 8 pages, 6 figures, accepted by Europhys. Lett. The latest pdf file
is available at http://www.physik.uni-kl.de/eggert/papers/interact2d.pd
Spin and Gauge Systems on Spherical Lattices
We present results for 2D and 4D systems on lattices with topology homotopic
to the surface of a (hyper) sphere or . Finite size scaling is
studied in situations with phase transitions of first and second order type.
The Ising and Potts models exhibit the expected behaviour; for the 4D pure
gauge theory we find consistent scaling indicative of a second order
phase transition with critical exponent .Comment: 4 pages, LaTeX, 3 POSTSCRIPT figures (uuencoded
Universality of the gauge-ball spectrum of the four-dimensional pure U(1) gauge theory
We continue numerical studies of the spectrum of the pure U(1) lattice gauge
theory in the confinement phase, initiated in our previous work. Using the
extended Wilson action we address the question of universality of the phase
transition line in the () plane between the confinement and the
Coulomb phases. Our present results at for the gauge-ball
spectrum are fully consistent with the previous results obtained at . Again, two different correlation length exponents,
and , are obtained in different channels. We also confirm
the stability of the values of these exponents with respect to the variation of
the distance from the critical point at which they are determined. These
results further demonstrate universal critical behaviour of the model at least
up to correlation lengths of 4 lattice spacings when the phase transition is
approached in some interval at .Comment: 16 page
A Lower Bound on in the O(4) Model on Anisotropic Lattices
Results of an investigation of the spin model at finite temperature
using anisotropic lattices are presented. In both the large approximation
and numerical simulations using the Wolff cluster algorithm we find that the
ratio of the symmetry restoration temperature to the Higgs mass
is independent of the anisotropy . From the numerical
simulations we obtain a lower bound of at a value for the Higgs mass , which is lowered
further by about at . Requiring certain timelike
correlation functions to coincide with their spacelike counterparts, quantum
and scaling corrections to the anisotropy are determined and are found to be
small, i.e., the anisotropy is found to be close to the ratio of spacelike and
timelike lattice spacings.Comment: 16 pages with 4 ps figures included. LaTeX file. BI-TP 92/27,
FSU-SCRI-92-101, HLRZ-92-40, TIFR/TH/92-4
Diffusion in the Continuous-Imaginary-Time Quantum World-Line Monte Carlo Simulations with Extended Ensembles
The dynamics of samples in the continuous-imaginary-time quantum world-line
Monte Carlo simulations with extended ensembles are investigated. In the case
of a conventional flat ensemble on the one-dimensional quantum S=1 bi-quadratic
model, the asymmetric behavior of Monte Carlo samples appears in the diffusion
process in the space of the number of vertices. We prove that a local
diffusivity is asymptotically proportional to the number of vertices, and we
demonstrate the asymmetric behavior in the flat ensemble case. On the basis of
the asymptotic form, we propose the weight of an optimal ensemble as
, where denotes the number of vertices in a sample. It is shown
that the asymmetric behavior completely vanishes in the case of the proposed
ensemble on the one-dimensional quantum S=1 bi-quadratic model.Comment: 4 pages, 2 figures, update a referenc
Monte Carlo Study of the Separation of Energy Scales in Quantum Spin 1/2 Chains with Bond Disorder
One-dimensional Heisenberg spin 1/2 chains with random ferro- and
antiferromagnetic bonds are realized in systems such as . We have investigated numerically the thermodynamic properties of a
generic random bond model and of a realistic model of by the quantum Monte Carlo loop algorithm. For the first time we
demonstrate the separation into three different temperature regimes for the
original Hamiltonian based on an exact treatment, especially we show that the
intermediate temperature regime is well-defined and observable in both the
specific heat and the magnetic susceptibility. The crossover between the
regimes is indicated by peaks in the specific heat. The uniform magnetic
susceptibility shows Curie-like behavior in the high-, intermediate- and
low-temperature regime, with different values of the Curie constant in each
regime. We show that these regimes are overlapping in the realistic model and
give numerical data for the analysis of experimental tests.Comment: 7 pages, 5 eps-figures included, typeset using JPSJ.sty, accepted for
publication in J. Phys. Soc. Jpn. 68, Vol. 3. (1999
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