4 research outputs found
Coarse-grained loop algorithms for Monte Carlo simulation of quantum spin systems
Recently, Syljuasen and Sandvik proposed a new framework for constructing
algorithms of quantum Monte Carlo simulation. While it includes new classes of
powerful algorithms, it is not straightforward to find an efficient algorithm
for a given model. Based on their framework, we propose an algorithm that is a
natural extension of the conventional loop algorithm with the split-spin
representation. A complete table of the vertex density and the worm-scattering
probability is presented for the general XXZ model of an arbitrary S with a
uniform magnetic field.Comment: 12 pages, 7 figures, insert a word "squared" in the first line of the
caption of Fig.7 and correct the label of vertical axis of Fig.
Quantum Monte Carlo with Directed Loops
We introduce the concept of directed loops in stochastic series expansion and
path integral quantum Monte Carlo methods. Using the detailed balance rules for
directed loops, we show that it is possible to smoothly connect generally
applicable simulation schemes (in which it is necessary to include
back-tracking processes in the loop construction) to more restricted loop
algorithms that can be constructed only for a limited range of Hamiltonians
(where back-tracking can be avoided). The "algorithmic discontinuities" between
general and special points (or regions) in parameter space can hence be
eliminated. As a specific example, we consider the anisotropic S=1/2 Heisenberg
antiferromagnet in an external magnetic field. We show that directed loop
simulations are very efficient for the full range of magnetic fields (zero to
the saturation point) and anisotropies. In particular for weak fields and
anisotropies, the autocorrelations are significantly reduced relative to those
of previous approaches. The back-tracking probability vanishes continuously as
the isotropic Heisenberg point is approached. For the XY-model, we show that
back-tracking can be avoided for all fields extending up to the saturation
field. The method is hence particularly efficient in this case. We use directed
loop simulations to study the magnetization process in the 2D Heisenberg model
at very low temperatures. For LxL lattices with L up to 64, we utilize the
step-structure in the magnetization curve to extract gaps between different
spin sectors. Finite-size scaling of the gaps gives an accurate estimate of the
transverse susceptibility in the thermodynamic limit: chi_perp = 0.0659 +-
0.0002.Comment: v2: Revised and expanded discussion of detailed balance, error in
algorithmic phase diagram corrected, to appear in Phys. Rev.
Entanglement and Spontaneous Symmetry Breaking in Quantum Spin Models
It is shown that spontaneous symmetry breaking does not modify the
ground-state entanglement of two spins, as defined by the concurrence, in the
XXZ- and the transverse field Ising-chain. Correlation function inequalities,
valid in any dimensions for these models, are presented outlining the regimes
where entanglement is unaffected by spontaneous symmetry breaking
Directed Loop Updates for Quantum Lattice Models
This article outlines how the quantum Monte Carlo directed loop update
recently introduced can be applied to a wide class of quantum lattice models.
Several models are considered: Spin-S XXZ models with longitudinal and
transverse magnetic fields, boson models with two-body interactions, and 1D
spinful fermion models. Expressions are given for the parameter regimes were
very efficient "no-bounce" quantum Monte Carlo algorithms can be found.Comment: 18 pages, 19 figure