960 research outputs found
Loop algorithm for classical Heisenberg models with spin-ice type degeneracy
In many frustrated Ising models, a single-spin flip dynamics is frozen out at
low temperatures compared to the dominant interaction energy scale because of
the discrete "multiple valley" structure of degenerate ground-state manifold.
This makes it difficult to study low-temperature physics of these frustrated
systems by using Monte Carlo simulation with the standard single-spin flip
algorithm. A typical example is the so-called spin ice model, frustrated
ferromagnets on the pyrochlore lattice. The difficulty can be avoided by a
global-flip algorithm, the loop algorithm, that enables to sample over the
entire discrete manifold and to investigate low-temperature properties. We
extend the loop algorithm to Heisenberg spin systems with strong easy-axis
anisotropy in which the ground-state manifold is continuous but still retains
the spin-ice type degeneracy. We examine different ways of loop flips and
compare their efficiency. The extended loop algorithm is applied to the
following two models, a Heisenberg antiferromagnet with easy-axis anisotropy
along the z axis, and a Heisenberg spin ice model with the local
easy-axis anisotropy. For both models, we demonstrate high efficiency of our
loop algorithm by revealing the low-temperature properties which were hard to
access by the standard single-spin flip algorithm. For the former model, we
examine the possibility of order-from-disorder and critically check its
absence. For the latter model, we elucidate a gas-liquid-solid transition,
namely, crossover or phase transition among paramagnet, spin-ice liquid, and
ferromagnetically-ordered ice-rule state.Comment: 12 pages, 11 figures, accepted for publication in Phys. Rev.
Semiclassical spin liquid state of easy axis Kagome antiferromagnets
Motivated by recent experiments on Nd-langasite, we consider the effect of
strong easy axis single-ion anisotropy on spins interacting with
antiferromagnetic exchange on the Kagome lattice. When , the
collinear low energy states selected by the anisotropy map on to configurations
of the classical Kagome lattice Ising antiferromagnet. However, the low
temperature limit is quite different from the cooperative Ising paramagnet that
obtains classically for . We find that sub-leading multi-spin interactions arising from the transverse quantum
dynamics result in a crossover from an intermediate temperature classical
cooperative Ising paramagnet to a semiclassical spin liquid with distinct
short-ranged correlations for .Comment: 4 pages, 3 eps figure
Classical dimer model with anisotropic interactions on the square lattice
We discuss phase transitions and the phase diagram of a classical dimer model
with anisotropic interactions defined on a square lattice. For the attractive
region, the perturbation of the orientational order parameter introduced by the
anisotropy causes the Berezinskii-Kosterlitz-Thouless transitions from a
dimer-liquid to columnar phases. According to the discussion by Nomura and
Okamoto for a quantum-spin chain system [J. Phys. A 27, 5773 (1994)], we
proffer criteria to determine transition points and also universal
level-splitting conditions. Subsequently, we perform numerical diagonalization
calculations of the nonsymmetric real transfer matrices up to linear dimension
specified by L=20 and determine the global phase diagram. For the repulsive
region, we find the boundary between the dimer-liquid and the strong repulsion
phases. Based on the dispersion relation of the one-string motion, which
exhibits a two-fold ``zero-energy flat band'' in the strong repulsion limit, we
give an intuitive account for the property of the strong repulsion phase.Comment: 11 pages, 8 figure
Quantum Monte Carlo study of the transverse-field Ising model on a frustrated checkerboard lattice
We present the numerical results for low temperature behavior of the
transverse-field Ising model on a frustrated checkerboard lattice, with focus
on the effect of both quantum and thermal fluctuations. Applying the
recently-developed continuous-time quantum Monte Carlo algorithm, we compute
the magnetization and susceptibility down to extremely low temperatures while
changing the magnitude of both transverse and longitudinal magnetic fields.
Several characteristic behaviors are observed, which were not inferred from the
previously studied quantum order from disorder at zero temperature, such as a
horizontal-type stripe ordering at a substantial longitudinal field and a
persistent critical behavior down to low temperature in a weak longitudinal
field region.Comment: 6 pages, 5 figures, accepted for publication in J. Phys.: Conf. Se
Nonintegrability of the two-body problem in constant curvature spaces
We consider the reduced two-body problem with the Newton and the oscillator
potentials on the sphere and the hyperbolic plane .
For both types of interaction we prove the nonexistence of an additional
meromorphic integral for the complexified dynamic systems.Comment: 20 pages, typos correcte
Numerical Linked-Cluster Approach to Quantum Lattice Models
We present a novel algorithm that allows one to obtain temperature dependent
properties of quantum lattice models in the thermodynamic limit from exact
diagonalization of small clusters. Our Numerical Linked Cluster (NLC) approach
provides a systematic framework to assess finite-size effects and is valid for
any quantum lattice model. Unlike high temperature expansions (HTE), which have
a finite radius of convergence in inverse temperature, these calculations are
accurate at all temperatures provided the range of correlations is finite. We
illustrate the power of our approach studying spin models on {\it kagom\'e},
triangular, and square lattices.Comment: 4 pages, 5 figures, published versio
Quantum Monte Carlo scheme for frustrated Heisenberg antiferromagnets
When one tries to simulate quantum spin systems by the Monte Carlo method,
often the 'minus-sign problem' is encountered. In such a case, an application
of probabilistic methods is not possible. In this paper the method has been
proposed how to avoid the minus sign problem for certain class of frustrated
Heisenberg models. The systems where this method is applicable are, for
instance, the pyrochlore lattice and the Heisenberg model. The method
works in singlet sector. It relies on expression of wave functions in dimer
(pseudo)basis and writing down the Hamiltonian as a sum over plaquettes. In
such a formulation, matrix elements of the exponent of Hamiltonian are
positive.Comment: 19 LaTeX pages, 6 figures, 1 tabl
Antiferromagnetic Quantum Spins on the Pyrochlore Lattice
The ground state of the S=1/2 Heisenberg antiferromagnet on the pyrochlore
lattice is theoretically investigated. Starting from the limit of isolated
tetrahedra, I include interactions between the tetrahedra and obtain an
effective model for the spin-singlet ground state multiplet by third-order
perturbation. I determine its ground state using the mean-field approximation
and found a dimerized state with a four-sublattice structure, which agrees with
the proposal by Harris et al. I also discuss chirality correlations and spin
correlations for this state.Comment: 4 pages in 2-column format, 5 figures; To appear in J. Phys. Soc.
Jpn. (Mar, 2001
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