470 research outputs found
Loop algorithm for Heisenberg models with biquadratic interaction and phase transitions in two dimensions
We present a new algorithm for quantum Monte Carlo simulation based on global
updating with loops. While various theoretical predictions are confirmed in one
dimension, we find, for S=1 systems on a square lattice with an
antiferromagnetic biquadratic interaction, that the intermediate phase between
the antiferromagnetic and the ferromagnetic phases is disordered and that the
two phase transitions are both of the first order in contrast to the
one-dimensional case. It is strongly suggested that the transition points
coincide those at which the algorithm changes qualitatively.Comment: 4 pages including 4 figures, to appear in JPS
Recent Developments of World-Line Monte Carlo Methods
World-line quantum Monte Carlo methods are reviewed with an emphasis on
breakthroughs made in recent years. In particular, three algorithms -- the loop
algorithm, the worm algorithm, and the directed-loop algorithm -- for updating
world-line configurations are presented in a unified perspective. Detailed
descriptions of the algorithms in specific cases are also given.Comment: To appear in Journal of Physical Society of Japa
Nematic, vector-multipole, and plateau-liquid states in the classical O(3) pyrochlore antiferromagnet with biquadratic interactions in applied magnetic field
The classical bilinear-biquadratic nearest-neighbor Heisenberg
antiferromagnet on the pyrochlore lattice does not exhibit conventional
Neel-type magnetic order at any temperature or magnetic field. Instead spin
correlations decay algebraically over length scales r ~ \sqrt{T}, behavior
characteristic of a Coulomb phase arising from a strong local constraint.
Despite this, its thermodynamic properties remain largely unchanged if Neel
order is restored by the addition of a degeneracy-lifting perturbation, e.g.,
further neighbor interactions. Here we show how these apparent contradictions
can be resolved by a proper understanding of way in which long-range Neel order
emerges out of well-formed local correlations, and identify nematic and
vector-multipole orders hidden in the different Coulomb phases of the model. So
far as experiment is concerned, our results suggest that where long range
interactions are unimportant, the magnetic properties of Cr spinels which
exhibit half-magnetization plateaux may be largely independent of the type of
magnetic order present.Comment: 27 pages latex, 25 eps figure
Order from structural disorder in pyrochlore antiferromagnet
Effect of structural disorder is investigated for an pyrochlore
antiferromagnet with continuous degeneracy of classical ground states. Two
types of disorder, vacancies and weakly fluctuating exchange bonds, lift
degeneracy selecting the same subset of classical ground states. Analytic and
numerical results demonstrate that such an "order by structural disorder"
mechanism competes with the effect of thermal and quantum fluctuations. Our
theory predicts that a small amount of nonmagnetic impurities in
will stabilize the coplanar ()
magnetic structure as opposed to the () state found in
pure material
Superconductivity in model cuprate as an S=1 pseudomagnon condensation
We make use of the S=1 pseudospin formalism to describe the charge degree of
freedom in a model high- cuprate with the on-site Hilbert space reduced to
the three effective valence centers, nominally Cu. Starting
with a parent cuprate as an analogue of the quantum paramagnet ground state and
using the Schwinger boson technique we found the pseudospin spectrum and
conditions for the pseudomagnon condensation with phase transition to a
superconducting state.Comment: Version to be published in JLT
Global phase diagram of the spin-1 antiferromagnet with uniaxial anisotropy on the kagome lattice
The phase diagram of the XXZ spin-1 quantum magnet on the kagome lattice is
studied for all cases where the coupling is antiferromagnetic. In the
zero magnetic field case, the six previously introduced phases, found using
various methods, are: the nondegenerate gapped photon phase which breaks no
space symmetry or spin symmetry; the six-fold degenerate phase with plaquette
order, which breaks both time reversal symmetry and translational symmetry; the
"superfluid" (ferromagnetic) phase with an in-plane global U(1) symmetry
broken, when ; the nematic phase when and large; and a phase with resonating
dimers on each hexagon. We obtain all of these phases and partial information
about their quantum phase transitions in a single framework by studying
condensation of defects in the six-fold plaquette phases. The transition
between nematic phase and the six-fold degenerate plaquette phase is
potentially an unconventional second-order critical point. In the case of a
nonzero magnetic field along , another ordered phase with translation
symmetry broken is opened up in the nematic phase. Due to the breaking of
time-reversal symmetry by the field, a supersolid phase emerges between the
six-fold plaquette order and the superfluid phase. This phase diagram might be
accessible in nickel compounds, BF salts, or optical lattices of atoms with
three degenerate states on every site.Comment: 17 pages, 16 figure
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