910 research outputs found
Master equation approach to computing RVB bond amplitudes
We describe a "master equation" analysis for the bond amplitudes h(r) of an
RVB wavefunction. Starting from any initial guess, h(r) evolves (in a manner
dictated by the spin hamiltonian under consideration) toward a steady-state
distribution representing an approximation to the true ground state. Unknown
transition coefficients in the master equation are treated as variational
parameters. We illustrate the method by applying it to the J1-J2
antiferromagnetic Heisenberg model. Without frustration (J2=0), the amplitudes
are radially symmetric and fall off as 1/r^3 in the bond length. As the
frustration increases, there are precursor signs of columnar or plaquette VBS
order: the bonds preferentially align along the axes of the square lattice and
weight accrues in the nearest-neighbour bond amplitudes. The Marshall sign rule
holds over a large range of couplings, J2/J1 < 0.418. It fails when the r=(2,1)
bond amplitude first goes negative, a point also marked by a cusp in the ground
state energy. A nonrigourous extrapolation of the staggered magnetic moment
(through this point of nonanalyticity) shows it vanishing continuously at a
critical value J2/J1 = 0.447. This may be preempted by a first-order transition
to a state of broken translational symmetry.Comment: 8 pages, 7 figure
Critical temperature and the transition from quantum to classical order parameter fluctuations in the three-dimensional Heisenberg antiferromagnet
We present results of extensive quantum Monte Carlo simulations of the
three-dimensional (3D) S=1/2 Heisenberg antiferromagnet. Finite-size scaling of
the spin stiffness and the sublattice magnetization gives the critical
temperature Tc/J = 0.946 +/- 0.001. The critical behavior is consistent with
the classical 3D Heisenberg universality class, as expected. We discuss the
general nature of the transition from quantum mechanical to classical (thermal)
order parameter fluctuations at a continuous Tc > 0 phase transition.Comment: 5 pages, Revtex, 4 PostScript figures include
Two-Dimensional Quantum XY Model with Ring Exchange and External Field
We present the zero-temperature phase diagram of a square lattice quantum
spin 1/2 XY model with four-site ring exchange in a uniform external magnetic
field. Using quantum Monte Carlo techniques, we identify various quantum phase
transitions between the XY-order, striped or valence bond solid, staggered Neel
antiferromagnet and fully polarized ground states of the model. We find no
evidence for a quantum spin liquid phase.Comment: 4 pages, 4 figure
Double-layer Heisenberg antiferromagnet at finite temperature: Brueckner Theory and Quantum Monte Carlo simulations
The double-layer Heisenberg antiferromagnet with intra- and inter-layer
couplings and exhibits a zero temperature quantum phase
transition between a quantum disordered dimer phase for and a Neel
phase with long range antiferromagnetic order for , where
and . We consider the behavior of the system at finite
temperature for using two different and complementary approaches;
an analytical Brueckner approximation and numerically exact quantum Monte Carlo
simulations. We calculate the temperature dependent spin excitation spectrum
(including the triplet gap), dynamic and static structure factors, the specific
heat, and the uniform magnetic susceptibility. The agreement between the
analytical and numerical approaches is excellent. For and , our analytical results for the specific heat and the magnetic
susceptibility coincide with those previously obtained within the nonlinear
model approach for . Our quantum Monte Carlo simulations
extend to significantly lower temperatures than previously, allowing us to
obtain accurate results for the asymptotic quantum critical behavior. We also
obtain an improved estimate for the critical coupling: .Comment: 23 pages, 12 figure
Effects of intrabilayer coupling on the magnetic properties of YBaCuO
A two-layer Heisenberg antiferromagnet is studied as a model of the bilayer
cuprate YBaCuO. Quantum Monte Carlo results are presented for the
temperature dependence of the spin correlation length, the static structure
factor, the magnetic susceptibility, and the Cu NMR spin-echo decay rate
. As expected, when the ratio of the intrabilayer and
in-plane coupling strengths is small, increasing pushes the system deeper
inside the renormalized classical regime. Even for as small as
the correlations are considerably enhanced at temperatures as high as . This has a significant effect on , and it is
suggested that measurements of this quantity at high temperatures can reveal
the strength of the intrabilayer coupling in YBaCuO.Comment: 10 pages (Revtex) + 5 uuencoded ps figures. To appear in Phys. Rev.
B, Rapid Com
Finite-Size Scaling of the Ground State Parameters of the Two-Dimensional Heisenberg Model
The ground state parameters of the two-dimensional S=1/2 antiferromagnetic
Heisenberg model are calculated using the Stochastic Series Expansion quantum
Monte Carlo method for L*L lattices with L up to 16. The finite-size results
for the energy E, the sublattice magnetization M, the long-wavelength
susceptibility chi_perp(q=2*pi/L), and the spin stiffness rho_s, are
extrapolated to the thermodynamic limit using fits to polynomials in 1/L,
constrained by scaling forms previously obtained from renormalization group
calculations for the nonlinear sigma model and chiral perturbation theory. The
results are fully consistent with the predicted leading finite-size corrections
and are of sufficient accuracy for extracting also subleading terms. The
subleading energy correction (proportional to 1/L^4) agrees with chiral
perturbation theory to within a statistical error of a few percent, thus
providing the first numerical confirmation of the finite-size scaling forms to
this order. The extrapolated ground state energy per spin, E=-0.669437(5), is
the most accurate estimate reported to date. The most accurate Green's function
Monte Carlo (GFMC) result is slightly higher than this value, most likely due
to a small systematic error originating from ``population control'' bias in
GFMC. The other extrapolated parameters are M=0.3070(3), rho_s = 0.175(2),
chi_perp = 0.0625(9), and the spinwave velocity c=1.673(7). The statistical
errors are comparable with those of the best previous estimates, obtained by
fitting loop algorithm quantum Monte Carlo data to finite-temperature scaling
forms. Both M and rho_s obtained from the finite-T data are, however, a few
error bars higher than the present estimates. It is argued that the T=0
extrapolations performed here are less sensitive to effects of neglectedComment: 16 pages, RevTex, 9 PostScript figure
Ground state parameters, finite-size scaling, and low-temperature properties of the two-dimensional S=1/2 XY model
We present high-precision quantum Monte Carlo results for the S=1/2 XY model
on a two-dimensional square lattice, in the ground state as well as at finite
temperature. The energy, the spin stiffness, the magnetization, and the
susceptibility are calculated and extrapolated to the thermodynamic limit. For
the ground state, we test a variety of finite-size scaling predictions of
effective Lagrangian theory and find good agreement and consistency between the
finite-size corrections for different quantities. The low-temperature behavior
of the susceptibility and the internal energy is also in good agreement with
theoretical predictions.Comment: 6 pages, 8 figure
Dark matter to dark energy transition in k-essence cosmologies
We implement the transition from dark matter to dark energy in k-essence
cosmologies for a very large set of kinetic functions , in a way alternative
to recent proposals which use generalized Chaplygin gas and transient models.
Here we require that the pressure admits a power-law expansion around some
value of the kinetic energy where the pressure vanishes. In addition, for
suitable values of the parameters of the model, the speed of sound of the dark
matter will be low. We first present the discussion in fairly general terms,
and later consider for illustration two examples.Comment: 5 pages, revte
Measurement properties of the high-level mobility assessment tool for mild traumatic brain injury
Background. The High-Level Mobility Assessment Tool (HiMAT) was developed
to quantify balance and mobility problems after traumatic brain injury (TBI). Measurement
properties of the HiMAT have not been tested in the mild TBI (MTBI)
population.
Objective. The aim of this study was to examine the reliability, validity, and
responsiveness of the HiMAT in a sample of the MTBI population.
Design. A cohort, pretest-posttest, comparison study was conducted.
Methods. Ninety-two patients (69% men, 31% women) with a mean age of 37.1
years (SD 13.8) and a mean Glasgow Coma Scale score of 14.7 (SD 0.7) were
recruited from Oslo University Hospital. All patients were tested with the HiMAT
(range of scores 0 [worst] to 54 [best]) at 3 months postinjury. Fifty-one patients
were retested at 6 months. A subgroup of 25 patients was selected for the reliability
testing. Balance function reported on the Rivermead Post Concussion Symptoms
Questionnaire was chosen as a criterion and anchor. Criterion-related validity was
studied with correlation analysis. Intraclass correlation coefficients (ICCs) were used
for assessing interrater and intrarater reliability. Minimal detectable change (MDC) for
the HiMAT was estimated. Responsiveness was assessed with receiver operating
characteristic curve analyses.
Results. The mean HiMAT sum score was 46.2 (95% confidence interval 44.4 to
48.1). The HiMAT had a ceiling effect of 22.8%. The correlation between HiMAT
scores and self-reported balance problems was large (r .63, P .001). Interrater
and intrarater reliability of the HiMAT sum score was high (interrater ICC .99,
intrarater ICC .95). The MDC was 3 to 4 points. Responsiveness was good, and
the HiMAT discriminated well between patients with self-perceived improved balance
function versus unchanged balance function (area under the curve 0.86).
Limitations. The small sample size, a ceiling effect, and lack of a gold standard
were limitations of the study.
Conclusions. The HiMAT demonstrated satisfactory measurement properties for
patients with MTBI. The HiMAT can be used as an outcome measure of balance and
mobility problems in patients with MTB
- …