4,115 research outputs found
Order and disorder in the triangular-lattice t-J-V model at 2/3 electron density
Motivated by the recent discovery of superconductivity in NaCoOHO, we use series expansion methods and cluster mean-field theory to
study spontaneous charge order, Neel order, ferromagnetic order, dimer order
and phase-separation in the triangular-lattice t-J-V model at 2/3 electron
density. We find that for t<0, the charge ordered state, with electrons
preferentially occupying a honeycomb lattice, is very robust. Quite
surprisingly, hopping to the third sublattice can even enhance Neel order. At
large negative t and small V, the Nagaoka ferromagnetic state is obtained. For
large positive t, charge and Neel order vanish below a critical V, giving rise
to an itinerant antiferromagnetically correlated state.Comment: 4 pages, 5 figure
Conformal invariance studies of the Baxter-Wu model and a related site-colouring problem
The partition function of the Baxter-Wu model is exactly related to the
generating function of a site-colouring problem on a hexagonal lattice. We
extend the original Bethe ansatz solution of these models in order to obtain
the eigenspectra of their transfer matrices in finite geometries and general
toroidal boundary conditions. The operator content of these models are studied
by solving numerically the Bethe-ansatz equations and by exploring conformal
invariance. Since the eigenspectra are calculated for large lattices, the
corrections to finite-size scaling are also calculated.Comment: 12 pages, latex, to appear in J. Phys. A: Gen. Mat
A standardised sampling protocol for robust assessment of reach-scale fish community diversity in wadeable New Zealand streams
The New Zealand fish fauna contains species that are affected not only by river system connectivity, but also by catchment and local-scale changes in landcover, water quality and habitat quality. Consequently, native fish have potential as multi-scale bioindicators of human pressure on stream ecosystems, yet no standardised, repeatable and scientifically defensible methods currently exist for effectively quantifying their abundance or diversity in New Zealand stream reaches. Here we report on the testing of a back-pack electrofishing method, modified from that used by the United States Environmental Protection Agency, on a wide variety of wadeable stream reaches throughout New Zealand. Seventy-three first- to third-order stream reaches were fished with a single pass over 150-345 m length. Time taken to sample a reach using single-pass electrofishing ranged from 1-8 h. Species accumulation curves indicated that, irrespective of location, continuous sampling of 150 stream metres is required to accurately describe reach-scale fish species richness using this approach. Additional species detection beyond 150 m was rare (<10%) with a single additional species detected at only two out of the 17 reaches sampled beyond this distance. A positive relationship was also evident between species detection and area fished, although stream length rather than area appeared to be the better predictor. The method tested provides a standardised and repeatable approach for regional and/or national reporting on the state of New Zealand's freshwater fish communities and trends in richness and abundance over time
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
Spin Dependence of Correlations in Two-Dimensional Quantum Heisenberg Antiferromagnets
We present a series expansion study of spin-S square-lattice Heisenberg
antiferromagnets. The numerical data are in excellent agreement with recent
neutron scattering measurements. Our key result is that the correlation length
for S>1/2 strongly deviates from the exact T->0 (renormalized classical, or RC)
scaling prediction for all experimentally and numerically accessible
temperatures. We note basic trends with S of the experimental and series
expansion correlation length data and propose a scaling crossover scenario to
explain them.Comment: 5 pages, REVTeX file. PostScript file for the paper with embedded
figures available via WWW at http://xxx.lanl.gov/ps/cond-mat/9503143
Series Expansions for three-dimensional QED
Strong-coupling series expansions are calculated for the Hamiltonian version
of compact lattice electrodynamics in (2+1) dimensions, with 4-component
fermions. Series are calculated for the ground-state energy per site, the
chiral condensate, and the masses of `glueball' and positronium states.
Comparisons are made with results obtained by other techniques.Comment: 13 figure
Dimensional Crossover in Quantum Antiferromagnets
The dimensional crossover in a spin- nearest neighbor Heisenberg
antiferromagnet is discussed as it is tuned from a two-dimensional square
lattice, of lattice spacing , towards a spin chain by varying the width
of a semi-infinite strip . For integer spins and arbitrary
, and for half integer spins with an arbitrary even integer,
explicit analytical expressions for the zero temperature correlation length and
the spin gap are given. For half integer spins and an odd inetger, it
is shown that the behavior of the WZW fixed point is squeezed
out as the width ; here is the conformal charge. The results
specialized to are relevant to spin-ladder systems.Comment: RevTeX, 4 pages, 1 embedded postscript figur
A Frustrated 3-Dimensional Antiferromagnet: Stacked Layers
We study a frustrated 3D antiferromagnet of stacked layers. The
intermediate 'quantum spin liquid' phase, present in the 2D case, narrows with
increasing interlayer coupling and vanishes at a triple point. Beyond this
there is a direct first-order transition from N{\' e}el to columnar order.
Possible applications to real materials are discussed.Comment: 11 pages,7 figure
Quantum Monte Carlo study of the one-dimensional Holstein model of spinless fermions
The Holstein model of spinless fermions interacting with dispersionless
phonons in one dimension is studied by a Green's function Monte Carlo
technique. The ground state energy, first fermionic excited state, density wave
correlations, and mean lattice displacement are calculated for lattices of up
to 16 sites, for one fermion per two sites, i.e., a half-filled band. Results
are obtained for values of the fermion hopping parameter of ,
, and where is the phonon frequency. At a finite
fermion-phonon coupling there is a transition from a metallic phase to an
insulating phase in which there is charge-density-wave order. Finite size
scaling is found to hold in the metallic phase and is used to extract the
coupling dependence of the Luttinger liquid parameters, and ,
the velocity of charge excitations and the correlation exponent, respectively.
For free fermions () and for strong coupling () our
results agree well with known analytic results. For and
our results are inconsistent with the metal-insulator transition being a
Kosterlitz-Thouless transition.\\Comment: 16 pages of ReVTeX, 11 figures in uuencoded compressed tar file.
Minor changes to text. Our results are inconsistent with the metal-insulator
transition studied being a Kosterlitz-Thouless transition. The figures are
now in the correct order. To appear in Physical Review B, April 15, 199
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