112 research outputs found
Does the Two-Dimensional t-J Model have Hole Pockets?
We have calculated the high temperature series for the momentum distribution
function n_k of the 2D t-J model to 12th order in inverse temperature. By
extrapolating the series to T=0.2J we investigate the possibility of hole
pockets in the t-J model. We find no indication of hole pockets at an electron
density of n=0.9 with J/t=0.5 or J/t=1.0.Comment: 2 pages, 2 figures. Contribution to Spectroscopies of Novel
Superconductors 97, Cape Cod, M
Scaling Regimes, Crossovers, and Lattice Corrections in 2D Heisenberg Antiferromagnets
We study scaling behavior in 2D, S=1/2 and S=1 Heisenberg antiferromagnets
using the data on full q-dependences of the equal time structure factor and the
static susceptibility, calculated through high temperature expansions. We also
carry out comparisons with a model of two coupled S=1/2 planes with the
interlayer coupling tuned to the T=0 critical point. We separately determine
the spin-wave velocity c and mass , in addition to the correlation
length, , and find that c is temperature dependent; only for T\alt JS,
it approaches its known T=0 value . Despite this temperature dependent
spin-wave velocity, full q- and -dependences of the dynamical
susceptibility agree with the universal scaling functions
computable for the -model, for temperatures upto .
Detailed comparisons show that below the S=1 model is in the renormalized
classical (RC) regime, the two plane model is in the quantum critical (QC)
regime, and the S=1/2 model exhibits a RC-QC crossover, centered at T=0.55J. In
particular, for the S=1/2 model above this crossover and for the two-plane
model at all T, the spin-wave mass is in excellent agreement with the universal
QC prediction, . In contrast, for the S=1/2 model below the
RC-QC crossover, and for the S=1 model at all T, the behavior agrees with the
known RC expression. For all models nonuniversal behavior occurs above . Our results strongly support the conjecture of Chubukov and Sachdev
that the S=1/2 model is close to the T=0 critical point to exhibit QC behavior.Comment: 13 pages, REVTeX with attached PostScript (see file for addl info
Studies of Quantum Spin Ladders at T=0 and at High Temperatures by Series Expansions
We have carried out extensive series studies, at T=0 and at high
temperatures, of 2-chain and 3-chain spin-half ladder systems with
antiferromagnetic intrachain and both antiferromagnetic and ferromagnetic
interchain couplings. Our results confirm the existence of a gap in the 2-chain
Heisenberg ladders for all non-zero values of the interchain couplings.
Complete dispersion relations for the spin-wave excitations are computed. For
3-chain systems, our results are consistent with a gapless spectrum. We also
calculate the uniform magnetic susceptibility and specific heat as a function
of temperature. We find that as , for the 2-chain system the uniform
susceptibility goes rapidly to zero, whereas for the 3-chain system it
approaches a finite value. These results are compared in detail with previous
studies of finite systems.Comment: RevTeX, 14 figure
Critical exponents in Ising spin glasses
We determine accurate values of ordering temperatures and critical exponents
for Ising Spin Glass transitions in dimension 4, using a combination of finite
size scaling and non-equilibrium scaling techniques. We find that the exponents
and vary with the form of the interaction distribution, indicating
non-universality at Ising spin glass transitions. These results confirm
conclusions drawn from numerical data for dimension 3.Comment: 6 pages, RevTeX (or Latex, etc), 10 figures, Submitted to PR
Order-Disorder Transition in a Two-Layer Quantum Antiferromagnet
We have studied the antiferromagnetic order -- disorder transition occurring
at in a 2-layer quantum Heisenberg antiferromagnet as the inter-plane
coupling is increased. Quantum Monte Carlo results for the staggered structure
factor in combination with finite-size scaling theory give the critical ratio
between the inter-plane and in-plane coupling constants.
The critical behavior is consistent with the 3D classical Heisenberg
universality class. Results for the uniform magnetic susceptibility and the
correlation length at finite temperature are compared with recent predictions
for the 2+1-dimensional nonlinear -model. The susceptibility is found
to exhibit quantum critical behavior at temperatures significantly higher than
the correlation length.Comment: 11 pages (5 postscript figures available upon request), Revtex 3.
Phase diagram for a class of spin-half Heisenberg models interpolating between the square-lattice, the triangular-lattice and the linear chain limits
We study the spin-half Heisenberg models on an anisotropic two-dimensional
lattice which interpolates between the square-lattice at one end, a set of
decoupled spin-chains on the other end, and the triangular-lattice Heisenberg
model in between. By series expansions around two different dimer ground states
and around various commensurate and incommensurate magnetically ordered states,
we establish the phase diagram for this model of a frustrated antiferromagnet.
We find a particularly rich phase diagram due to the interplay of magnetic
frustration, quantum fluctuations and varying dimensionality. There is a large
region of the usual 2-sublattice Ne\'el phase, a 3-sublattice phase for the
triangular-lattice model, a region of incommensurate magnetic order around the
triangular-lattice model, and regions in parameter space where there is no
magnetic order. We find that the incommensurate ordering wavevector is in
general altered from its classical value by quantum fluctuations. The regime of
weakly coupled chains is particularly interesting and appears to be nearly
critical.Comment: RevTeX, 15 figure
Systematic 1/S study of the 2D Hubbard model at half-filling
The 2D Hubbard model is extended by placing 2S orbitals at each lattice site
and studied in a systematic 1/S expansion. The 1/S results for the magnetic
susceptibility and the spectra of spin-wave excitations at half-filling are
consistent with the large S calculations for the Heisenberg antiferromagnet.
The 1/S corrections to the fermionic spectrum lift the degeneracy along the
edge of the magnetic Brillouin zone yielding minima at (+- pi/2, +- pi/2).
Relation to previous papers on the subject is discussed.Comment: 18 pages, emTex version 3.
Spin Excitations in La2CuO4: Consistent Description by Inclusion of Ring-Exchange
We consider the square lattice Heisenberg antiferromagnet with plaquette ring
exchange and a finite interlayer coupling leading to a consistent description
of the spin-wave excitation spectrum in La2CuO4. The values of the in-plane
exchange parameters, including ring-exchange J_{\Box}, are obtained
consistently by an accurate fit to the experimentally observed in-plane
spin-wave dispersion, while the out-of-plane exchange interaction is found from
the temperature dependence of the sublattice magnetization at low temperatures.
The fitted exchange interactions J=151.9 meV and J_{\Box}=0.24 J give values
for the spin stiffness and the Neel temperature in excellent agreement with the
experimental data.Comment: 4 pages, 1 figure, RevTe
Spin-wave spectrum in La2CuO4 -- double occupancy and competing interaction effects
The recently observed spin-wave energy dispersion along the AF zone boundary
in La2CuO4 is discussed in terms of double occupancy and competing interaction
effects in the Hubbard model on a square lattice.Comment: 4 pages, 2 figure
Spin diffusion of the t-J model
The spin-diffusion constant of the 2D model is calculated for the first
time using an analytical approach at high temperatures and a recently-developed
numerical method based on the Lanczos technique combined with random sampling
in the intermediate temperature regime. A simple relation, ,
between spin conductivity and spin diffusion is established and used to
calculate the latter. In the high-temperature and low-doping limit the
calculated diffusion constant agrees with known results for the Heisenberg
model. At small hole doping, increases approximately linearly with
doping, which leads us to an important conclusion that hopping processes
enhance spin diffusion at high temperatures. At modest hole doping, , diffusion exhibits a nonmonotonic temperature dependence, which
indicates anomalous spin dynamics at small frequencies.Comment: 12 pages with figure
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