3,256 research outputs found
Effect of low-level, low-frequency electric fields on EEG and behavior in Macaca nemestrina
Effect of low level, low frequency electric fields on EEG and behavior of Macaca nemestrin
A closer look at symmetry breaking in the collinear phase of the Heisenberg Model
The large limit of the square-lattice Heisenberg
antiferromagnet is a classic example of order by disorder where quantum
fluctuations select a collinear ground state. Here, we use series expansion
methods and a meanfield spin-wave theory to study the excitation spectra in
this phase and look for a finite temperature Ising-like transition,
corresponding to a broken symmetry of the square-lattice, as first proposed by
Chandra et al. (Phys. Rev. Lett. 64, 88 (1990)). We find that the spectra
reveal the symmetries of the ordered phase. However, we do not find any
evidence for a finite temperature phase transition. Based on an effective field
theory we argue that the Ising-like transition occurs only at zero temperature.Comment: 4 pages and 5 figure
Critical Behaviour of Structure Factors at a Quantum Phase Transition
We review the theoretical behaviour of the total and one-particle structure
factors at a quantum phase transition for temperature T=0. The predictions are
compared with exact or numerical results for the transverse Ising model, the
alternating Heisenberg chain, and the bilayer Heisenberg model. At the critical
wavevector, the results are generally in accord with theoretical expectations.
Away from the critical wavevector, however, different models display quite
different behaviours for the one-particle residues and structure factors.Comment: 17 pp, 10 figure
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
Realization of a large J_2 quasi-2D spin-half Heisenberg system: Li2VOSiO4
Exchange couplings are calculated for Li2VOSiO4 using LDA. While the sum of
in-plane couplings J_1 + J_2 = 9.5 \pm 1.5 K and the inter-plane coupling
J_{perp} \sim 0.2 - 0.3 K agree with recent experimental data, the ratio
J_2/J_1 \sim 12 exceeds the reported value by an order of magnitude. Using
geometrical considerations, high temperature expansions and perturbative mean
field theory, we show that the LDA derived exchange constants lead to a
remarkably accurate description of the properties of these materials including
specific heat, susceptibility, Neel temperature and NMR spectra.Comment: 4 two-column pages, 4 embedded postscript figure
Low energy states with different symmetries in the t-J model with two holes on a 32-site lattice
We study the low energy states of the t-J model with two holes on a 32-site
lattice with periodic boundary conditions. In contrary to common belief, we
find that the state with d_{x^2-y^2} symmetry is not always the ground state in
the realistic parameter range 0.2\le J/t\le 0.4. There exist low-lying
finite-momentum p-states whose energies are lower than the d_{x^2-y^2} state
when J/t is small enough. We compare various properties of these low energy
states at J/t=0.3 where they are almost degenerate, and find that those
properties associated with the holes (such as the hole-hole correlation and the
electron momentum distribution function) are very different between the
d_{x^2-y^2} and p states, while their spin properties are very similar.
Finally, we demonstrate that by adding ``realistic'' terms to the t-J model
Hamiltonian, we can easily destroy the d_{x^2-y^2} ground state. This casts
doubt on the robustness of the d_{x^2-y^2} state as the ground state in a
microscopic model for the high temperature superconductors
Single hole dynamics in the t-J model on a square lattice
We present quantum Monte Carlo (QMC) simulations for a single hole in a t-J
model from J=0.4t to J=4t on square lattices with up to 24 x 24 sites. The
lower edge of the spectrum is directly extracted from the imaginary time
Green's function. In agreement with earlier calculations, we find flat bands
around , and the minimum of the dispersion at
. For small J both self-consistent Born approximation and
series expansions give a bandwidth for the lower edge of the spectrum in
agreement with the simulations, whereas for J/t > 1, only series expansions
agree quantitatively with our QMC results. This band corresponds to a coherent
quasiparticle. This is shown by a finite size scaling of the quasiparticle
weight that leads to a finite result in the thermodynamic limit for
the considered values of . The spectral function is
obtained from the imaginary time Green's function via the maximum entropy
method. Resonances above the lowest edge of the spectrum are identified, whose
J-dependence is quantitatively described by string excitations up to J/t=2
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
Impurity in a Luttinger liquid away from half-filling: a numerical study
Conformal field theory gives quite detailed predictions for the low energy
spectrum and scaling exponents of a massless Luttinger liquid at generic
filling in the presence of an impurity. While these predictions were verified
for half-filled systems, there was till now no analysis away from this
particular filling. Here, we fill in this gap by numerically investigating a
quarter-filled system using the density matrix renormalization group technique.
Our results confirm conformal field theory predictions, and suggest that they
are indeed valid for arbitrary fillings.Comment: 9 pages (include figures), one reference added in this new versio
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