16 research outputs found
A Monte Carlo Study of Correlations in Quantum Spin Ladders
We study antiferromagnetic spin--1/2 Heisenberg ladders, comprised of
chains () with ratio of inter-- to
intra--chain couplings. From measurements of the correlation function we deduce
the correlation length . For even , the static structure factor
exhibits a peak at a temperature below the corresponding spin gap. Results for
isotropically coupled ladders () are compared to those for
the single chain and the square lattice. For , the
correlation function of the two--chain ladder is in excellent agreement with
analytic results from conformal field theory, and exhibits simple
scaling behavior.Comment: 4 pages, 5 EPS figures, submitted to Phys. Rev. Let
Thickness-Magnetic Field Phase Diagram at the Superconductor-Insulator Transition in 2D
The superconductor-insulator transition in ultrathin films of amorphous Bi
was tuned by changing the film thickness, with and without an applied magnetic
field. The first experimentally obtained phase diagram is mapped as a function
of thickness and magnetic field in the T=0 limit. A finite size scaling
analysis has been carried out to determine the critical exponent product vz,
which was found to be 1.2 for the zero field transition, and 1.4 for the finite
field transition. Both results are different from the exponents found for the
magnetic field tuned transition in the same system, 0.7.Comment: 4 pages, 4 figure
Critical behavior at superconductor-insulator phase transitions near one dimension
I argue that the system of interacting bosons at zero temperature and in
random external potential possesses a simple critical point which describes the
proliferation of disorder-induced topological defects in the superfluid ground
state, and which is located at weak disorder close to and above one dimension.
This makes it possible to address the critical behavior at the superfluid-Bose
glass transition in dirty boson systems by expanding around the lower critical
dimension d=1. Within the formulated renormalization procedure near d=1 the
dynamical critical exponent is obtained exactly and the correlation length
exponent is calculated as a Laurent series in the parameter \sqrt{\epsilon},
with \epsilon=d-1: z=d, \nu=1/\sqrt{3\epsilon} for the short range, and z=1,
\nu=\sqrt{2/3\epsilon}, for the long-range Coulomb interaction between bosons.
The identified critical point should be stable against the residual
perturbations in the effective action for the superfluid, at least in
dimensions 1\leq d \leq 2, for both short-range and Coulomb interactions. For
the superfluid-Mott insulator transition in the system in a periodic potential
and at a commensurate density of bosons I find \nu=(1/2\sqrt{\epsilon})+
1/4+O(\sqrt{\epsilon}), which yields a result reasonably close to the known XY
critical exponent in d=2+1. The critical behavior of the superfluid density,
phonon velocity and the compressibility in the system with the short-range
interactions is discussed.Comment: 23 pages, 1 Postscript figure, LaTe
Quantum magnetism in the stripe phase: bond- versus site order
It is argued that the spin dynamics in the charge-ordered stripe phase might
be revealing with regards to the nature of the anomalous spin dynamics in
cuprate superconductors. Specifically, if the stripes are bond ordered much of
the spin fluctuation will originate in the spin sector itself, while site
ordered stripes require the charge sector as the driving force for the strong
quantum spin fluctuations.Comment: 4 pages, 3 figures, LaTe
Numerical Calculations of the B1g Raman Spectrum of the Two-Dimensional Heisenberg Model
The B1g Raman spectrum of the two-dimensional S=1/2 Heisenberg model is
discussed within Loudon-Fleury theory at both zero and finite temperature. The
exact T=0 spectrum for lattices with up to 6*6 sites is computed using Lanczos
exact diagonalization. A quantum Monte Carlo (QMC) method is used to calculate
the corresponding imaginary-time correlation function and its first two
derivatives for lattices with up to 16*16 spins. The imaginary-time data is
continued to real frequency using the maximum-entropy method, as well as a fit
based on spinwave theory. The numerical results are compared with spinwave
calculations for finite lattices. There is a surprisingly large change in the
exact spectrum going from 4*4 to 6*6 sites. In the former case there is a
single dominant two-magnon peak at frequency w/J appr. 3.0, whereas in the
latter case there are two approximately equal-sized peaks at w/J appr. 2.7 and
3.9. This is in good qualitative agreement with the spinwave calculations
including two-magnon processes on the same lattices. Both the Lanczos and the
QMC results indicate that the actual infinite-size two-magnon profile is
broader than the narrow peak obtained in spinwave theory, but the positions of
the maxima agree to within a few percent. The higher-order contributions
present in the numerical results are merged with the two-magnon profile and
extend up to frequencies w/J appr. 7. The first three frequency cumulants of
the spectrum are in excellent agreement with results previously obtained from a
series expansion around the Ising limit. Typical experimental B1g$ spectra for
La2CuO4 are only slightly broader than what we obtain here. The exchange
constant extracted from the peak position is J appr. 1400K, in good agreement
with values obtained from neutron scattering and NMR experiments.Comment: 15 pages, Revtex, 13 PostScript figure
The superconductor-insulator transition in 2D dirty boson systems
Universal properties of the zero temperature superconductor-insulator
transition in two-dimensional amorphous films are studied by extensive Monte
Carlo simulations of bosons in a disordered medium. We report results for both
short-range and long-range Coulomb interactions for several different points in
parameter space. In all cases we observe a transition from a superconducting
phase to an insulating Bose glass phase. {}From finite-size scaling of our
Monte Carlo data we determine the universal conductivity and the
critical exponents at the transition. The result for bosons with long-range Coulomb interaction is roughly consistent
with experiments reported so far. We also find for bosons with short-range interactions.Comment: Revtex 3.0, 54 pages, 17 figures included, UBCTP-93-01
Spin dynamics of the 2D spin 1/2 quantum antiferromagnet copper deuteroformate tetradeuterate (CFTD)
The magnetic excitation spectrum in the two-dimensional (2D) S = 1/2 Heisenberg antiferromagnet copper deuteroformate tetradeuterate has been measured for temperatures up to T similar to J/2, where J = 6.31 +/- 0.02 meV is the 2D exchange coupling. For T << J, a dispersion of the zone boundary energy is observed, which is attributed to a wave vector dependent quantum renormalization. At higher temperatures, spin-wavelike excitations persist, but are found to broaden and soften. By combining our data with numerical calculations, and with existing theoretical work, a consistent description of the behavior of the model system is found over the whole temperature interval investigated