3,580 research outputs found
Density of critical clusters in strips of strongly disordered systems
We consider two models with disorder dominated critical points and study the
distribution of clusters which are confined in strips and touch one or both
boundaries. For the classical random bond Potts model in the large-q limit we
study optimal Fortuin-Kasteleyn clusters by combinatorial optimization
algorithm. For the random transverse-field Ising chain clusters are defined and
calculated through the strong disorder renormalization group method. The
numerically calculated density profiles close to the boundaries are shown to
follow scaling predictions. For the random bond Potts model we have obtained
accurate numerical estimates for the critical exponents and demonstrated that
the density profiles are well described by conformal formulae.Comment: 9 pages, 9 figure
Multiple Magnon Modes and Consequences for the Bose-Einstein Condensed Phase in BaCuSi2O6
The compound BaCuSi2O6 is a quantum magnet with antiferromagnetic dimers of S
= 1/2 moments on a quasi-2D square lattice. We have investigated its spin
dynamics by inelastic neutron scattering experiments on single crystals with an
energy resolution considerably higher than in an earlier study. We observe
multiple magnon modes, indicating clearly the presence of magnetically
inequivalent dimer sites. This more complex spin Hamiltonian leads to a
distinct form of magnon Bose-Einstein condensate (BEC) phase with a spatially
modulated condensate amplitude.Comment: 5 pages, 4 figures, to be published in Phys. Rev. Let
Universality and the five-dimensional Ising model
We solve the long-standing discrepancy between Monte Carlo results and the
renormalization prediction for the Binder cumulant of the five-dimensional
Ising model. Our conclusions are based on accurate Monte Carlo data for systems
with linear sizes up to L=22. A detailed analysis of the corrections to scaling
allows the extrapolation of these results to L=\infinity. Our determination of
the critical point, K_c=0.1139150 (4), is more than an order of magnitude more
accurate than previous estimates.Comment: 6 pages LaTeX, 1 PostScript figure. Uses cite.sty (included) and
epsf.sty. Also available as PostScript and PDF file at
http://www.tn.tudelft.nl/tn/erikpubs.htm
Existence of temperature on the nanoscale
We consider a regular chain of quantum particles with nearest neighbour
interactions in a canonical state with temperature . We analyse the
conditions under which the state factors into a product of canonical density
matrices with respect to groups of particles each and under which these
groups have the same temperature . In quantum mechanics the minimum group
size depends on the temperature , contrary to the classical case.
We apply our analysis to a harmonic chain and find that for
temperatures above the Debye temperature and below.Comment: Version that appeared in PR
Universality of the Ising Model on Sphere-like Lattices
We study the 2D Ising model on three different types of lattices that are
topologically equivalent to spheres. The geometrical shapes are reminiscent of
the surface of a pillow, a 3D cube and a sphere, respectively. Systems of
volumes ranging up to O() sites are simulated and finite size scaling is
analyzed. The partition function zeros and the values of various cumulants at
their respective peak positions are determined and they agree with the scaling
behavior expected from universality with the Onsager solution on the torus
(). For the pseudocritical values of the coupling we find significant
anomalies indicating a shift exponent for sphere-like lattice
topology.Comment: 24 pages, LaTeX, 8 figure
Collective pinning of a frozen vortex liquid in ultrathin superconducting YBa_2Cu_3O_7 films
The linear dynamic response of the two-dimensional (2D) vortex medium in
ultrathin YBa_2Cu_3O_7 films was studied by measuring their ac sheet impedance
Z over a broad range of frequencies \omega. With decreasing temperature the
dissipative component of Z exhibits, at a temperature T*(\omega) well above the
melting temperature of a 2D vortex crystal, a crossover from a thermally
activated regime involving single vortices to a regime where the response has
features consistent with a description in terms of a collectively pinned vortex
manifold. This suggests the idea of a vortex liquid which, below T*(\omega),
appears to be frozen at the time scales 1/\omega of the experiments.Comment: 4 pages, 3 figures, submitted to Phys. Rev. Let
Quantum Critical Point of the XY Model and Condensation of Field-Induced Quasiparticles in Dimer Compounds
The quantum critical point of the three-dimensional XY model in a
symmetry-preserving field is investigated. The results of Monte Carlo
simulations with the directed-loop algorithm show that the quantum critical
behavior is characterized by the mean-field values of critical exponents. The
system-size dependence of various quantities is compared to a simple
field-theoretical argument that supports the mean-field scaling
Nonlinear AC resistivity in s-wave and d-wave disordered granular superconductors
We model s-wave and d-wave disordered granular superconductors with a
three-dimensional lattice of randomly distributed Josephson junctions with
finite self-inductance. The nonlinear ac resistivity of these systems was
calculated using Langevin dynamical equations. The current amplitude dependence
of the nonlinear resistivity at the peak position is found to be a power law
characterized by exponent . The later is not universal but depends on
the self-inductance and current regimes. In the weak current regime is
independent of the self-inductance and equal to 0.5 or both of s- and d-wave
materials. In the strong current regime this exponent depends on the screening.
We find for some interval of inductance which agrees with
the experimental finding for d-wave ceramic superconductors.Comment: 4 pages, 5 figures, to appear in Phys. Rev. Let
Mesoscopic phase separation in La2CuO4.02 - a 139La NQR study
In crystals of La2CuO4.02 oxygen diffusion can be limited to such small
length scales, that the resulting phase separation is invisible for neutrons.
Decomposition of the 139La NQR spectra shows the existence of three different
regions, of which one orders antiferromagnetically below 17K concomitantly with
the onset of a weak superconductivity in the crystal. These regions are
compared to the macroscopic phases seen previously in the title compound and
the cluster-glass and striped phases reported for the underdoped Sr-doped
cuprates.Comment: 4 pages, RevTeX, 5 figures, to be published in PR
Random walks and polymers in the presence of quenched disorder
After a general introduction to the field, we describe some recent results
concerning disorder effects on both `random walk models', where the random walk
is a dynamical process generated by local transition rules, and on `polymer
models', where each random walk trajectory representing the configuration of a
polymer chain is associated to a global Boltzmann weight. For random walk
models, we explain, on the specific examples of the Sinai model and of the trap
model, how disorder induces anomalous diffusion, aging behaviours and Golosov
localization, and how these properties can be understood via a strong disorder
renormalization approach. For polymer models, we discuss the critical
properties of various delocalization transitions involving random polymers. We
first summarize some recent progresses in the general theory of random critical
points : thermodynamic observables are not self-averaging at criticality
whenever disorder is relevant, and this lack of self-averaging is directly
related to the probability distribution of pseudo-critical temperatures
over the ensemble of samples of size . We describe the
results of this analysis for the bidimensional wetting and for the
Poland-Scheraga model of DNA denaturation.Comment: 17 pages, Conference Proceedings "Mathematics and Physics", I.H.E.S.,
France, November 200
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