13,317 research outputs found
Defect energy of infinite-component vector spin glasses
We compute numerically the zero temperature defect energy, Delta E, of the
vector spin glass in the limit of an infinite number of spin components m, for
a range of dimensions 2 <= d <= 5. Fitting to Delta E ~ L^theta, where L is the
system size, we obtain: theta = -1.54 (d=2), theta = -1.04 (d=3), theta = -0.67
(d=4) and theta = -0.37 (d=5). These results show that the lower critical
dimension, d_l (the dimension where theta changes sign), is significantly
higher for m=infinity than for finite m (where 2 < d_l < 3).Comment: 4 pages, 5 figure
Critical behavior of the three- and ten-state short-range Potts glass: A Monte Carlo study
We study the critical behavior of the short-range p-state Potts spin glass in
three and four dimensions using Monte Carlo simulations. In three dimensions,
for p = 3, a finite-size scaling analysis of the correlation length shows clear
evidence of a transition to a spin-glass phase at T_c = 0.273(5) for a Gaussian
distribution of interactions and T_c = 0.377(5) for a bimodal distribution.
These results indicate that the lower critical dimension of the 3-state Potts
glass is below three. By contrast, the correlation length of the ten-state (p =
10) Potts glass in three dimensions remains small even at very low temperatures
and thus shows no sign of a transition. In four dimensions we find that the p =
3 Potts glass with Gaussian interactions has a spin-glass transition at T_c
=0.536(3).Comment: 11 pages, 13 figures, 6 table
Spin glasses in the limit of an infinite number of spin components
We consider the spin glass model in which the number of spin components, m,
is infinite. In the formulation of the problem appropriate for numerical
calculations proposed by several authors, we show that the order parameter
defined by the long-distance limit of the correlation functions is actually
zero and there is only "quasi long range order" below the transition
temperature. We also show that the spin glass transition temperature is zero in
three dimensions.Comment: 9 pages, 13 figure
Large-scale Monte Carlo simulations of the isotropic three-dimensional Heisenberg spin glass
We study the Heisenberg spin glass by large-scale Monte Carlo simulations for
sizes up to 32^3, down to temperatures below the transition temperature claimed
in earlier work. The data for the larger sizes show more marginal behavior than
that for the smaller sizes, indicating the lower critical dimension is close
to, and possibly equal to three. We find that the spins and chiralities behave
in a quite similar manner.Comment: 8 pages, 8 figures. Replaced with published versio
Single spin- and chiral-glass transition in vector spin glasses in three-dimensions
Results of Monte Carlo simulations of XY and Heisenberg spin glass models in
three dimensions are presented. A finite size scaling analysis of the
correlation length of the spins and chiralities of both models shows that there
is a single, finite-temperature transition at which both spins and chiralities
order.Comment: 5 pages, 5 figures. Replaced by published versio
Orientational Melting in Carbon Nanotube Ropes
Using Monte Carlo simulations, we investigate the possibility of an
orientational melting transition within a "rope" of (10,10) carbon nanotubes.
When twisting nanotubes bundle up during the synthesis, orientational
dislocations or twistons arise from the competition between the anisotropic
inter-tube interactions, which tend to align neighboring tubes, and the torsion
rigidity that tends to keep individual tubes straight. We map the energetics of
a rope containing twistons onto a lattice gas model and find that the onset of
a free "diffusion" of twistons, corresponding to orientational melting, occurs
at T_OM > 160 K.Comment: 4 page LaTeX file with 3 figures (10 PostScript files
Criticality in the two-dimensional random-bond Ising model
The two-dimensional (2D) random-bond Ising model has a novel multicritical
point on the ferromagnetic to paramagnetic phase boundary. This random phase
transition is one of the simplest examples of a 2D critical point occurring at
both finite temperatures and disorder strength. We study the associated
critical properties, by mapping the random 2D Ising model onto a network model.
The model closely resembles network models of quantum Hall plateau transitions,
but has different symmetries. Numerical transfer matrix calculations enable us
to obtain estimates for the critical exponents at the random Ising phase
transition. The values are consistent with recent estimates obtained from
high-temperature series.Comment: minor changes, 7 pages LaTex, 8 postscript figures included using
epsf; to be published Phys. Rev. B 55 (1997
Theory of Tunneling Anomaly in Superconductor above Paramagnetic Limit
We study the tunneling density of states (DoS) in the superconducting systems
driven by Zeeman splitting into the paramagnetic phase. We show that,
even though the BCS gap disappears, superconducting fluctuations cause a strong
DoS singularity in the vicinity of energies for electrons polarized
along the magnetic field and for the opposite polarization. The position
of this singularity E^*=\case{1}{2}(E_Z + \sqrt{E_Z^2- \Delta^2}) (where
is BCS gap at ) is universal. We found analytically the shape
of the DoS for different dimensionality of the system. For ultra-small grains
the singularity has the shape of the hard gap, while in higher dimensions it
appears as a significant though finite dip. The spin-orbit scattering, and the
orbital magnetic field suppress the singularity. Our results are qualitatively
consistent with recent experiments in superconducting films.Comment: 29 pages, 17 figures include
Parton energy loss limits and shadowing in Drell-Yan dimuon production
A precise measurement of the ratios of the Drell-Yan cross section per
nucleon for an 800 GeV/c proton beam incident on Be, Fe and W targets is
reported. The behavior of the Drell-Yan ratios at small target parton momentum
fraction is well described by an existing fit to the shadowing observed in
deep-inelastic scattering. The cross section ratios as a function of the
incident parton momentum fraction set tight limits on the energy loss of quarks
passing through a cold nucleus
Ordering of the Heisenberg spin glass in two dimensions
The spin and the chirality orderings of the Heisenberg spin glass in two
dimensions with the nearest-neighbor Gaussian coupling are investigated by
equilibrium Monte Carlo simulations. Particular attention is paid to the
behavior of the spin and the chirality correlation lengths. In order to observe
the true asymptotic behavior, fairly large system size L\gsim 20 (L the linear
dimension of the system) appears to be necessary. It is found that both the
spin and the chirality order only at zero temperature. At high temperatures,
the chiral correlation length stays shorter than spin correlation length,
whereas at lower temperatures below the crossover temperature T_\times, the
chiral correlation length exceeds the spin correlation length. The spin and the
chirality correlation-length exponents are estimated above T_\times to be
\nu_SG=0.9+-0.2 and \nu_CG=2.1+-0.3, respectively. These values are close to
the previous estimates on the basis of the domain-wall-energy calculation.
Discussion is given about the asymptotic critical behavior realized below
T_\times.Comment: to appear in a special issue of J. Phys.
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