229 research outputs found
Zero Temperature Glass Transition in the Two-Dimensional Gauge Glass Model
We investigate dynamic scaling properties of the two-dimensional gauge glass
model for the vortex glass phase in superconductors with quenched disorder.
From extensive Monte Carlo simulations we obtain static and dynamic finite
size scaling behavior, where the static simulations use a temperature exchange
method to ensure convergence at low temperatures. Both static and dynamic
scaling of Monte Carlo data is consistent with a glass transition at zero
temperature. We study a dynamic correlation function for the superconducting
order parameter, as well as the phase slip resistance. From the scaling of
these two functions, we find evidence for two distinct diverging correlation
times at the zero temperature glass transition. The longer of these time scales
is associated with phase slip fluctuations across the system that lead to
finite resistance at any finite temperature, while the shorter time scale is
associated with local phase fluctuations.Comment: 8 pages, 10 figures; v2: some minor correction
On the existence of a finite-temperature transition in the two-dimensional gauge glass
Results from Monte Carlo simulations of the two-dimensional gauge glass
supporting a zero-temperature transition are presented. A finite-size scaling
analysis of the correlation length shows that the system does not exhibit
spin-glass order at finite temperatures. These results are compared to earlier
claims of a finite-temperature transition.Comment: 4 pages, 2 figure
Molecular Dynamics Study of the Nematic-Isotropic Interface
We present large-scale molecular dynamics simulations of a nematic-isotropic
interface in a system of repulsive ellipsoidal molecules, focusing in
particular on the capillary wave fluctuations of the interfacial position. The
interface anchors the nematic phase in a planar way, i.e., the director aligns
parallel to the interface. Capillary waves in the direction parallel and
perpendicular to the director are considered separately. We find that the
spectrum is anisotropic, the amplitudes of capillary waves being larger in the
direction perpendicular to the director. In the long wavelength limit, however,
the spectrum becomes isotropic and compares well with the predictions of a
simple capillary wave theory.Comment: to appear in Phys. Rev.
Numerical Study of Spin and Chiral Order in a Two-Dimensional XY Spin Glass
The two dimensional XY spin glass is studied numerically by a finite size
scaling method at T=0 in the vortex representation which allows us to compute
the exact (in principle) spin and chiral domain wall energies. We confirm
earlier predictions that there is no glass phase at any finite T. Our results
strongly support the conjecture that both spin and chiral order have the same
correlation length exponent . We obtain preliminary results
in 3d.Comment: 4 pages, 2 figures, revte
Numerical Study of Order in a Gauge Glass Model
The XY model with quenched random phase shifts is studied by a T=0 finite
size defect energy scaling method in 2d and 3d. The defect energy is defined by
a change in the boundary conditions from those compatible with the true ground
state configuration for a given realization of disorder. A numerical technique,
which is exact in principle, is used to evaluate this energy and to estimate
the stiffness exponent . This method gives in
2d and in 3d, which are considerably larger than
previous estimates, strongly suggesting that the lower critical dimension is
less than three. Some arguments in favor of these new estimates are given.Comment: 4 pages, 2 figures, revtex. Submitted to Phys. Rev. Let
Domain Wall Renormalization Group Study of XY Model with Quenched Random Phase Shifts
The XY model with quenched random disorder is studied by a zero temperature
domain wall renormalization group method in 2D and 3D. Instead of the usual
phase representation we use the charge (vortex) representation to compute the
domain wall, or defect, energy. For the gauge glass corresponding to the
maximum disorder we reconfirm earlier predictions that there is no ordered
phase in 2D but an ordered phase can exist in 3D at low temperature. However,
our simulations yield spin stiffness exponents in 2D
and in 3D, which are considerably larger than
previous estimates and strongly suggest that the lower critical dimension is
less than three. For the XY spin glass in 3D, we obtain a spin
stiffness exponent which supports the existence of
spin glass order at finite temperature in contrast with previous estimates
which obtain . Our method also allows us to study
renormalization group flows of both the coupling constant and the disorder
strength with length scale . Our results are consistent with recent analytic
and numerical studies suggesting the absence of a re-entrant transition in 2D
at low temperature. Some possible consequences and connections with real vortex
systems are discussed.Comment: 14 pages, 9 figures, revtex
Evolution of defence portfolios in exploiter-victim systems
Some organisms maintain a battery of defensive strategies against their exploiters (predators, parasites or parasitoids), while others fail to employ a defence that seems obvious. In this paper, we shall investigate the circumstances under which defence strategies might be expected to evolve. Brood parasites and their hosts provide our main motivation, and we shall discuss why the reed warbler Acrocephalus scirpaceus has evolved an egg-rejection but not a chick-rejection strategy as a defence against the common (Eurasian) cuckoo Cuculus canorus, while the superb fairy-wren Malurus cyaneus has evolved a chick-rejection but not an egg-rejection strategy as a defence against Horsfield's bronze-cuckoo Chrysococcyx basalis. We suggest that the answers lie in strategy-blocking, where one strategy (the blocking strategy) prevents the appearance of another (the blocked strategy) that would be adaptive in its absence. This may be common in exploiter-victim systems. © 2006 Springer Science+Business Media, Inc
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