2,359 research outputs found

### Vortex glass transitions in disordered three-dimensional XY models: Simulations for several different sets of parameters

The anisotropic frustrated 3D XY model with strong disorder in the coupling
constants is studied as a model of a disordered superconductor in an applied
magnetic field. Simulations with the exchange Monte Carlo method are performed
for frustrations f=1/5 and f=1/4, corresponding to two different values of the
magnetic field along the z direction. The anisotropy is also varied. The
determination of the helicity modulus from twist histograms is discussed in
some detail and the helicity modulus is used in finite size scaling analyses of
the vortex glass transition. The general picture is that the behavior in [Phys.
Rev. Lett. 91, 077002 (2003)] is confirmed. For strong (e.g. isotropic)
coupling in the z direction the helicity modulus fails to scale and it is
argued that this is due to a too small effective randomness of such systems for
the accessible system sizes

### Counterflow measurements in strongly correlated GaAs hole bilayers: evidence for electron-hole pairing

We study interacting GaAs bilayer hole systems, with very small interlayer
tunneling, in a counterflow geometry where equal currents are passed in
opposite directions in the two, independently contacted layers. At low
temperatures, both the longitudinal and Hall counterflow resistances tend to
vanish in the quantum Hall state at total bilayer filling $\nu=1$,
demonstrating the pairing of oppositely charged carriers in opposite layers.
The temperature dependence of the counterflow Hall resistance is anomalous
compared to the other transport coefficients: even at relatively high
temperatures ($\sim$600mK), it develops a very deep minimum, with a value that
is about an order of magnitude smaller than the longitudinal counterflow
resistivity.Comment: 4+ pages, 4 figure

### Dynamics of vortex glass phase in strongly type II superconductors

Dynamics of vortices in strongly type-II superconductors with strong disorder
is investigated within the frustrated three-dimensional XY model. For two
typical models in [Phys. Rev. Lett. {\bf 91}, 077002 (2003)] and [Phys. Rev. B
{\bf 68}, 220502(R) (2003)], a strong evidence for the finite temperature
vortex glass transition in the unscreened limit is provided by performing
large-scale dynamical simulations. The obtained correlation length exponents
and the dynamic exponents in both models are different from each other and from
those in the three-dimensional gauge glass model. In addition, a genuine
continuous depinning transition is observed at zero temperature for both
models. A scaling analysis for the thermal rounding of the depinning transition
shows a non-Arrhenius type creep motion in the vortex glass phase, contrarily
to the recent studies..Comment: 6 pages, 5 figure

### Localized systems coupled to small baths: from A$_{nderson}$ to Z$_{eno}$

We investigate what happens if an Anderson localized system is coupled to a
small bath, with a discrete spectrum, when the coupling between system and bath
is specially chosen so as to never localize the bath. We find that the effect
of the bath on localization in the system is a non-monotonic function of the
coupling between system and bath. At weak couplings, the bath facilitates
transport by allowing the system to 'borrow' energy from the bath. But above a
certain coupling the bath produces localization, because of an orthogonality
catastrophe, whereby the bath 'dresses' the system and hence suppresses the
hopping matrix element. We call this last regime the regime of
"Zeno-localization", since the physics of this regime is akin to the quantum
Zeno effect, where frequent measurements of the position of a particle impede
its motion. We confirm our results by numerical exact diagonalization

### Permutation-Symmetric Multicritical Points in Random Antiferromagnetic Spin Chains

The low-energy properties of a system at a critical point may have additional
symmetries not present in the microscopic Hamiltonian. This letter presents the
theory of a class of multicritical points that provide an interesting example
of this in the phase diagrams of random antiferromagnetic spin chains. One case
provides an analytic theory of the quantum critical point in the random
spin-3/2 chain, studied in recent work by Refael, Kehrein and Fisher
(cond-mat/0111295).Comment: Revtex, 4 pages (2 column format), 2 eps figure

### Universality and Crossover of Directed Polymers and Growing Surfaces

We study KPZ surfaces on Euclidean lattices and directed polymers on
hierarchical lattices subject to different distributions of disorder, showing
that universality holds, at odds with recent results on Euclidean lattices.
Moreover, we find the presence of a slow (power-law) crossover toward the
universal values of the exponents and verify that the exponent governing such
crossover is universal too. In the limit of a 1+epsilon dimensional system we
obtain both numerically and analytically that the crossover exponent is 1/2.Comment: LateX file + 5 .eps figures; to appear on Phys. Rev. Let

### Zero Temperature Dynamics of the Weakly Disordered Ising Model

The Glauber dynamics of the pure and weakly disordered random-bond 2d Ising
model is studied at zero-temperature. A single characteristic length scale,
$L(t)$, is extracted from the equal time correlation function. In the pure
case, the persistence probability decreases algebraically with the coarsening
length scale. In the disordered case, three distinct regimes are identified: a
short time regime where the behaviour is pure-like; an intermediate regime
where the persistence probability decays non-algebraically with time; and a
long time regime where the domains freeze and there is a cessation of growth.
In the intermediate regime, we find that $P(t)\sim L(t)^{-\theta'}$, where
$\theta' = 0.420\pm 0.009$. The value of $\theta'$ is consistent with that
found for the pure 2d Ising model at zero-temperature. Our results in the
intermediate regime are consistent with a logarithmic decay of the persistence
probability with time, $P(t)\sim (\ln t)^{-\theta_d}$, where $\theta_d =
0.63\pm 0.01$.Comment: references updated, very minor amendment to abstract and the
labelling of figures. To be published in Phys Rev E (Rapid Communications), 1
March 199

### Numerical Results for the Ground-State Interface in a Random Medium

The problem of determining the ground state of a $d$-dimensional interface
embedded in a $(d+1)$-dimensional random medium is treated numerically. Using a
minimum-cut algorithm, the exact ground states can be found for a number of
problems for which other numerical methods are inexact and slow. In particular,
results are presented for the roughness exponents and ground-state energy
fluctuations in a random bond Ising model. It is found that the roughness
exponent $\zeta = 0.41 \pm 0.01, 0.22 \pm 0.01$, with the related energy
exponent being $\theta = 0.84 \pm 0.03, 1.45 \pm 0.04$, in $d = 2, 3$,
respectively. These results are compared with previous analytical and numerical
estimates.Comment: 10 pages, REVTEX3.0; 3 ps files (separate:tar/gzip/uuencoded) for
figure

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