9,518 research outputs found
Ising metamagnets in thin film geometry: equilibrium properties
Artificial antiferromagnets and synthetic metamagnets have attracted much
attention recently due to their potential for many different applications.
Under some simplifying assumptions these systems can be modeled by thin Ising
metamagnetic films. In this paper we study, using both the Wang/Landau scheme
and importance sampling Monte Carlo simulations, the equilibrium properties of
these films. On the one hand we discuss the microcanonical density of states
and its prominent features. On the other we analyze canonically various global
and layer quantities. We obtain the phase diagram of thin Ising metamagnets as
a function of temperature and external magnetic field. Whereas the phase
diagram of the bulk system only exhibits one phase transition between the
antiferromagnetic and paramagnetic phases, the phase diagram of thin Ising
metamagnets includes an additional intermediate phase where one of the surface
layers has aligned itself with the direction of the applied magnetic field.
This additional phase transition is discontinuous and ends in a critical end
point. Consequently, it is possible to gradually go from the antiferromagnetic
phase to the intermediate phase without passing through a phase transition.Comment: 8 figures, accepted for publication in Physical Review
On locations and properties of the multicritical point of Gaussian and +/-J Ising spin glasses
We use transfer-matrix and finite-size scaling methods to investigate the
location and properties of the multicritical point of two-dimensional Ising
spin glasses on square, triangular and honeycomb lattices, with both binary and
Gaussian disorder distributions. For square and triangular lattices with binary
disorder, the estimated position of the multicritical point is in numerical
agreement with recent conjectures regarding its exact location. For the
remaining four cases, our results indicate disagreement with the respective
versions of the conjecture, though by very small amounts, never exceeding 0.2%.
Our results for: (i) the correlation-length exponent governing the
ferro-paramagnetic transition; (ii) the critical domain-wall energy amplitude
; (iii) the conformal anomaly ; (iv) the finite-size susceptibility
exponent ; and (v) the set of multifractal exponents
associated to the moments of the probability distribution of spin-spin
correlation functions at the multicritical point, are consistent with
universality as regards lattice structure and disorder distribution, and in
good agreement with existing estimates.Comment: RevTeX 4, 9 pages, 2 .eps figure
Survival probabilities in time-dependent random walks
We analyze the dynamics of random walks in which the jumping probabilities
are periodic {\it time-dependent} functions. In particular, we determine the
survival probability of biased walkers who are drifted towards an absorbing
boundary. The typical life-time of the walkers is found to decrease with an
increment of the oscillation amplitude of the jumping probabilities. We discuss
the applicability of the results in the context of complex adaptive systems.Comment: 4 pages, 3 figure
Survival Probabilities of History-Dependent Random Walks
We analyze the dynamics of random walks with long-term memory (binary chains
with long-range correlations) in the presence of an absorbing boundary. An
analytically solvable model is presented, in which a dynamical phase-transition
occurs when the correlation strength parameter \mu reaches a critical value
\mu_c. For strong positive correlations, \mu > \mu_c, the survival probability
is asymptotically finite, whereas for \mu < \mu_c it decays as a power-law in
time (chain length).Comment: 3 pages, 2 figure
Non-universal dynamics of dimer growing interfaces
A finite temperature version of body-centered solid-on-solid growth models
involving attachment and detachment of dimers is discussed in 1+1 dimensions.
The dynamic exponent of the growing interface is studied numerically via the
spectrum gap of the underlying evolution operator. The finite size scaling of
the latter is found to be affected by a standard surface tension term on which
the growth rates depend. This non-universal aspect is also corroborated by the
growth behavior observed in large scale simulations. By contrast, the
roughening exponent remains robust over wide temperature ranges.Comment: 11 pages, 7 figures. v2 with some slight correction
Quasiperiodic spin-orbit motion and spin tunes in storage rings
We present an in-depth analysis of the concept of spin precession frequency
for integrable orbital motion in storage rings. Spin motion on the periodic
closed orbit of a storage ring can be analyzed in terms of the Floquet theorem
for equations of motion with periodic parameters and a spin precession
frequency emerges in a Floquet exponent as an additional frequency of the
system. To define a spin precession frequency on nonperiodic synchro-betatron
orbits we exploit the important concept of quasiperiodicity. This allows a
generalization of the Floquet theorem so that a spin precession frequency can
be defined in this case too. This frequency appears in a Floquet-like exponent
as an additional frequency in the system in analogy with the case of motion on
the closed orbit. These circumstances lead naturally to the definition of the
uniform precession rate and a definition of spin tune. A spin tune is a uniform
precession rate obtained when certain conditions are fulfilled. Having defined
spin tune we define spin-orbit resonance on synchro--betatron orbits and
examine its consequences. We give conditions for the existence of uniform
precession rates and spin tunes (e.g. where small divisors are controlled by
applying a Diophantine condition) and illustrate the various aspects of our
description with several examples. The formalism also suggests the use of
spectral analysis to ``measure'' spin tune during computer simulations of spin
motion on synchro-betatron orbits.Comment: 62 pages, 1 figure. A slight extension of the published versio
Phase-Transition in Binary Sequences with Long-Range Correlations
Motivated by novel results in the theory of correlated sequences, we analyze
the dynamics of random walks with long-term memory (binary chains with
long-range correlations). In our model, the probability for a unit bit in a
binary string depends on the fraction of unities preceding it. We show that the
system undergoes a dynamical phase-transition from normal diffusion, in which
the variance D_L scales as the string's length L, into a super-diffusion phase
(D_L ~ L^{1+|alpha|}), when the correlation strength exceeds a critical value.
We demonstrate the generality of our results with respect to alternative
models, and discuss their applicability to various data, such as coarse-grained
DNA sequences, written texts, and financial data.Comment: 4 pages, 4 figure
On the finite-size behavior of systems with asymptotically large critical shift
Exact results of the finite-size behavior of the susceptibility in
three-dimensional mean spherical model films under Dirichlet-Dirichlet,
Dirichlet-Neumann and Neumann-Neumann boundary conditions are presented. The
corresponding scaling functions are explicitly derived and their asymptotics
close to, above and below the bulk critical temperature are obtained. The
results can be incorporated in the framework of the finite-size scaling theory
where the exponent characterizing the shift of the finite-size
critical temperature with respect to is smaller than , with
being the critical exponent of the bulk correlation length.Comment: 24 pages, late
Continuous phase transitions with a convex dip in the microcanonical entropy
The appearance of a convex dip in the microcanonical entropy of finite
systems usually signals a first order transition. However, a convex dip also
shows up in some systems with a continuous transition as for example in the
Baxter-Wu model and in the four-state Potts model in two dimensions. We
demonstrate that the appearance of a convex dip in those cases can be traced
back to a finite-size effect. The properties of the dip are markedly different
from those associated with a first order transition and can be understood
within a microcanonical finite-size scaling theory for continuous phase
transitions. Results obtained from numerical simulations corroborate the
predictions of the scaling theory.Comment: 8 pages, 7 figures, to appear in Phys. Rev.
Symmetries and noise in quantum walk
We study some discrete symmetries of unbiased (Hadamard) and biased quantum
walk on a line, which are shown to hold even when the quantum walker is
subjected to environmental effects. The noise models considered in order to
account for these effects are the phase flip, bit flip and generalized
amplitude damping channels. The numerical solutions are obtained by evolving
the density matrix, but the persistence of the symmetries in the presence of
noise is proved using the quantum trajectories approach. We also briefly extend
these studies to quantum walk on a cycle. These investigations can be relevant
to the implementation of quantum walks in various known physical systems. We
discuss the implementation in the case of NMR quantum information processor and
ultra cold atoms.Comment: 19 pages, 24 figures : V3 - Revised version to appear in Phys. Rev.
A. - new section on quantum walk in a cycle include
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