2,199 research outputs found
Fluctuation-dissipation relation and the Edwards entropy for a glassy granular compaction model
We analytically study a one dimensional compaction model in the glassy
regime. Both correlation and response functions are calculated exactly in the
evolving dense and low tapping strength limit, where the density relaxes in a
fashion. The response and correlation functions turn out to be
connected through a non-equilibrium generalisation of the
fluctuation-dissipation theorem. The initial response in the average density to
an increase in the tapping strength is shown to be negative, while on longer
timescales it is shown to be positive. On short time scales the
fluctuation-dissipation theorem governs the relation between correlation and
response, and we show that such a relationship also exists for the slow degrees
of freedom, albeit with a different temperature. The model is further studied
within the statistical theory proposed by Edwards and co-workers, and the
Edwards entropy is calculated in the large system limit. The fluctuations
described by this approach turn out to match the fluctuations as calculated
through the dynamical consideration. We believe this to be the first time these
ideas have been analytically confirmed in a non-mean-field model.Comment: 4 pages, 3 figure
Domain wall theory and non-stationarity in driven flow with exclusion
We study the dynamical evolution toward steady state of the stochastic
non-equilibrium model known as totally asymmetric simple exclusion process, in
both uniform and non-uniform (staggered) one-dimensional systems with open
boundaries. Domain-wall theory and numerical simulations are used and, where
pertinent, their results are compared to existing mean-field predictions and
exact solutions where available. For uniform chains we find that the inclusion
of fluctuations inherent to the domain-wall formulation plays a crucial role in
providing good agreement with simulations, which is severely lacking in the
corresponding mean-field predictions. For alternating-bond chains the
domain-wall predictions for the features of the phase diagram in the parameter
space of injection and ejection rates turn out to be realized only in an
incipient and quantitatively approximate way. Nevertheless, significant
quantitative agreement can be found between several additional domain-wall
theory predictions and numerics.Comment: 12 pages, 12 figures (published version
Correlation--function distributions at the Nishimori point of two-dimensional Ising spin glasses
The multicritical behavior at the Nishimori point of two-dimensional Ising
spin glasses is investigated by using numerical transfer-matrix methods to
calculate probability distributions and associated moments of spin-spin
correlation functions on strips. The angular dependence of the shape of
correlation function distributions provides a stringent test of how well
they obey predictions of conformal invariance; and an even symmetry of reflects the consequences of the Ising spin-glass gauge (Nishimori)
symmetry. We show that conformal invariance is obeyed in its strictest form,
and the associated scaling of the moments of the distribution is examined, in
order to assess the validity of a recent conjecture on the exact localization
of the Nishimori point. Power law divergences of are observed near C=1
and C=0, in partial accord with a simple scaling scheme which preserves the
gauge symmetry.Comment: Final version to be published in Phys Rev
Dilution effect in correlated electron system with orbital degeneracy
Theory of dilution effect in orbital ordered system is presented. The
orbital model without spin degree of freedom and the spin-orbital coupled model
in a three-dimensional simple-cubic lattice are analyzed by the Monte-Carlo
simulation and the cluster expansion method. In the orbital model without
spin degree of freedom, reduction of the orbital ordering temperature due to
dilution is steeper than that in the dilute magnet. This is attributed to a
modification of the orbital wave-function around vacant sites. In the
spin-orbital coupled model, it is found that magnetic structure is changed from
the A-type antiferromagnetic order into the ferromagnetic one. Orbital
dependent exchange interaction and a sign change of this interaction around
vacant sites bring about this novel phenomena. Present results explain the
recent experiments in transition-metal compounds with orbital dilution.Comment: 12 pages, 16 figure
Scaling treatment of the random field Ising model
Analytic phenomenological scaling is carried out for the random field Ising
model in general dimensions using a bar geometry. Domain wall configurations
and their decorated profiles and associated wandering and other exponents
are obtained by free energy minimization. Scaling
between different bar widths provides the renormalization group (RG)
transformation. Its consequences are (1) criticality at in
with correlation length diverging like for and
for , where is a decoration constant; (2) criticality in dimensions at , , where
, .
Finite temperature generalizations are outlined. Numerical transfer matrix
calculations and results from a ground state algorithm adapted for strips in
confirm the ingredients which provide the RG description.Comment: RevTex v3.0, 5 pages, plus 4 figures uuencode
Randomly Diluted e_g Orbital-Ordered Systems
Dilution effects on the long-range ordered state of the doubly degenerate
orbital are investigated. Quenched impurities without the orbital degree
of freedom are introduced in the orbital model where the long-range order is
realized by the order-from-disorder mechanism. It is shown by the Monte-Carlo
simulation and the cluster-expansion method that a decrease in the orbital
ordering temperature by dilution is remarkable in comparison with that in the
randomly diluted spin models. Tiltings of orbitals around impurity cause this
unique dilution effects on the orbital systems. The present theory provides a
new view point for the recent experiments in KCuZnF.Comment: 4 pages, 4 figure
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