107 research outputs found
Novel glassy behavior in a ferromagnetic p-spin model
Recent work has suggested the existence of glassy behavior in a ferromagnetic
model with a four-spin interaction. Motivated by these findings, we have
studied the dynamics of this model using Monte Carlo simulations with
particular attention being paid to two-time quantities. We find that the system
shares many features in common with glass forming liquids. In particular, the
model exhibits: (i) a very long-lived metastable state, (ii) autocorrelation
functions that show stretched exponential relaxation, (iii) a non-equilibrium
timescale that appears to diverge at a well defined temperature, and (iv) low
temperature aging behaviour characteristic of glasses.Comment: 6 pages, 5 figure
Comments on "Note on varying speed of light theories"
In a recent note Ellis criticizes varying speed of light theories on the
grounds of a number of foundational issues. His reflections provide us with an
opportunity to clarify some fundamental matters pertaining to these theories
Persistence in higher dimensions : a finite size scaling study
We show that the persistence probability , in a coarsening system of
linear size at a time , has the finite size scaling form where is the persistence exponent and
is the coarsening exponent. The scaling function for
and is constant for large . The scaling form implies a fractal
distribution of persistent sites with power-law spatial correlations. We study
the scaling numerically for Glauber-Ising model at dimension to 4 and
extend the study to the diffusion problem. Our finite size scaling ansatz is
satisfied in all these cases providing a good estimate of the exponent
.Comment: 4 pages in RevTeX with 6 figures. To appear in Phys. Rev.
Tensionless structure of glassy phase
We study a class of homogeneous finite-dimensional Ising models which were
recently shown to exhibit glassy properties. Monte Carlo simulations of a
particular three-dimensional model in this class show that the glassy phase
obtained under slow cooling is dominated by large scale excitations whose
energy scales with their size as with
. Simulations suggest that in another model of this class,
namely the four-spin model, energy is concentrated mainly in linear defects
making also in this case domain walls tensionless. Two-dimensinal variants of
these models are trivial and energy of excitations scales with the exponent
.Comment: 5 page
Absence of Dipole Transitions in Vortices of Type II Superconductors
The response of a single vortex to a time dependent field is examined
microscopically and an equation of motion for vortex motion at non-zero
frequencies is derived. Of interest are frequencies near ,
where is the bulk energy gap and is the fermi energy. The low
temperature, clean, extreme type II limit and maintaining of equilibrium with
the lattice are assumed. A simplification occurs for large planar mass
anisotropy. Thus the results may be pertinent to materials such as and
high temperature superconductors. The expected dipole transition between core
states is hidden because of the self consistent nature of the vortex potential.
Instead the vortex itself moves and has a resonance at the frequency of the
transition.Comment: 12 pages, no figure
SO(5) theory of insulating vortex cores in high- materials
We study the fermionic states of the antiferromagnetically ordered vortex
cores predicted to exist in the superconducting phase of the newly proposed
SO(5) model of strongly correlated electrons. Our model calculation gives a
natural explanation of the recent STM measurements on BSCCO, which in
surprising contrast to YBCO revealed completely insulating vortex cores.Comment: 4 pages, 1 figur
Crystallization of a supercooled liquid and of a glass - Ising model approach
Using Monte Carlo simulations we study crystallization in the
three-dimensional Ising model with four-spin interaction. We monitor the
morphology of crystals which grow after placing crystallization seeds in a
supercooled liquid. Defects in such crystals constitute an intricate and very
stable network which separate various domains by tensionless domain walls. We
also show that the crystallization which occurs during the continuous heating
of the glassy phase takes place at a heating-rate dependent temperature.Comment: 7 page
Slow dynamics in the 3--D gonihedric model
We study dynamical aspects of three--dimensional gonihedric spins by using
Monte--Carlo methods. The interest of this family of models (parametrized by
one self-avoidance parameter ) lies in their capability to show
remarkably slow dynamics and seemingly glassy behaviour below a certain
temperature without the need of introducing disorder of any kind. We
consider first a hamiltonian that takes into account only a four--spin term
(), where a first order phase transition is well established. By
studying the relaxation properties at low temperatures we confirm that the
model exhibits two distinct regimes. For , with long lived
metastability and a supercooled phase, the approach to equilibrium is well
described by a stretched exponential. For the dynamics appears to be
logarithmic. We provide an accurate determination of . We also determine
the evolution of particularly long lived configurations. Next, we consider the
case , where the plaquette term is absent and the gonihedric action
consists in a ferromagnetic Ising with fine-tuned next-to-nearest neighbour
interactions. This model exhibits a second order phase transition. The
consideration of the relaxation time for configurations in the cold phase
reveals the presence of slow dynamics and glassy behaviour for any .
Type II aging features are exhibited by this model.Comment: 13 pages, 12 figure
A Field-theoretical Interpretation of the Holographic Renormalization Group
A quantum-field theoretical interpretation is given to the holographic RG
equation by relating it to a field-theoretical local RG equation which
determines how Weyl invariance is broken in a quantized field theory. Using
this approach we determine the relation between the holographic C theorem and
the C theorem in two-dimensional quantum field theory which relies on the
Zamolodchikov metric. Similarly we discuss how in four dimensions the
holographic C function is related to a conjectured field-theoretical C
function. The scheme dependence of the holographic RG due to the possible
presence of finite local counterterms is discussed in detail, as well as its
implications for the holographic C function. We also discuss issues special to
the situation when mass deformations are present. Furthermore we suggest that
the holographic RG equation may also be obtained from a bulk diffeomorphism
which reduces to a Weyl transformation on the boundary.Comment: 24 pages, LaTeX, no figures; references added, typos corrected,
paragraph added to section
Aging without disorder on long time scales
We study the Metropolis dynamics of a simple spin system without disorder,
which exhibits glassy dynamics at low temperatures. We use an implementation of
the algorithm of Bortz, Kalos and Lebowitz \cite{bortz}. This method turns out
to be very efficient for the study of glassy systems, which get trapped in
local minima on many different time scales. We find strong evidence of aging
effects at low temperatures. We relate these effects to the distribution
function of the trapping times of single configurations.Comment: 8 pages Revtex, 7 figures uuencoded (Revised version: the figures are
now present
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