39 research outputs found

### Aging in a topological spin glass

We have examined the nonconventional spin glass phase of the 2-dimensional
kagome antiferromagnet (H_3 O) Fe_3 (SO_4)_2 (OH)_6 by means of ac and dc
magnetic measurements. The frequency dependence of the ac susceptibility peak
is characteristic of a critical slowing down at Tg ~ 18K. At fixed temperature
below Tg, aging effects are found which obey the same scaling law as in spin
glasses or polymers. However, in clear contrast with conventional spin glasses,
aging is remarkably insensitive to temperature changes. This particular type of
dynamics is discussed in relation with theoretical predictions for highly
frustrated non-disordered systems.Comment: 4 pages, 4 figure

### Rejuvenation in the Random Energy Model

We show that the Random Energy Model has interesting rejuvenation properties
in its frozen phase. Different `susceptibilities' to temperature changes, for
the free-energy and for other (`magnetic') observables, can be computed
exactly. These susceptibilities diverge at the transition temperature, as
(1-T/T_c)^-3 for the free-energy.Comment: 9 pages, 1 eps figur

### Finite-Temperature Transition into a Power-Law Spin Phase with an Extensive Zero-Point Entropy

We introduce an $xy$ generalization of the frustrated Ising model on a
triangular lattice. The presence of continuous degrees of freedom stabilizes a
{\em finite-temperature} spin state with {\em power-law} discrete spin
correlations and an extensive zero-point entropy. In this phase, the unquenched
degrees of freedom can be described by a fluctuating surface with logarithmic
height correlations. Finite-size Monte Carlo simulations have been used to
characterize the exponents of the transition and the dynamics of the
low-temperature phase

### Fluctuation-dissipation ratio of a spin glass in the aging regime

We present the first experimental determination of the time autocorrelation
$C(t',t)$ of magnetization in the non-stationary regime of a spin glass.
Quantitative comparison with the response, the magnetic susceptibility
$\chi(t',t)$, is made using a new experimental setup allowing both measurements
in the same conditions. Clearly, we observe a non-linear
fluctuation-dissipation relation between $C$ and $\chi$, depending weakly on
the waiting time $t'$. Following theoretical developments on mean-field models,
and lately on short range models, it is predicted that in the limit of long
times, the $\chi(C)$ relationship should become independent on $t'$. A scaling
procedure allows us to extrapolate to the limit of long waiting times.Comment: 4 pages, 3 figure

### Chaos and Universality in a Four-Dimensional Spin Glass

We present a finite size scaling analysis of Monte Carlo simulation results
on a four dimensional Ising spin glass. We study chaos with both coupling and
temperature perturbations, and find the same chaos exponent in each case. Chaos
is investigated both at the critical temperature and below where it seems to be
more efficient (larger exponent). Dimension four seems to be above the critical
dimension where chaos with temperature is no more present in the critical
region. Our results are consistent with the Gaussian and bimodal coupling
distributions being in the same universality class.Comment: 11 pages, including 6 postscript figures. Latex with revtex macro

### Numerical Study on Aging Dynamics in the 3D Ising Spin-Glass Model. II. Quasi-Equilibrium Regime of Spin Auto-Correlation Function

Using Monte Carlo simulations, we have studied isothermal aging of
three-dimensional Ising spin-glass model focusing on quasi-equilibrium behavior
of the spin auto-correlation function. Weak violation of the time translational
invariance in the quasi-equilibrium regime is analyzed in terms of {\it
effective stiffness} for droplet excitations in the presence of domain walls.
Within the range of computational time window, we have confirmed that the
effective stiffness follows the expected scaling behavior with respect to the
characteristic length scales associated with droplet excitations and domain
walls, whose growth law has been extracted from our simulated data. Implication
of the results are discussed in relation to experimental works on ac
susceptibilities.Comment: 18 pages, 6 figure

### Numerical Study of Aging in the Generalized Random Energy Model

Magnetizations are introduced to the Generalized Random Energy Model (GREM)
and numerical simulations on ac susceptibility is made for direct comparison
with experiments in glassy materials. Prominent dynamical natures of spin
glasses, {\it i.e.}, {\em memory} effect and {\em reinitialization}, are
reproduced well in the GREM. The existence of many layers causing continuous
transitions is very important for the two natures. Results of experiments in
other glassy materials such as polymers, supercooled glycerol and orientational
glasses, which are contrast to those in spin glasses, are interpreted well by
the Single-layer Random Energy Model.Comment: 8 pages, 9 figures, to be submitted to J. Phys. Soc. Jp

### Aging in Spin Glasses in three, four and infinite dimensions

The SUE machine is used to extend by a factor of 1000 the time-scale of
previous studies of the aging, out-of-equilibrium dynamics of the
Edwards-Anderson model with binary couplings, on large lattices (L=60). The
correlation function, $C(t+t_w,t_w)$, $t_w$ being the time elapsed under a
quench from high-temperature, follows nicely a slightly-modified power law for
$t>t_w$. Very tiny (logarithmic), yet clearly detectable deviations from the
full-aging $t/t_w$ scaling can be observed. Furthermore, the $t<t_w$ data shows
clear indications of the presence of more than one time-sector in the aging
dynamics. Similar results are found in four-dimensions, but a rather different
behaviour is obtained in the infinite-dimensional $z=6$ Viana-Bray model. Most
surprisingly, our results in infinite dimensions seem incompatible with
dynamical ultrametricity. A detailed study of the link correlation function is
presented, suggesting that its aging-properties are the same as for the spin
correlation-function.Comment: J.P.A special issue on glasses and spin-glasses. Some improvements in
citations over printed versio

### Aging, rejuvenation and memory phenomena in spin glasses

In this paper, we review several important features of the out-of-equilibrium
dynamics of spin glasses. Starting with the simplest experiments, we discuss
the scaling laws used to describe the isothermal aging observed in spin glasses
after a quench down to the low temperature phase. We report in particular new
results on the sub-aging behaviour of spin glasses. We then discuss the
rejuvenation and memory effects observed when a spin glass is submitted to
temperature variations during aging, from the point of view of both energy
landscape pictures and of real space pictures. We highlight the fact that both
approaches point out the necessity of hierarchical processes involved in aging.
Finally, we report an investigation of the effect of small temperature
variations on aging in spin glass samples with various anisotropies which
indicates that this hierarchy depends on the spin anisotropy.Comment: submitted for the Proceedings of Stat Phys 22, Bangalore (India

### Temperature shifts in the Sinai model: static and dynamical effects

We study analytically and numerically the role of temperature shifts in the
simplest model where the energy landscape is explicitely hierarchical, namely
the Sinai model. This model has both attractive features (there are valleys
within valleys in a strict self similar sense), but also one important
drawback: there is no phase transition so that the model is, in the large size
limit, effectively at zero temperature. We compute various static chaos
indicators, that are found to be trivial in the large size limit, but exhibit
interesting features for finite sizes. Correspondingly, for finite times, some
interesting rejuvenation effects, related to the self similar nature of the
potential, are observed. Still, the separation of time scales/length scales
with temperatures in this model is much weaker that in experimental
spin-glasses.Comment: 19 pages, Revtex4, eps figure