17,595 research outputs found
Spin glass behavior in an interacting gamma-Fe2O3 nanoparticle system
In this paper we investigate the superspin glass behavior of a concentrated
assembly of interacting maghemite nanoparticles and compare it to that of
canonical atomic spin glass systems. ac versus temperature and frequency
measurements show evidence of a superspin glass transition taking place at low
temperature. In order to fully characterize the superspin glass phase, the
aging behavior of both the thermo-remanent magnetization (TRM) and ac
susceptibility has been investigated. It is shown that the scaling laws obeyed
by superspin glasses and atomic spin glasses are essentially the same, after
subtraction of a superparamagnetic contribution from the superspin glass
response functions. Finally, we discuss a possible origin of this
superparamagnetic contribution in terms of dilute spin glass models
The relative influences of disorder and of frustration on the glassy dynamics in magnetic systems
The magnetisation relaxations of three different types of geometrically
frustrated magnetic systems have been studied with the same experimental
procedures as previously used in spin glasses. The materials investigated are
YMoO (pyrochlore system), SrCrGaO (piled
pairs of Kagom\'e layers) and (HO)Fe(SO)(OH) (jarosite
compound). Despite a very small amount of disorder, all the samples exhibit
many characteristic features of spin glass dynamics below a freezing
temperature , much smaller than their Curie-Weiss temperature .
The ageing properties of their thermoremanent magnetization can be well
accounted for by the same scaling law as in spin glasses, and the values of the
scaling exponents are very close. The effects of temperature variations during
ageing have been specifically investigated. In the pyrochlore and the
bi-Kagom\'e compounds, a decrease of temperature after some waiting period at a
certain temperature re-initializes ageing and the evolution at the new
temperature is the same as if the system were just quenched from above .
However, as the temperature is raised back to , the sample recovers the
state it had previously reached at that temperature. These features are known
in spin glasses as rejuvenation and memory effects. They are clear signatures
of the spin glass dynamics. In the Kagom\'e compound, there is also some
rejuvenation and memory, but much larger temperature changes are needed to
observe the effects. In that sense, the behaviour of this compound is
quantitatively different from that of spin glasses.Comment: latex VersionCorrigee4.tex, 4 files, 3 figures, 5 pages (Proceedings
of the International Conference on Highly Frustrated Magnetism (HFM2003),
August 26-30, 2003, Institut Laue Langevin (ILL), Grenoble, France
Linear and non linear response in the aging regime of the 1D trap model
We investigate the behaviour of the response function in the one dimensional
trap model using scaling arguments that we confirm by numerical simulations. We
study the average position of the random walk at time tw+t given that a small
bias h is applied at time tw. Several scaling regimes are found, depending on
the relative values of t, tw and h. Comparison with the diffusive motion in the
absence of bias allows us to show that the fluctuation dissipation relation is,
in this case, valid even in the aging regime.Comment: 5 pages, 3 figures, 3 references adde
A microscopic description of the aging dynamics: fluctuation-dissipation relations, effective temperature and heterogeneities
We consider the dynamics of a diluted mean-field spin glass model in the
aging regime. The model presents a particularly rich heterogeneous behavior. In
order to catch this behavior, we perform a **spin-by-spin analysis** for a
**given disorder realization**. The results compare well with the outcome of a
static calculation which uses the ``survey propagation'' algorithm of Mezard,
Parisi, and Zecchina [Sciencexpress 10.1126/science.1073287 (2002)]. We thus
confirm the connection between statics and dynamics at the level of single
degrees of freedom. Moreover, working with single-site quantities, we can
introduce a new response-vs-correlation plot, which clearly shows how
heterogeneous degrees of freedom undergo coherent structural rearrangements.
Finally we discuss the general scenario which emerges from our work and
(possibly) applies to more realistic glassy models. Interestingly enough, some
features of this scenario can be understood recurring to thermometric
considerations.Comment: 4 pages, 5 figures (7 eps files
Response of non-equilibrium systems at criticality: Exact results for the Glauber-Ising chain
We investigate the non-equilibrium two-time correlation and response
functions and the associated fluctuation-dissipation ratio for the
ferromagnetic Ising chain with Glauber dynamics. The scaling behavior of these
quantities at low temperature and large times is studied in detail. This
analysis encompasses the self-similar domain-growth (aging) regime, the spatial
and temporal Porod regimes, and the convergence toward equilibrium. The
fluctuation-dissipation ratio admits a non-trivial limit value
at zero temperature, and more generally in the aging regime.Comment: 27 pages. 3 figures. To appear in Journal of Physics
Response properties in a model for granular matter
We investigate the response properties of granular media in the framework of
the so-called {\em Random Tetris Model}. We monitor, for different driving
procedures, several quantities: the evolution of the density and of the density
profiles, the ageing properties through the two-times correlation functions and
the two-times mean-square distance between the potential energies, the response
function defined in terms of the difference in the potential energies of two
replica driven in two slightly different ways. We focus in particular on the
role played by the spatial inhomogeneities (structures) spontaneously emerging
during the compaction process, the history of the sample and the driving
procedure. It turns out that none of these ingredients can be neglected for the
correct interpretation of the experimental or numerical data. We discuss the
problem of the optimization of the compaction process and we comment on the
validity of our results for the description of granular materials in a
thermodynamic framework.Comment: 22 pages, 35 eps files (21 figures
Ground state of the Kagome-like S=1/2 antiferromagnet, Volborthite Cu3V2O7(OH)2.2H2O
Volborthite compound is one of the very few realizations of S=1/2 quantum
spins on a highly frustrated kagome-like lattice. Low-T SQUID measurements
reveal a broad magnetic transition below 2K which is further confirmed by a
peak in the 51V nuclear spin relaxation rate (1/T1) at 1.4K0.2K. Through
51V NMR, the ground state (GS) appears to be a mixture of different spin
configurations, among which 20% correspond to a well defined short range order,
possibly of the type. While the freezing involve all
the Cu spins, only 40% of the copper moment is actually frozen which
suggests that quantum fluctuations strongly renormalize the GS.Comment: 4 pages, 4 figures, to appear in PR
Aging in lattice-gas models with constrained dynamics
We investigate the aging behavior of lattice-gas models with constrained
dynamics in which particle exchange with a reservoir is allowed. Such models
provide a particularly simple interpretation of aging phenomena as a slow
approach to criticality. They appear as the simplest three dimensional models
exhibiting a glassy behavior similar to that of mean field (low temperature
mode-coupling) models.Comment: 5 pages and 3 figures, REVTeX. Submitted to Europhysics Letter
Dynamical ultrametricity in the critical trap model
We show that the trap model at its critical temperature presents dynamical
ultrametricity in the sense of Cugliandolo and Kurchan [CuKu94]. We use the
explicit analytic solution of this model to discuss several issues that arise
in the context of mean-field glassy dynamics, such as the scaling form of the
correlation function, and the finite time (or finite forcing) corrections to
ultrametricity, that are found to decay only logarithmically with the
associated time scale, as well as the fluctuation dissipation ratio. We also
argue that in the multilevel trap model, the short time dynamics is dominated
by the level which is at its critical temperature, so that dynamical
ultrametricity should hold in the whole glassy temperature range. We revisit
some experimental data on spin-glasses in light of these results.Comment: 7 pages, 4 .eps figures. submitted to J. Phys.
Time and length scales in spin glasses
We discuss the slow, nonequilibrium, dynamics of spin glasses in their glassy
phase. We briefly review the present theoretical understanding of the
spectacular phenomena observed in experiments and describe new numerical
results obtained in the first large-scale simulation of the nonequilibrium
dynamics of the three dimensional Heisenberg spin glass.Comment: Paper presented at "Highly Frustrated Magnetism 2003", Grenoble,
August 200
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