1,131 research outputs found
Characterizing dynamic length scales in glass-forming liquids
Reply to Comment by Flenner and Szamel on our paper in Nature Physics 8, 164
(2012).Comment: 1 pag
Real spin glasses relax slowly in the shade of hierarchical trees
The Parisi solution of the mean-field spin glass has been widely accepted and
celebrated. Its marginal stability in 3d and its complexity however raised the
question of its relevance to real spin glasses. This paper gives a short
overview of the important experimental results which could be understood within
the mean-field solution. The existence of a true phase transition and the
particular behaviour of the susceptibility below the freezing temperature,
predicted by the theory, are clearly confirmed by the experimental results. The
behaviour of the complex order parameter and of the Fluctuation Dissipation
ratio are in good agreement with results of spontaneous noise measurements. The
very particular ultrametric symmetry, the key feature of the theory, provided
us with a simple description of the rejuvenation and memory effects observed in
experiment. Finally, going a step beyond mean-field, the paper shortly
discusses new analyses in terms of correlated domains characterized by their
length scales, as well as new experiments on superspin glasses which compare
well with recent theoretical simulations.Comment: To appear in the proceedings of "Wandering with Curiosity in Complex
Landscapes", a scientific conference in honour of Giorgio Parisi for his 60th
birthday, Roma, September 8-10 2008 (submitted for the special issue of the
Journal of Statistical Physics, 2009
Phase separation in a chaotic flow
The phase separation between two immiscible liquids advected by a
bidimensional velocity field is investigated numerically by solving the
corresponding Cahn-Hilliard equation. We study how the spinodal decomposition
process depends on the presence -or absence- of Lagrangian chaos. A fully
chaotic flow, in particular, limits the growth of domains and for unequal
volume fractions of the liquids, a characteristic exponential distribution of
droplet sizes is obtained. The limiting domain size results from a balance
between chaotic mixing and spinodal decomposition, measured in terms of
Lyapunov exponent and diffusivity constant, respectively.Comment: Minor changes - Version accepted for publication - Physical Review
Letter
Non-equilibrium dynamics of spin facilitated glass models
We consider the dynamics of spin facilitated models of glasses in the
non-equilibrium aging regime following a sudden quench from high to low
temperatures. We briefly review known results obtained for the broad class of
kinetically constrained models, and then present new results for the behaviour
of the one-spin facilitated Fredrickson-Andersen and East models in various
spatial dimensions. The time evolution of one-time quantities, such as the
energy density, and the detailed properties of two-time correlation and
response functions are studied using a combination of theoretical approaches,
including exact mappings of master operators and reductions to integrable
quantum spin chains, field theory and renormalization group, and independent
interval and timescale separation methods. The resulting analytical predictions
are confirmed by means of detailed numerical simulations. The models we
consider are characterized by trivial static properties, with no finite
temperature singularities, but they nevertheless display a surprising variety
of dynamic behaviour during aging, which can be directly related to the
existence and growth in time of dynamic lengthscales. Well-behaved
fluctuation-dissipation ratios can be defined for these models, and we study
their properties in detail. We confirm in particular the existence of negative
fluctuation-dissipation ratios for a large number of observables. Our results
suggest that well-defined violations of fluctuation-dissipation relations, of a
purely dynamic origin and unrelated to the thermodynamic concept of effective
temperatures, could in general be present in non-equilibrium glassy materials.Comment: 72 pages, invited contribution to special issue of JSTAT on
"Principles of Dynamics of Nonequilibrium Systems" (Programme at Newton
Institute Cambridge). v2: New data added to Figs. 11, 23, 24, new Fig. 26 on
East model in d=3, minor improvements to tex
NMR Determination of 2D Electron Spin Polarization at
Using a `standard' NMR spin-echo technique we determined the spin
polarization of two-dimensional electrons, confined to GaAs quantum wells, from
the hyperfine shift of Ga nuclei in the wells. Concentrating on the temperature
and magnetic field dependencies of spin polarization at Landau level filling
factor , we find that the results are described well by a simple
model of non-interacting composite fermions, although some inconsistencies
remain when the two-dimensional electron system is tilted in the magnetic
field.Comment: 4 pages (REVTEX) AND 4 figures (PS
Phase separation in an homogeneous shear flow: Morphology, growth laws and dynamic scaling
We investigate numerically the influence of an homogeneous shear flow on the
spinodal decomposition of a binary mixture by solving the Cahn-Hilliard
equation in a two-dimensional geometry. Several aspects of this much studied
problem are clarified. Our numerical data show unambiguously that, in the shear
flow, the domains have on average an elliptic shape. The time evolution of the
three parameters describing this ellipse are obtained for a wide range of shear
rates. For the lowest shear rates investigated, we find the growth laws for the
two principal axis , , while
the mean orientation of the domains with respect to the flow is inversely
proportional to the strain. This implies that when hydrodynamics is neglected a
shear flow does not stop the domain growth process. We investigate also the
possibility of dynamic scaling, and show that only a non trivial form of
scaling holds, as predicted by a recent analytical approach to the case of a
non-conserved order parameter. We show that a simple physical argument may
account for these results.Comment: Version accepted for publication - Physical Review
Observation of a new phase transition between fully and partially polarized quantum Hall states with charge and spin gaps at
The average electron spin-polarization of two-dimensional electron
gas confined in multiple quantum-wells was measured by
nuclear magnetic resonance (NMR) near the fractional quantum Hall state with
filling factor . Above this filling factor (), a strong depolarization is observed corresponding to two spin flips per
additional flux quantum. The most remarkable behavior of the polarization is
observed at , where a quantum phase transition from a partially
polarized () to a fully polarized ()
state can be driven by increasing the ratio between the Zeeman and the Coulomb
energy above a critical value .Comment: 4 pages including 4 figure
The Old Host-Galaxy Environment of SSS17a, the First Electromagnetic Counterpart to a Gravitational Wave Source
We present an analysis of the host-galaxy environment of Swope Supernova
Survey 2017a (SSS17a), the discovery of an electromagnetic counterpart to a
gravitational wave source, GW170817. SSS17a occurred 1.9 kpc (in projection;
10.2") from the nucleus of NGC 4993, an S0 galaxy at a distance of 40 Mpc. We
present a Hubble Space Telescope (HST) pre-trigger image of NGC 4993, Magellan
optical spectroscopy of the nucleus of NGC 4993 and the location of SSS17a, and
broad-band UV through IR photometry of NGC 4993. The spectrum and broad-band
spectral-energy distribution indicate that NGC 4993 has a stellar mass of log
(M/M_solar) = 10.49^{+0.08}_{-0.20} and star formation rate of 0.003
M_solar/yr, and the progenitor system of SSS17a likely had an age of >2.8 Gyr.
There is no counterpart at the position of SSS17a in the HST pre-trigger image,
indicating that the progenitor system had an absolute magnitude M_V > -5.8 mag.
We detect dust lanes extending out to almost the position of SSS17a and >100
likely globular clusters associated with NGC 4993. The offset of SSS17a is
similar to many short gamma-ray burst offsets, and its progenitor system was
likely bound to NGC 4993. The environment of SSS17a is consistent with an old
progenitor system such as a binary neutron star system.Comment: ApJL in pres
Dense active matter model of motion patterns in confluent cell monolayers
Epithelial cell monolayers show remarkable displacement and velocity
correlations over distances of ten or more cell sizes that are reminiscent of
supercooled liquids and active nematics. We show that many observed features
can be described within the framework of dense active matter, and argue that
persistent uncoordinated cell motility coupled to the collective elastic modes
of the cell sheet is sufficient to produce swirl-like correlations. We obtain
this result using both continuum active linear elasticity and a normal modes
formalism, and validate analytical predictions with numerical simulations of
two agent-based cell models, soft elastic particles and the self-propelled
Voronoi model together with in-vitro experiments of confluent corneal
epithelial cell sheets. Simulations and normal mode analysis perfectly match
when tissue-level reorganisation occurs on times longer than the persistence
time of cell motility. Our analytical model quantitatively matches measured
velocity correlation functions over more than a decade with a single fitting
parameter.Comment: updated version accepted for publication in Nat. Com
Spatial fluctuations in transient creep deformation
We study the spatial fluctuations of transient creep deformation of materials
as a function of time, both by Digital Image Correlation (DIC) measurements of
paper samples and by numerical simulations of a crystal plasticity or discrete
dislocation dynamics model. This model has a jamming or yielding phase
transition, around which power-law or Andrade creep is found. During primary
creep, the relative strength of the strain rate fluctuations increases with
time in both cases - the spatially averaged creep rate obeys the Andrade law
, while the time dependence of the spatial
fluctuations of the local creep rates is given by . A similar scaling for the fluctuations is found in the logarithmic
creep regime that is typically observed for lower applied stresses. We review
briefly some classical theories of Andrade creep from the point of view of such
spatial fluctuations. We consider these phenomenological, time-dependent creep
laws in terms of a description based on a non-equilibrium phase transition
separating evolving and frozen states of the system when the externally applied
load is varied. Such an interpretation is discussed further by the data
collapse of the local deformations in the spirit of absorbing state/depinning
phase transitions, as well as deformation-deformation correlations and the
width of the cumulative strain distributions. The results are also compared
with the order parameter fluctuations observed close to the depinning
transition of the 2 Linear Interface Model or the quenched Edwards-Wilkinson
equation.Comment: 27 pages, 18 figure
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