52,012 research outputs found
Caging and mosaic lengthscales in plaquette spin models of glasses
We consider two systems of Ising spins with plaquette interactions. They are
simple models of glasses which have dual representations as kinetically
constrained systems. These models allow an explicit analysis using the mosaic,
or entropic droplet, approach of the random first-order transition theory of
the glass transition. We show that the low temperature states of these systems
resemble glassy mosaic states, despite the fact that excitations are localized
and that there are no static singularities. By means of finite size
thermodynamics we study a generalised caging effect whereby the system is
frozen on short lengthscales, but free at larger lengthscales. We find that the
freezing lengthscales obtained from statics coincide with those relevant to
dynamic correlations, as expected in the mosaic view. The simple nucleation
arguments of the mosaic approach, however, do not give the correct relation
between freezing lengths and relaxation times, as they do not capture the
transition states for relaxation. We discuss how these results make a
connection between the mosaic and the dynamic facilitation views of glass
formers.Comment: 14 pages, 14 figs, some typos correcte
Effects of Interactions on the Critical Temperature of a Trapped Bose Gas
We perform high-precision measurements of the condensation temperature of a
harmonically-trapped atomic Bose gas with widely-tuneable interactions. For
weak interactions we observe a negative shift of the critical temperature in
excellent agreement with mean-field theory. However for sufficiently strong
interactions we clearly observe an additional positive shift, characteristic of
beyond-mean-field critical correlations. We also discuss non-equilibrium
effects on the apparent critical temperature for both very weak and very strong
interactions.Comment: 4 pages, 4 figure
Static and dynamic lengthscales in a simple glassy plaquette model
We study static and dynamic spatial correlations in a two-dimensional spin
model with four-body plaquette interactions and standard Glauber dynamics by
means of analytic arguments and Monte Carlo simulations. We study in detail the
dynamical behaviour which becomes glassy at low temperatures due to the
emergence of effective kinetic constraints in a dual representation where spins
are mapped to plaquette variables. We study the interplay between non-trivial
static correlations of the spins and the dynamic `four-point' correlations
usually studied in the context of supercooled liquids. We show that slow
dynamics is spatially heterogeneous due to the presence of diverging
lengthscales and scaling, as is also found in kinetically constrained models.
This analogy is illustrated by a comparative study of a froth model where the
kinetic constraints are imposed.Comment: 12 pages, 13 figs; published versio
Model choice versus model criticism
The new perspectives on ABC and Bayesian model criticisms presented in
Ratmann et al.(2009) are challenging standard approaches to Bayesian model
choice. We discuss here some issues arising from the authors' approach,
including prior influence, model assessment and criticism, and the meaning of
error in ABC.Comment: This is a comment on the recent paper by Ratmann, Andrieu, Wiuf, and
Richardson (PNAS, 106), submitted too late for PNAS to consider i
Metastable states and space-time phase transitions in a spin-glass model
We study large deviations of the dynamical activity in the random orthogonal
model (ROM). This is a fully connected spin-glass model with one-step replica
symmetry breaking behaviour, consistent with the random first-order transition
scenario for structural glasses. We show that this model displays dynamical
(space-time) phase-transitions between active and inactive phases, as
demonstrated by singularities in large deviation functions. We argue that such
transitions are generic in systems with long-lived metastable states.Comment: 4.1 pages, 4 fig
Fiscal Federalism in Europe: Lessons From the United States Experience
The existing political and legal institutions of fiscal policy-making are under challenge. As the United States and the eastern European and Soviet states experiment with policy decentralization, the states of western Europe are looking to a more centralized policy structure via the E.E.C.. This paper seeks to raise issues of importance to all such reform efforts--notably, the need to consider, and balance, the inefficiencies of fiscal policy decentralization (spillovers and wasteful fiscal competition) against the inefficiencies of fiscal policy centralization (policy cycles and localized 'pork barrel' spending and taxes). The need to develop new fiscal policy institutions emphasizing voluntary agreements and responsive 'agenda-setters' is stressed.
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