777 research outputs found
Elasticity and metastability limit in supercooled liquids: a lattice model
We present Monte Carlo simulations on a lattice system that displays a first
order phase transition between a disordered phase (liquid) and an ordered phase
(crystal). The model is augmented by an interaction that simulates the effect
of elasticity in continuum models. The temperature range of stability of the
liquid phase is strongly increased in the presence of the elastic interaction.
We discuss the consequences of this result for the existence of a kinetic
spinodal in real systems.Comment: 8 pages, 5 figure
Replica symmetry breaking in long-range glass models without quenched disorder
We discuss mean field theory of glasses without quenched disorder focusing on
the justification of the replica approach to thermodynamics. We emphasize the
assumptions implicit in this method and discuss how they can be verified. The
formalism is applied to the long range Ising model with orthogonal coupling
matrix. We find the one step replica-symmetry breaking solution and show that
it is stable in the intermediate temperature range that includes the glass
state but excludes very low temperatures. At very low temperatures this
solution becomes unstable and this approach fails.Comment: 6 pages, 2 figure
Frequency-domain study of relaxation in a spin glass model for the structural glass transition
We have computed the time-dependent susceptibility for the finite-size
mean-field Random Orthogonal model (ROM). We find that for temperatures above
the mode-coupling temperature the imaginary part of the susceptibility
obeys the scaling forms proposed for glass-forming liquids.
Furthermore, as the temperature is lowered the peak frequency of
decreases following a Vogel-Fulcher law with a critical temperature remarkably
close to the known critical temperature where the configurational entropy
vanishes.Comment: 7 pages, 4 figures, epl LaTeX packag
Fermion kinetics in the Falicov-Kimball limit of the three-band Emery model
The three-band Emery model is reduced to a single-particle quantum model of
Falicov-Kimball type, by allowing only up-spins to hop, and forbidding double
occupation by projection. It is used to study the effects of geometric
obstruction on mobile fermions in thermodynamic equilibrium. For low hopping
overlap, there appears a plateau in the entropy, due to charge correlations,
and related to real-space disorder. For large overlap, the equilibrium
thermopower susceptibility remains anomalous, with a sign opposite to the one
predicted from the single-particle density of states. The heat capacity and
non-Fermi liquid response are discussed in the context of similar results in
the literature. All results are obtained by evaluation of an effective
single-particle free-energy operator in closed form. The method to obtain this
operator is described in detail.Comment: New calculations, method explained in detail, 16 pages, 9 figure
Solvent-induced micelle formation in a hydrophobic interaction model
We investigate the aggregation of amphiphilic molecules by adapting the
two-state Muller-Lee-Graziano model for water, in which a solvent-induced
hydrophobic interaction is included implicitly. We study the formation of
various types of micelle as a function of the distribution of hydrophobic
regions at the molecular surface. Successive substitution of non-polar surfaces
by polar ones demonstrates the influence of hydrophobicity on the upper and
lower critical solution temperatures. Aggregates of lipid molecules, described
by a refinement of the model in which a hydrophobic tail of variable length
interacts with different numbers of water molecules, are stabilized as the
length of the tail increases. We demonstrate that the essential features of
micelle formation are primarily solvent-induced, and are explained within a
model which focuses only on the alteration of water structure in the vicinity
of the hydrophobic surface regions of amphiphiles in solution.Comment: 11 pages, 10 figures; some rearrangement of introduction and
discussion sections, streamlining of formalism and general compression; to
appear in Phys. Rev.
Hydrophobic interactions: an overview
We present an overview of the recent progress that has been made in
understanding the origin of hydrophobic interactions. We discuss the different
character of the solvation behavior of apolar solutes at small and large length
scales. We emphasize that the crossover in the solvation behavior arises from a
collective effect, which means that implicit solvent models should be used with
care. We then discuss a recently developed explicit solvent model, in which the
solvent is not described at the atomic level, but rather at the level of a
density field. The model is based upon a lattice-gas model, which describes
density fluctuations in the solvent at large length scales, and a Gaussian
model, which describes density fluctuations at smaller length scales. By
integrating out the small length scale field, a Hamiltonian is obtained, which
is a function of the binary, large-length scale field only. This makes it
possible to simulate much larger systems than hitherto possible as demonstrated
by the application of the model to the collapse of an ideal hydrophobic
polymer. The results show that the collapse is dominated by the dynamics of the
solvent, in particular the formation of a vapor bubble of critical size.
Implications of these findings to the understanding of pressure denaturation of
proteins are discussed.Comment: 10 pages, 4 figure
Geometry and symmetry presculpt the free-energy landscape of proteins
We present a simple physical model which demonstrates that the native state
folds of proteins can emerge on the basis of considerations of geometry and
symmetry. We show that the inherent anisotropy of a chain molecule, the
geometrical and energetic constraints placed by the hydrogen bonds and sterics,
and hydrophobicity are sufficient to yield a free energy landscape with broad
minima even for a homopolymer. These minima correspond to marginally compact
structures comprising the menu of folds that proteins choose from to house
their native-states in. Our results provide a general framework for
understanding the common characteristics of globular proteins.Comment: 23 pages, 5 figure
A teaching guide of nuclear physics: the concept of bonds
We propose discussions and hands-on activities for GCSE and A-level students,
covering a fundamental aspect of nuclear physics: the concept of bond and the
energy released (absorbed) when a bond is created (broken). This is the first
of the series of papers named "A teaching guide of nuclear physics", whose main
goal is to provide teaching tools and ideas to GCSE and A-level teachers,
within a consistent and complete curriculum
Configurational Entropy and its Crisis in Metastable States: Ideal Glass Transition in a Dimer Model as a Paragidm of a Molecular Glass
We discuss the need for discretization to evaluate the configurational
entropy in a general model. We also discuss the prescription using restricted
partition function formalism to study the stationary limit of metastable
states. We introduce a lattice model of dimers as a paradigm of molecular fluid
and study metastability in it to investigate the root cause of glassy behavior.
We demonstrate the existence of the entropy crisis in metastable states, from
which it follows that the entropy crisis is the root cause underlying the ideal
glass transition in systems with particles of all sizes. The orientational
interactions in the model control the nature of the liquid-liquid transition
observed in recent years in molecular glasses.Comment: 36 pages, 9 figure
Mean field theory of hard sphere glasses and jamming
Hard spheres are ubiquitous in condensed matter: they have been used as
models for liquids, crystals, colloidal systems, granular systems, and powders.
Packings of hard spheres are of even wider interest, as they are related to
important problems in information theory, such as digitalization of signals,
error correcting codes, and optimization problems. In three dimensions the
densest packing of identical hard spheres has been proven to be the FCC
lattice, and it is conjectured that the closest packing is ordered (a regular
lattice, e.g, a crystal) in low enough dimension. Still, amorphous packings
have attracted a lot of interest, because for polydisperse colloids and
granular materials the crystalline state is not obtained in experiments for
kinetic reasons. We review here a theory of amorphous packings, and more
generally glassy states, of hard spheres that is based on the replica method:
this theory gives predictions on the structure and thermodynamics of these
states. In dimensions between two and six these predictions can be successfully
compared with numerical simulations. We will also discuss the limit of large
dimension where an exact solution is possible. Some of the results we present
here have been already published, but others are original: in particular we
improved the discussion of the large dimension limit and we obtained new
results on the correlation function and the contact force distribution in three
dimensions. We also try here to clarify the main assumptions that are beyond
our theory and in particular the relation between our static computation and
the dynamical procedures used to construct amorphous packings.Comment: 59 pages, 25 figures. Final version published on Rev.Mod.Phy
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