443 research outputs found
Locally preferred structure in simple atomic liquids
We propose a method to determine the locally preferred structure of model
liquids. This latter is obtained numerically as the global minimum of the
effective energy surface of clusters formed by small numbers of particles
embedded in a liquid-like environment. The effective energy is the sum of the
intra-cluster interaction potential and of an external field that describes the
influence of the embedding bulk liquid at a mean-field level. Doing so we
minimize the surface effects present in isolated clusters without introducing
the full blown geometrical frustration present in bulk condensed phases. We
find that the locally preferred structure of the Lennard-Jones liquid is an
icosahedron, and that the liquid-like environment only slightly reduces the
relative stability of the icosahedral cluster. The influence of the boundary
conditions on the nature of the ground-state configuration of Lennard-Jones
clusters is also discussed.Comment: RevTeX 4, 17 pages, 6 eps figure
Finite TYCZ expansions and cscK metrics
Let be a Kaehler manifold whose associated Kaehler form is
integral and let be a quantization hermitian
line bundle. In this paper we study those Kaehler manifolds admitting
a finite TYCZ expansion. We show that if the TYCZ expansion is finite then
is indeed a polynomial in of degree , , and the
log-term of the Szeg\"{o} kernel of the disc bundle vanishes
(where is the dual bundle of ). Moreover, we provide a complete
classification of the Kaehler manifolds admitting finite TYCZ expansion either
when is a complex curve or when is a complex surface with a cscK metric
which admits a radial Kaehler potential
Dynamic force spectroscopy of DNA hairpins. II. Irreversibility and dissipation
We investigate irreversibility and dissipation in single molecules that
cooperatively fold/unfold in a two state manner under the action of mechanical
force. We apply path thermodynamics to derive analytical expressions for the
average dissipated work and the average hopping number in two state systems. It
is shown how these quantities only depend on two parameters that characterize
the folding/unfolding kinetics of the molecule: the fragility and the
coexistence hopping rate. The latter has to be rescaled to take into account
the appropriate experimental setup. Finally we carry out pulling experiments
with optical tweezers in a specifically designed DNA hairpin that shows
two-state cooperative folding. We then use these experimental results to
validate our theoretical predictions.Comment: 28 pages, 12 figure
Liquid stability in a model for ortho-terphenyl
We report an extensive study of the phase diagram of a simple model for
ortho-terphenyl, focusing on the limits of stability of the liquid state.
Reported data extend previous studies of the same model to both lower and
higher densities and to higher temperatures. We estimate the location of the
homogeneous liquid-gas nucleation line and of the spinodal locus. Within the
potential energy landscape formalism, we calculate the distributions of depth,
number, and shape of the potential energy minima and show that the statistical
properties of the landscape are consistent with a Gaussian distribution of
minima over a wide range of volumes. We report the volume dependence of the
parameters entering in the Gaussian distribution (amplitude, average energy,
variance). We finally evaluate the locus where the configurational entropy
vanishes, the so-called Kauzmann line, and discuss the relative location of the
spinodal and Kauzmann loci.Comment: RevTeX 4, 8 pages, 8 eps figure
Molecular dynamics simulation study of the high frequency sound waves in the fragile glass former ortho-terphenyl
Using a realistic flexible molecule model of the fragile glass former
orthoterphenyl, we calculate via molecular dynamics simulation the collective
dynamic structure factor, recently measured in this system by Inelastic X-ray
Scattering. The comparison of the simulated and measured dynamic structure
factor, and the study of its properties in an extended momentum, frequency and
temperature range allows: i) to conclude that the utilized molecular model
gives rise to a dynamic structure factor in agreement with the experimental
data, for those thermodynamic states and momentum values where the latter are
available; ii) to confirm the existence of a slope discontinuity on the
T-dependence of the sound velocity that, at finite Q, takes place at a
temperature T_x higher than the calorimetric glass transition temperature T_g;
iii) to find that the values of T_x is Q-dependent and that its vanishing Q
limit is consistent with T_g. The latter finding is interpreted within the
framework of the current description of the dynamics of supercooled liquids in
terms of exploration of the potential energy landscape.Comment: RevTex, 9 pages, 10 eps figure
Vibrational origin of the fast relaxation processes in molecular glass-formers
We study the interaction of the relaxation processes with the density
fluctuations by molecular dynamics simulation of a flexible molecule model for
o-terphenyl (oTP) in the liquid and supercooled phases. We find evidence,
besides the structural relaxation, of a secondary vibrational relaxation whose
characteristic time, few ps, is slightly temperature dependent. This i)
confirms the result by Monaco et al. [Phys. Rev, E 62, 7595 (2000)] of the
vibrational nature of the fast relaxation observed in Brillouin Light
Scattering (BLS) experiments in oTP; and ii) poses a caveat on the
interpretation of the BLS spectra of molecular systems in terms of a purely
center of mass dynamics.Comment: RevTeX, 5 pages, 4 eps figure
Effective temperature of active matter
We follow the dynamics of an ensemble of interacting self-propelled motorized
particles in contact with an equilibrated thermal bath. We find that the
fluctuation-dissipation relation allows for the definition of an effective
temperature that is compatible with the results obtained using a tracer
particle as a thermometer. The effective temperature takes a value which is
higher than the temperature of the bath and it is continuously controlled by
the motor intensity
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