9,100 research outputs found
Glass Transition of the Monodisperse Gaussian Core Model
We numerically study dynamical properties of the one-component Gaussian Core
Model in the supercooled states. We find that nucleation is suppressed as
density increases. Concomitantly the system exhibits glassy slow dynamics
characterized by the two-step and stretched exponential relaxation of the
density correlation as well as drastic increase of the relaxation time. It is
found that violation of the Stokes-Einstein relation is weaker and the
non-Gaussian parameter is smaller than typical model glass formers, implying
weaker dynamic heterogeneities. Besides, agreement of simulation data with the
prediction of mode-coupling theory is exceptionally good, indicating that the
nature of slow dynamics of this ultra-soft particle fluid is mean-field-like.
This fact may be understood as the consequences of multiple overlaps of the
constituent particles at high densities.Comment: 5 pages, 4 figure
Thermodynamics and Structural Properties of the High Density Gaussian Core Model
We numerically study thermodynamic and structural properties of the
one-component Gaussian core model (GCM) at very high densities. The solid-fluid
phase boundary is carefully determined. We find that the density dependence of
both the freezing and melting temperatures obey the asymptotic relation, , , where is the number density, which
is consistent with Stillinger's conjecture. Thermodynamic quantities such as
the energy and pressure and the structural functions such as the static
structure factor are also investigated in the fluid phase for a wide range of
temperature above the phase boundary. We compare the numerical results with the
prediction of the liquid theory with the random phase approximation (RPA). At
high temperatures, the results are in almost perfect agreement with RPA for a
wide range of density, as it has been already shown in the previous studies. In
the low temperature regime close to the phase boundary line, although RPA fails
to describe the structure factors and the radial distribution functions at the
length scales of the interparticle distance, it successfully predicts their
behaviors at shorter length scales. RPA also predicts thermodynamic quantities
such as the energy, pressure, and the temperature at which the thermal
expansion coefficient becomes negative, almost perfectly. Striking ability of
RPA to predict thermodynamic quantities even at high densities and low
temperatures is understood in terms of the decoupling of the length scales
which dictate thermodynamic quantities from the interparticle distance which
dominates the peak structures of the static structure factor due to the
softness of the Gaussian core potential.Comment: 10 pages, 10 figure
Ab initio studies of the spin-transfer torque in tunnel junctions
We calculate the spin-transfer torque in Fe/MgO/Fe tunnel junctions and
compare the results to those for all-metallic junctions. We show that the
spin-transfer torque is interfacial in the ferromagnetic layer to a greater
degree than in all-metallic junctions. This result originates in the half
metallic behavior of Fe for the states at the Brillouin zone center;
in contrast to all-metallic structures, dephasing does not play an important
role. We further show that it is possible to get a component of the torque that
is out of the plane of the magnetizations and that is linear in the bias.
However, observation of such a torque requires highly ideal samples. In samples
with typical interfacial roughness, the torque is similar to that in
all-metallic multilayers, although for different reasons.Comment: 5 pages, 4 figure
Slow Dynamics of the High Density Gaussian Core Model
We numerically study crystal nucleation and glassy slow dynamics of the
one-component Gaussian core model (GCM) at high densities. The nucleation rate
at a fixed supersaturation is found to decrease as the density increases. At
very high densities, the nucleation is not observed at all in the time window
accessed by long molecular dynamics (MD) simulation. Concomitantly, the system
exhibits typical slow dynamics of the supercooled fluids near the glass
transition point. We compare the simulation results of the supercooled GCM with
the predictions of mode-coupling theory (MCT) and find that the agreement
between them is better than any other model glassformers studied numerically in
the past. Furthermore, we find that a violation of the Stokes-Einstein relation
is weaker and the non-Gaussian parameter is smaller than canonical
glassformers. Analysis of the probability distribution of the particle
displacement clearly reveals that the hopping effect is strongly suppressed in
the high density GCM. We conclude from these observations that the GCM is more
amenable to the mean-field picture of the glass transition than other models.
This is attributed to the long-ranged nature of the interaction potential of
the GCM in the high density regime. Finally, the intermediate scattering
function at small wavevectors is found to decay much faster than its self part,
indicating that dynamics of the large-scale density fluctuations decouples with
the shorter-ranged caging motion.Comment: 15 pages, 13 figure
Mode coupling theory in the FDR-preserving field theory of interacting Brownian particles
We develop a renormalized perturbation theory for the dynamics of interacting
Brownian particles, which preserves the fluctuation-dissipation relation order
by order. We then show that the resulting one-loop theory gives a closed
equation for the density correlation function, which is identical with that in
the standard mode coupling theory.Comment: version to be published in Fast Track Communication in Journal of
Physics A:Math. Theo
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