8,469 research outputs found
Entropic Origin of the Growth of Relaxation Times in Simple Glassy Liquids
Transitions between ``glassy'' local minima of a model free-energy functional
for a dense hard-sphere system are studied numerically using a
``microcanonical'' Monte Carlo method that enables us to obtain the transition
probability as a function of the free energy and the Monte Carlo ``time''. The
growth of the height of the effective free energy barrier with density is found
to be consistent with a Vogel-Fulcher law. The dependence of the transition
probability on time indicates that this growth is primarily due to entropic
effects arising from the difficulty of finding low-free-energy saddle points
connecting glassy minima.Comment: Four pages, plus three postscript figure
Phase Diagram Of A Hard-sphere System In A Quenched Random Potential: A Numerical Study
We report numerical results for the phase diagram in the density-disorder
plane of a hard sphere system in the presence of quenched, random, pinning
disorder. Local minima of a discretized version of the Ramakrishnan-Yussouff
free energy functional are located numerically and their relative stability is
studied as a function of the density and the strength of disorder. Regions in
the phase diagram corresponding to liquid, glassy and nearly crystalline states
are mapped out, and the nature of the transitions is determined. The liquid to
glass transition changes from first to second order as the strength of the
disorder is increased. For weak disorder, the system undergoes a first order
crystallization transition as the density is increased. Beyond a critical value
of the disorder strength, this transition is replaced by a continuous glass
transition. Our numerical results are compared with those of analytical work on
the same system. Implications of our results for the field-temperature phase
diagram of type-II superconductors are discussed.Comment: 14 pages, 10 postscript figures (included), submitted to Phys. Rev.
Structure and Magnetization of Two-Dimensional Vortex Arrays in the Presence of Periodic Pinning
Ground-state properties of a two-dimensional system of superconducting
vortices in the presence of a periodic array of strong pinning centers are
studied analytically and numerically. The ground states of the vortex system at
different filling ratios are found using a simple geometric argument under the
assumption that the penetration depth is much smaller than the spacing of the
pin lattice. The results of this calculation are confirmed by numerical studies
in which simulated annealing is used to locate the ground states of the vortex
system. The zero-temperature equilibrium magnetization as a function of the
applied field is obtained by numerically calculating the energy of the ground
state for a large number of closely spaced filling ratios. The results show
interesting commensurability effects such as plateaus in the B-H diagram at
simple fractional filling ratios.Comment: 12 pages, 19 figures, submitted for publicatio
Na2V3O7, a frustrated nanotubular system with spin-1/2 diamond rings
Following the recent discussion on the puzzling nature of the interactions in
the nanotubular system Na2V3O7, we present a detailed ab-initio microscopic
analysis of its electronic and magnetic properties. By means of a non-trivial
downfolding study we propose an effective model in terms of tubes of nine-site
rings with the geometry of a spin-diamond necklace with frustrated inter-ring
interactions. We show that this model provides a quantitative account of the
observed magnetic behavior.Comment: 5 pages, 5 figures. Phys. Rev. Lett. (in press
Role of break-up processes in fusion enhancement of drip-line nuclei at energies below the Coulomb barrier
We carry out realistic coupled-channels calculations for
Be + Pb reaction in order to discuss the effects of break-up
of the projectile nucleus on sub-barrier fusion.
We discretize in energy the particle continuum states, which are associated
with the break-up process, and construct the coupling form factors to these
states on a microscopic basis.
The incoming boundary condition is employed in solving coupled-channels
equations, which enables us to define the flux for complete fusion inside the
Coulomb barrier. It is shown that complete fusion cross sections are
significantly enhanced due to the couplings to the continuum states compared
with the no coupling case at energies below the Coulomb barrier, while they are
hindered at above barrier energies.Comment: RevTex, 3 pages, 5 figure
Electronic structure and exchange interactions of the ladder vanadates CaV2O5 and MgV2O5
We have performed ab-initio calculations of the electronic structure and
exchange couplings in the layered vanadates CaV2O5 and MgV2O5. Based on our
results we provide a possible explanation of the unusual magnetic properties of
these materials, in particular the large difference in the spin gap between
CaV2O5 and MgV2O5
Construction of the free energy landscape by the density functional theory
On the basis of the density functional theory, we give a clear definition of
the free energy landscape. To show the usefulness of the definition, we
construct the free energy landscape for rearrangement of atoms in an FCC
crystal of hard spheres. In this description, the cooperatively rearranging
region (CRR) is clealy related to the hard spheres involved in the saddle
between two adjacent basins. A new concept of the simultaneously rearranging
region (SRR) emerges naturally as spheres defined by the difference between two
adjacent basins. We show that the SRR and the CRR can be determined explicitly
from the free energylandscape.Comment: 11 pages, 3 figures, submitted to J. Chem. Phy
Evidence of Double Phonon Excitations in ^{16}O + ^{208}Pb Reaction
The fusion cross-sections for ^{16}O + ^{208}Pb, measured to high precision,
enable the extraction of the distribution of fusion barriers. This shows a
structure markedly different from the single-barrier which might be expected
for fusion of two doubly-closed shell nuclei. The results of exact coupled
channel calculations performed to understand the observations are presented.
These calculations indicate that coupling to a double octupole phonon excited
state in ^{208}Pb is necessary to explain the experimental barrier
distributions.Comment: 6 pages, 2 figures, To be published in the Proceedings of the FUSION
97 Conference, South Durras, Australia, March 1997 (J. Phys. G
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