950 research outputs found

### Ground-state properties and superfluidity of two- and quasi two-dimensional solid 4He

In a recent study we have reported a new type of trial wave function
symmetric under the exchange of particles and which is able to describe a
supersolid phase. In this work, we use the diffusion Monte Carlo method and
this model wave function to study the properties of solid 4He in two- and quasi
two-dimensional geometries. In the purely two-dimensional case, we obtain
results for the total ground-state energy and freezing and melting densities
which are in good agreement with previous exact Monte Carlo calculations
performed with a slightly different interatomic potential model. We calculate
the value of the zero-temperature superfluid fraction \rho_{s} / \rho of 2D
solid 4He and find that it is negligible in all the considered cases, similarly
to what is obtained in the perfect (free of defects) three-dimensional crystal
using the same computational approach. Interestingly, by allowing the atoms to
move locally in the perpendicular direction to the plane where they are
confined to zero-point oscillations (quasi two-dimensional crystal) we observe
the emergence of a finite superfluid density that coexists with the periodicity
of the system.Comment: 16 pages, 8 figure

### Two-dimensional molecular para-hydrogen and ortho-deuterium at zero temperature

We study molecular para-hydrogen (p-${\rm H_{2}}$) and ortho-deuterium
(o-${\rm D_{2}}$) in two dimensions and in the limit of zero temperature by
means of the diffusion Monte Carlo method. We report energetic and structural
properties of both systems like the total and kinetic energy per particle,
radial pair distribution function, and Lindemann's ratio in the low pressure
regime. By comparing the total energy per particle as a function of the density
in liquid and solid p-${\rm H_{2}}$, we show that molecular para-hydrogen, and
also ortho-deuterium, remain solid at zero temperature. Interestingly, we
assess the quality of three different symmetrized trial wave functions, based
on the Nosanow-Jastrow model, in the p-${\rm H_{2}}$ solid film at the
variational level. In particular, we analyze a new type of symmetrized trial
wave function which has been used very recently to describe solid $^{4}$He and
found that also characterizes hydrogen satisfactorily. With this wave function,
we show that the one-body density matrix $\varrho_{1} (r)$ of solid p-${\rm
H_{2}}$ possesses off-diagonal long range order, with a condensate fraction
that increases sizably in the negative pressure regime.Comment: 11 pages, 9 figure

### Comment on 'Molybdenum at High Pressure and Temperature: Melting from Another Solid Phase'

There has been a major controversy over the past seven years about the
high-pressure melting curves of transition metals. Static compression
(diamond-anvil cell: DAC) experiments up to the Mbar region give very low
melting slopes dT_m/dP, but shock-wave (SW) data reveal transitions indicating
much larger dT_m/dP values. Ab initio calculations support the correctness of
the shock data. In a very recent letter, Belonoshko et al. propose a simple and
elegant resolution of this conflict for molybdenum. Using ab initio
calculations based on density functional theory (DFT), they show that the
high-P/high-T phase diagram of Mo must be more complex than was hitherto
thought. Their calculations give convincing evidence that there is a transition
boundary between the normal bcc structure of Mo and a high-T phase, which they
suggest could be fcc. They propose that this transition was misinterpreted as
melting in DAC experiments. In confirmation, they note that their boundary also
explains a transition seen in the SW data. We regard Belonoshko et al.'s Letter
as extremely important, but we note that it raises some puzzling questions, and
we believe that their proposed phase diagram cannot be completely correct. We
have calculated the Helmholtz and Gibbs free energies of the bcc, fcc and hcp
phases of Mo, using essentially the same quasiharmonic methods as used by
Belonoshko et al.; we find that at high-P and T Mo in the hcp structure is more
stable than in bcc or fcc.Comment: 1 page, 1 figure. submitted to Phys. Rev. Let

### Ab initio melting curve of molybdenum by the phase coexistence method

We report ab initio calculations of the melting curve of molybdenum for the
pressure range 0-400 GPa. The calculations employ density functional theory
(DFT) with the Perdew-Burke-Ernzerhof exchange-correlation functional in the
projector augmented wave (PAW) implementation. We present tests showing that
these techniques accurately reproduce experimental data on low-temperature
b.c.c. Mo, and that PAW agrees closely with results from the full-potential
linearized augmented plane-wave implementation. The work attempts to overcome
the uncertainties inherent in earlier DFT calculations of the melting curve of
Mo, by using the ``reference coexistence'' technique to determine the melting
curve. In this technique, an empirical reference model (here, the embedded-atom
model) is accurately fitted to DFT molecular dynamics data on the liquid and
the high-temperature solid, the melting curve of the reference model is
determined by simulations of coexisting solid and liquid, and the ab initio
melting curve is obtained by applying free-energy corrections. Our calculated
melting curve agrees well with experiment at ambient pressure and is consistent
with shock data at high pressure, but does not agree with the high pressure
melting curve deduced from static compression experiments. Calculated results
for the radial distribution function show that the short-range atomic order of
the liquid is very similar to that of the high-T solid, with a slight decrease
of coordination number on passing from solid to liquid. The electronic
densities of states in the two phases show only small differences. The results
do not support a recent theory according to which very low dTm/dP values are
expected for b.c.c. transition metals because of electron redistribution
between s-p and d states.Comment: 27 pages, 10 figures. to be published in Journal of Chemical Physic

### Superfluidity versus localization in bulk 4He at zero temperature

We present a zero-temperature quantum Monte Carlo calculation of liquid
$^4$He immersed in an array of confining potentials. These external potentials
are centered in the lattice sites of a fcc solid geometry and, by modifying
their well depth and range, the system evolves from a liquid phase towards a
progressively localized system which mimics a solid phase. The superfluid
density decreases with increasing order, reaching a value $\rho_{\rm s}/\rho =
0.079(16)$ when the Lindemann's ratio of the model equals the experimental
value for solid $^4$He.Comment: 5 pages,5 figure

### Supersolidity in quantum films adsorbed on graphene and graphite

Using quantum Monte Carlo we have studied the superfluid density of the first
layer of $^4$He and H$_2$ adsorbed on graphene and graphite. Our main focus has
been on the equilibrium ground state of the system, which corresponds to a
registered $\sqrt3 \times \sqrt3$ phase. The perfect solid phase of H$_2$ shows
no superfluid signal whereas $^4$He has a finite but small superfluid fraction
(0.67%). The introduction of vacancies in the crystal makes the superfluidity
increase, showing values as large as 14% in $^4$He without destroying the
spatial solid order.Comment: 5 pages, accepted for publication in PR

### The kinetics of homogeneous melting beyond the limit of superheating

Molecular dynamics simulation is used to study the time-scales involved in
the homogeneous melting of a superheated crystal. The interaction model used is
an embedded-atom model for Fe developed in previous work, and the melting
process is simulated in the microcanonical $(N, V, E)$ ensemble. We study
periodically repeated systems containing from 96 to 7776 atoms, and the initial
system is always the perfect crystal without free surfaces or other defects.
For each chosen total energy $E$ and number of atoms $N$, we perform several
hundred statistically independent simulations, with each simulation lasting for
between 500 ps and 10 ns, in order to gather statistics for the waiting time
$\tau_{\rm w}$ before melting occurs. We find that the probability distribution
of $\tau_{\rm w}$ is roughly exponential, and that the mean value $<\tau_{\rm
w} >$ depends strongly on the excess of the initial steady temperature of the
crystal above the superheating limit identified by other researchers. The mean
$$ also depends strongly on system size in a way that we have
quantified. For very small systems of $\sim 100$ atoms, we observe a persistent
alternation between the solid and liquid states, and we explain why this
happens. Our results allow us to draw conclusions about the reliability of the
recently proposed Z method for determining the melting properties of simulated
materials, and to suggest ways of correcting for the errors of the method.Comment: 19 pages, 8 figure

### Melting properties of a simple tight-binding model of transition metals: I.The region of half-filled d-band

We present calculations of the free energy, and hence the melting properties,
of a simple tight-binding model for transition metals in the region of d-band
filling near the middle of a d-series, the parameters of the model being
designed to mimic molybdenum. The melting properties are calculated for
pressures ranging from ambient to several Mbar. The model is intended to be the
simplest possible tight-binding representation of the two basic parts of the
energy: first, the pairwise repulsion due to Fermi exclusion; and second, the
d-band bonding energy described in terms of an electronic density of states
that depends on structure. In addition to the number of d-electrons, the model
contains four parameters, which are adjusted to fit the pressure dependent
d-band width and the zero-temperature pressure-volume relation of Mo. We show
that the resulting model reproduces well the phonon dispersion relations of Mo
in the body-centred-cubic structure, as well as the radial distribution
function of the high-temperature solid and liquid given by earlier
first-principles simulations. Our free-energy calculations start from the free
energy of the liquid and solid phases of the purely repulsive pair-potential
model, without d-band bonding. The free energy of the full tight-binding model
is obtained from this by thermodynamic integration. The resulting melting
properties of the model are quite close to those given by earlier
first-principles work on Mo. An interpretation of these melting properties is
provided by showing how they are related to those of the purely repulsive
model.Comment: 34 pages, 12 figures. Accepted for publication in Journal of Chemical
Physic

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