1,287 research outputs found
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
First-principles prediction of extraordinary thermoelectric efficiency in superionic Li2SnX3(X=S,Se)
Thermoelectric materials create an electric potential when subject to a
temperature gradient and vice versa hence they can be used to harvest waste
heat into electricity and in thermal management applications. However, finding
highly efficient thermoelectrics with high figures of merit, zT1, is very
challenging because the combination of high power factor and low thermal
conductivity is rare in materials. Here, we use first-principles methods to
analyze the thermoelectric properties of LiSn (=S,Se), a recently
synthesized class of lithium fast-ion conductors presenting high thermal
stability. In p-type LiSn, we estimate highly flat electronic valence
bands that render high Seebeck coefficients exceeding 400 VK at
700K. In n-type LiSn, the electronic conduction bands are slightly
dispersive however the accompanying weak electron-acoustic phonon scattering
induces high electrical conductivity. The combination of high Seebeck
coefficient and electrical conductivity gives rise to high power factors,
reaching a maximum of 4 mWmK in p-type LiSnS and 8
mWmK in n-type LiSnSe at 300 K. Likewise, the thermal
conductivity in LiSn is low as compared to conventional thermoelectric
materials, 2-5 WmK at room temperature. As a result, we estimate
a maximum zT = 1.05 in p-type LiSnS at 700 K and an extraordinary 3.07
(1.5) in n-type LiSnSe at the same temperature (300 K). Our findings of
huge zT in LiSn suggest that lithium fast-ion conductors, typically
employed as electrolytes in solid-state batteries, hold exceptional promise as
thermoelectric materials.Comment: 21 Page
Two-dimensional molecular para-hydrogen and ortho-deuterium at zero temperature
We study molecular para-hydrogen (p-) and ortho-deuterium
(o-) 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-, 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- 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 He and
found that also characterizes hydrogen satisfactorily. With this wave function,
we show that the one-body density matrix of solid p- possesses off-diagonal long range order, with a condensate fraction
that increases sizably in the negative pressure regime.Comment: 11 pages, 9 figure
First-Principles Modeling of Pt/LaAlO3/SrTiO3 Capacitors Under an External Bias Potential
We study the electrical properties of Pt/LaAlO3/SrTiO3 capacitors under the
action of an external bias potential, using first-principles simulations
performed at constrained electric displacement field. A complete set of band
diagrams, together with the relevant electrical characteristics (capacitance
and built-in fields), are determined as a function of LaAlO3 thickness and the
applied potential.We find that the internal field in LaAlO3 monotonically
decreases with increasing thickness; hence, the occurrence of spontaneous Zener
tunneling is ruled out in this system.We discuss the implications of our
results in the light of recent experimental observations on biased
LaAlO3/SrTiO3 junctions involving metallic top electrodes.Comment: 5 pages, 4 figure
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
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 when the Lindemann's ratio of the model equals the experimental
value for solid He.Comment: 5 pages,5 figure
The biting and predaceous midges of Guadeloupe (Diptera: Ceratopogonidae). I. Species of the subfamily Ceratopogoninae
We provide new records of biting and predaceous midges (Diptera: Ceratopogonidae) from Guadeloupe in the subfamily Ceratopogoninae, including descriptions and illustrations of three new predaceous species in the genera, Parabezzia Malloch, Stilobezzia Kieffer and Palpomyia Meigen, respectively, and the first records of the New World predaceous genus, Amerohelea Grogan and Wirth, from the Caribbean region. We also provide the first Guadeloupe records of the biting midges, Culicoides (Anilomyia) decor (Williston), C. (Avaritia) pusillus Lutz, C. (Drymodesmyia) bredini Wirth and Blanton, C. (D.) poikilonotus Macfie, C. (Haematomyidium) hoffmani Fox, C. (Hoffmania) insignis Lutz, C. rangeli Ortiz and Mirsa and C. trilineatus Fox, and the predaceous midges, Brachypogon (Brachypogon) bifidus Spinelli and Grogan, B. (B.) telesfordi Spinelli and Grogan, B. (B.) woodruffi Spinelli and Grogan, Monohelea maya Felippe-Bauer, Huerta and Ibáñez-Bernal, Stilobezzia (Stilobezzia) diminuta Lane and Forattini, S. (S.) thomsenae Wirth, Amerohelea galindoi Grogan and Wirth, Bezzia (Bezzia) flinti Spinelli and Wirth, B. (Homobezzia) venustula (Williston) and Palpomyia insularis Spinelli and Grogan
Measurement of Ozone Production Sensor
A new ambient air monitor, the Measurement of Ozone Production Sensor (MOPS), measures directly the rate of ozone production in the atmosphere. The sensor consists of two 11.3 L environmental chambers made of UV-transmitting Teflon film, a unit to convert NO<sub>2</sub> to O<sub>3</sub>, and a modified ozone monitor. In the sample chamber, flowing ambient air is exposed to the sunlight so that ozone is produced just as it is in the atmosphere. In the second chamber, called the reference chamber, a UV-blocking film over the Teflon film prevents ozone formation but allows other processes to occur as they do in the sample chamber. The air flows that exit the two chambers are sampled by an ozone monitor operating in differential mode so that the difference between the two ozone signals, divided by the exposure time in the chambers, gives the ozone production rate. High-efficiency conversion of NO<sub>2</sub> to O<sub>3</sub> prior to detection in the ozone monitor accounts for differences in the NO<sub>x</sub> photostationary state that can occur in the two chambers. The MOPS measures the ozone production rate, but with the addition of NO to the sampled air flow, the MOPS can be used to study the sensitivity of ozone production to NO. Preliminary studies with the MOPS on the campus of the Pennsylvania State University show the potential of this new technique
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 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 and number of atoms , 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
before melting occurs. We find that the probability distribution
of is roughly exponential, and that the mean value 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 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
Supersolidity in quantum films adsorbed on graphene and graphite
Using quantum Monte Carlo we have studied the superfluid density of the first
layer of He and H adsorbed on graphene and graphite. Our main focus has
been on the equilibrium ground state of the system, which corresponds to a
registered phase. The perfect solid phase of H shows
no superfluid signal whereas 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 He without destroying the
spatial solid order.Comment: 5 pages, accepted for publication in PR
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