1,687 research outputs found
Superconductivity in Boron under pressure - why are the measured T's so low?
Using the full potential linear muffin-tin orbitals (FP-LMTO) method we
examine the pressure-dependence of superconductivity in the two metallic phases
of Boron: bct and fcc. Linear response calculations are carried out to examine
the phonon frequencies and electron-phonon coupling for various lattice
parameters, and superconducting transition temperatures are obtained from the
Eliashberg equation. In both bct and fcc phases the superconducting transition
temperature T is found to decrease with increasing pressure, due to
stiffening of phonons with an accompanying decrease in electron-phonon
coupling. This is in contrast to a recent report, where T is found to
increase with pressure. Even more drastic is the difference between the
measured T, in the range 4-11 K, and the calculated values for both bct and
fcc phases, in the range 60-100 K. The calculation reveals that the transition
from the fcc to bct phase, as a result of increasing volume or decreasing
pressure, is caused by the softening of the X-point transverse phonons. This
phonon softening also causes large electron-phonon coupling for high volumes in
the fcc phase, resulting in coupling constants in excess of 2.5 and T
nearing 100 K. We discuss possible causes as to why the experiment might have
revealed T's much lower than what is suggested by the present study. The
main assertion of this paper is that the possibility of high T, in excess
of 50 K, in high pressure pure metallic phases of boron cannot be ruled out,
thus substantiating the need for further experimental investigations of the
superconducting properties of high pressure pure phases of boron.Comment: 16 pages, 8 figures, 1 Tabl
Electronic Structure of the Chevrel-Phase Compounds SnMoSe: Photoemission Spectroscopy and Band-structure Calculations
We have studied the electronic structure of two Chevrel-phase compounds,
MoSe and SnMoSe, by combining photoemission
spectroscopy and band-structure calculations. Core-level spectra taken with
x-ray photoemission spectroscopy show systematic core-level shifts, which do
not obey a simple rigid-band model. The inverse photoemission spectra imply the
existence of an energy gap located eV above the Fermi level, which is
a characteristic feature of the electronic structure of the Chevrel compounds.
Quantitative comparison between the photoemission spectra and the
band-structure calculations have been made. While good agreement between theory
and experiment in the wide energy range was obtained as already reported in
previous studies, we found that the high density of states near the Fermi level
predicted theoretically due to the Van Hove singularity is considerably reduced
in the experimental spectra taken with higher energy resolution than in the
previous reports. Possible origins are proposed to explain this observation.Comment: 8 pages, 5 figure
Developing the MTO Formalism
We review the simple linear muffin-tin orbital method in the atomic-spheres
approximation and a tight-binding representation (TB-LMTO-ASA method), and show
how it can be generalized to an accurate and robust Nth order muffin-tin
orbital (NMTO) method without increasing the size of the basis set and without
complicating the formalism. On the contrary, downfolding is now more efficient
and the formalism is simpler and closer to that of screened multiple-scattering
theory. The NMTO method allows one to solve the single-electron Schroedinger
equation for a MT-potential -in which the MT-wells may overlap- using basis
sets which are arbitrarily minimal. The substantial increase in accuracy over
the LMTO-ASA method is achieved by substitution of the energy-dependent partial
waves by so-called kinked partial waves, which have tails attached to them, and
by using these kinked partial waves at N+1 arbitrary energies to construct the
set of NMTOs. For N=1 and the two energies chosen infinitesimally close, the
NMTOs are simply the 3rd-generation LMTOs. Increasing N, widens the energy
window, inside which accurate results are obtained, and increases the range of
the orbitals, but it does not increase the size of the basis set and therefore
does not change the number of bands obtained. The price for reducing the size
of the basis set through downfolding, is a reduction in the number of bands
accounted for and -unless N is increased- a narrowing of the energy window
inside which these bands are accurate. A method for obtaining orthonormal NMTO
sets is given and several applications are presented.Comment: 85 pages, Latex2e, Springer style, to be published in: Lecture notes
in Physics, edited by H. Dreysse, (Springer Verlag
Combined density-functional and dynamical cluster quantum Monte Carlo calculations for three-band Hubbard models for hole-doped cuprate superconductors
Using a combined local density functional theory (LDA-DFT) and quantum Monte
Carlo (QMC) dynamic cluster approximation approach, the parameter dependence of
the superconducting transition temperature Tc of several single-layer
hole-doped cuprate superconductors with experimentally very different Tcmax is
investigated. The parameters of two different three-band Hubbard models are
obtained using the LDA and the downfolding Nth-order muffin-tin orbital
technique with N=0 and 1 respectively. QMC calculations on 4-site clusters show
that the d-wave transition temperature Tc depends sensitively on the
parameters. While the N=1 MTO basis set which reproduces all three
bands leads to a d-wave transition, the N=0 set which merely reproduces the LDA
Fermi surface and velocities does not
Third-Generation TB-LMTO
We describe the screened Korringa-Kohn-Rostoker (KKR) method and the
third-generation linear muffin-tin orbital (LMTO) method for solving the
single-particle Schroedinger equation for a MT potential. The simple and
popular formalism which previously resulted from the atomic-spheres
approximation (ASA) now holds in general, that is, it includes downfolding and
the combined correction. Downfolding to few-orbital, possibly short-ranged,
low-energy, and possibly orthonormal Hamiltonians now works exceedingly well,
as is demonstrated for a high-temperature superconductor. First-principles sp3
and sp3d5 TB Hamiltonians for the valence and lowest conduction bands of
silicon are derived. Finally, we prove that the new method treats overlap of
the potential wells correctly to leading order and we demonstrate how this can
be exploited to get rid of the empty spheres in the diamond structure.Comment: latex2e, 32 printed pages, Postscript figs, to be published in:
Tight-Binding Approach to Computational Materials Science, MRS Symposia
Proceedings No. 491 (MRS, Pittsburgh, 1998
New directions for hydrogen storage: Sulphur destabilised sodium aluminium hydride
Aluminium sulphide (Al2S3) is predicted to effectively destabilise sodium aluminium hydride (NaAlH4) in a single-step endothermic hydrogen release reaction. The experimental results show unexpectedly complex desorption processes and a range of new sulphur containing hydrogen storage materials have been observed. The NaAlH4-Al 2S3 system releases a total of 4.9 wt% of H2 that begins below 100°C without the need for a catalyst. Characterisation via temperature programmed desorption, in situ synchrotron powder X-ray diffraction, ex situ x-ray diffraction, ex situ Fourier transform infrared spectroscopy and hydrogen sorption measurements reveal complex decomposition processes that involve multiple new sulphur-containing hydride compounds. The system shows partial H2 reversibility, without the need for a catalyst, with a stable H2 capacity of ~1.6 wt% over 15 cycles in the temperature range of 200°C to 300°C. This absorption capacity is limited by the need for high H2 pressures (>280 bar) to drive the absorption process at the high temperatures required for reasonable absorption kinetics. The large number of new phases discovered in this system suggests that destabilisation of complex hydrides with metal sulphides is a novel but unexplored research avenue for hydrogen storage materials
Lattice vibrations and structural instability in Cesium near the cubic to tetragonal transition
Under pressure cesium undergoes a transition from a high-pressure fcc phase
(Cs-II) to a collapsed fcc phase (Cs-III) near 4.2GPa. At 4.4GPa there follows
a transition to the tetragonal Cs-IV phase. In order to investigate the lattice
vibrations in the fcc phase and seek a possible dynamical instability of the
lattice, the phonon spectra of fcc-Cs at volumes near the III-IV transition are
calculated using Savrasov's density functional linear-response LMTO method.
Compared with quasiharmonic model calculations including non-central
interatomic forces up to second neighbours, at the volume (
is the experimental volume of bcc-Cs with =6.048{\AA}), the
linear-response calculations show soft intermediate wavelength
phonons. Similar softening is also observed for
short wavelength and phonons and intermediate
wavelength phonons. The Born-von K\'{a}rm\'{a}n analysis of
dispersion curves indicates that the interplanar force constants exhibit
oscillating behaviours against plane spacing and the large softening of
intermediate wavelength phonons results from a
negative (110)-interplanar force-constant . The frequencies of the
phonons with around 1/3 become imaginary
and the fcc structure becomes dynamically unstable for volumes below .
It is suggested that superstructures corresponding to the
soft mode should be present as a precursor of tetragonal Cs-IV structure.Comment: 12 pages, 5 figure
Heat Conduction and Entropy Production in a One-Dimensional Hard-Particle Gas
We present large scale simulations for a one-dimensional chain of hard-point
particles with alternating masses. We correct several claims in the recent
literature based on much smaller simulations. Both for boundary conditions with
two heat baths at different temperatures at both ends and from heat current
autocorrelations in equilibrium we find heat conductivities kappa to diverge
with the number N of particles. These depended very strongly on the mass
ratios, and extrapolation to N -> infty resp. t -> infty is difficult due to
very large finite-size and finite-time corrections. Nevertheless, our data seem
compatible with a universal power law kappa ~ N^alpha with alpha approx 0.33.
This suggests a relation to the Kardar-Parisi-Zhang model. We finally show that
the hard-point gas with periodic boundary conditions is not chaotic in the
usual sense and discuss why the system, when kept out of equilibrium, leads
nevertheless to energy dissipation and entropy production.Comment: 4 pages (incl. 5 figures), RevTe
A simple one-dimensional model of heat conduction which obeys Fourier's law
We present the computer simulation results of a chain of hard point particles
with alternating masses interacting on its extremes with two thermal baths at
different temperatures. We found that the system obeys Fourier's law at the
thermodynamic limit. This result is against the actual belief that one
dimensional systems with momentum conservative dynamics and nonzero pressure
have infinite thermal conductivity. It seems that thermal resistivity occurs in
our system due to a cooperative behavior in which light particles tend to
absorb much more energy than the heavier ones.Comment: 5 pages, 4 figures, to be published in PR
The Prevalence of Latent Mycobacterium Tuberculosis Infection Based on an Interferon-γ Release Assay: A Cross-Sectional Survey Among Urban Adults in Mwanza, Tanzania.
One third of the world's population is estimated to be latently infected with Mycobacterium tuberculosis (LTBI). Surveys of LTBI are rarely performed in resource poor TB high endemic countries like Tanzania although low-income countries harbor the largest burden of the worlds LTBI. The primary objective was to estimate the prevalence of LTBI in household contacts of pulmonary TB cases and a group of apparently healthy neighborhood controls in an urban setting of such a country. Secondly we assessed potential impact of LTBI on inflammation by quantitating circulating levels of an acute phase reactant: alpha-1-acid glycoprotein (AGP) in neighborhood controls. The study was nested within the framework of two nutrition studies among TB patients in Mwanza, Tanzania. Household contacts- and neighborhood controls were invited to participate. The study involved a questionnaire, BMI determination and blood samples to measure AGP, HIV testing and a Quantiferon Gold In tube (QFN-IT) test to detect signs of LTBI. 245 household contacts and 192 neighborhood controls had available QFN-IT data. Among household contacts, the proportion of QFT-IT positive was 59% compared to 41% in the neighborhood controls (p = 0.001). In a linear regression model adjusted for sex, age, CD4 and HIV, a QFT-IT positive test was associated with a 10% higher level of alpha-1-acid glycoprotein(AGP) (10(B) 1.10, 95% CI 1.01; 1.20, p = 0.03), compared to individuals with a QFT-IT negative test. LTBI is highly prevalent among apparently healthy urban Tanzanians even without known exposure to TB in the household. LTBI was found to be associated with elevated levels of AGP. The implications of this observation merit further studies
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