965 research outputs found
Zero-temperature generalized phase diagram of the 4d transition metals under pressure
We use an accurate implementation of density functional theory (DFT) to
calculate the zero-temperature generalized phase diagram of the 4 series of
transition metals from Y to Pd as a function of pressure and atomic number
. The implementation used is full-potential linearized augmented plane waves
(FP-LAPW), and we employ the exchange-correlation functional recently developed
by Wu and Cohen. For each element, we obtain the ground-state energy for
several crystal structures over a range of volumes, the energy being converged
with respect to all technical parameters to within meV/atom. The
calculated transition pressures for all the elements and all transitions we
have found are compared with experiment wherever possible, and we discuss the
origin of the significant discrepancies. Agreement with experiment for the
zero-temperature equation of state is generally excellent. The generalized
phase diagram of the 4 series shows that the major boundaries slope towards
lower with increasing for the early elements, as expected from the
pressure induced transfer of electrons from states to states, but are
almost independent of for the later elements. Our results for Mo indicate a
transition from bcc to fcc, rather than the bcc-hcp transition expected from
- transfer.Comment: 28 pages and 10 figures. Submitted to Phys. Rev.
Effect of pressure on the Raman modes of antimony
The effect of pressure on the zone-center optical phonon modes of antimony in
the A7 structure has been investigated by Raman spectroscopy. The A_g and E_g
frequencies exhibit a pronounced softening with increasing pressure, the effect
being related to a gradual suppression of the Peierls-like distortion of the A7
phase relative to a cubic primitive lattice. Also, both Raman modes broaden
significantly under pressure. Spectra taken at low temperature indicate that
the broadening is at least partly caused by phonon-phonon interactions. We also
report results of ab initio frozen-phonon calculations of the A_g and E_g mode
frequencies. Presence of strong anharmonicity is clearly apparent in calculated
total energy versus atom displacement relations. Pronounced nonlinearities in
the force versus displacement relations are observed. Structural instabilities
of the Sb-A7 phase are briefly addressed in the Appendix.Comment: 10 pages, 8 figure
Tight-binding study of high-pressure phase transitions in titanium: alpha to omega and beyond
We use a tight-binding total energy method, with parameters determined from a
fit to first-principles calculations, to examine the newly discovered gamma
phase of titanium. Our parameters were adjusted to accurately describe the
alpha Ti-omega Ti phase transition, which is misplaced by density functional
calculations. We find a transition from omega Ti to gamma Ti at 102 GPa, in
good agreement with the experimental value of 116 GPa. Our results suggest that
current density functional calculations will not reproduce the omega Ti-gamma
Ti phase transition, but will instead predict a transition from omega Ti to the
bcc beta Ti phase.Comment: 3 encapsulated Postscript figures, submitted to Phyical Review
Letter
Strained tetragonal states and Bain paths in metals
Paths of tetragonal states between two phases of a material, such as bcc and
fcc, are called Bain paths. Two simple Bain paths can be defined in terms of
special imposed stresses, one of which applies directly to strained epitaxial
films. Each path goes far into the range of nonlinear elasticity and reaches a
range of structural parameters in which the structure is inherently unstable.
In this paper we identify and analyze the general properties of these paths by
density functional theory. Special examples include vanadium, cobalt and
copper, and the epitaxial path is used to identify an epitaxial film as related
uniquely to a bulk phase.Comment: RevTeX, 4 pages, 4 figures, submitted to Phys. Rev. Let
Boron in copper: a perfect misfit in the bulk and cohesion enhancer at a grain boundary
Our ab initio study suggests that boron segregation to the Sigma 5(310)[001]
grain boundary should strengthen the boundary up to 1.5 ML coverage (15.24
at/nm^2). The maximal effect is observed at 0.5 ML and corresponds to boron
atoms filling exclusively grain boundary interstices. In copper bulk, B causes
significant distortion both in interstitial and regular lattice sites for which
boron atoms are either too big or too small. The distortion is compensated to
large extent when the interstitial and substitutional boron combine together to
form a strongly bound dumbell. Our prediction is that bound boron impurities
should appear in sizable proportion if not dominate in most experimental
conditions. A large discrepancy between calculated heats of solution and
experimental terminal solubility of B in Cu is found, indicating either a sound
failure of the local density approximation or, more likely, strongly
overestimated solubility limits in the existing B-Cu phase diagram.Comment: 16 pages, 9 figure
The QCD Phase Structure at High Baryon Density
We consider the possibility that color deconfinement and chiral symmetry
restoration do not coincide in dense baryonic matter at low temperature. As a
consequence, a state of massive "constituent" quarks would exist as an
intermediate phase between confined nuclear matter and the plasma of deconfined
massless quarks and gluons. We discuss the properties of this state and its
relation to the recently proposed quarkyonic matter.Comment: 17 pages, 9 figure
Impact testing to determine the mechanical properties of articular cartilage in isolation and on bone
The original publication is available at www.springerlink.comNon peer reviewedPostprin
Pseudogap formation of four-layer BaRuO and its electrodynamic response changes
We investiaged the optical properties of four-layer BaRuO, which shows
a fermi-liquid-like behavior at low temperature. Its optical conductivity
spectra clearly displayed the formation of a pseudogap and the development of a
coherent peak with decreasing temperature. Temperature-dependences of the
density and the scattering rate of the coherent component were
also derived. As the temperature decreases, both and decrease for
four-layer BaRuO. These electrodynamic responses were compared with those
of nine-layer BaRuO, which also shows a pseudogap formation but has an
insulator-like state at low temperature. It was found that the relative rates
of change of both and determine either metallic or insulator-like
responses in the ruthenates. The optical properties of the four-layer ruthenate
were also compared with those of other pseudogap systems, such as high
cuprates and heavy electron systems.Comment: 7 figures. submitted to Phys. Rev.
Calculations of the A_1 phonon frequency in photoexcited Tellurium
Calculations of the A_1 phonon frequency in photoexcited tellurium are
presented. The phonon frequency as a function of photoexcited carrier density
and phonon amplitude is determined. Recent pump probe experiments are
interpreted in the light of these calculatons. It is proposed that, in
conjunction with measurements of the phonon period in ultra-fast pump-probe
reflectivity experiments, the calculated frequency shifts can be used to infer
the evolution of the density of photoexcited carriers on a sub-picosecond
time-scale.Comment: 15 pages Latex, 3 postscript figure
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A new interpretation of total column BrO during Arctic spring
Emission of bromine from sea-salt aerosol, frost flowers, ice leads, and snow results in the nearly complete removal of surface ozone during Arctic spring. Regions of enhanced total column BrO observed by satellites have traditionally been associated with these emissions. However, airborne measurements of BrO and O3 within the convective boundary layer (CBL) during the ARCTAS and ARCPAC field campaigns at times bear little relation to enhanced column BrO. We show that the locations of numerous satellite BrO âhotspotsâ during Arctic spring are consistent with observations of total column ozone and tropopause height, suggesting a stratospheric origin to these regions of elevated BrO. Tropospheric enhancements of BrO large enough to affect the column abundance are also observed, with important contributions originating from above the CBL. Closure of the budget for total column BrO, albeit with significant uncertainty, is achieved by summing observed tropospheric partial columns with calculated stratospheric partial columns provided that natural, short-lived biogenic bromocarbons supply between 5 and 10 ppt of bromine to the Arctic lowermost stratosphere. Proper understanding of bromine and its effects on atmospheric composition requires accurate treatment of geographic variations in column BrO originating from both the stratosphere and troposphere
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