110,587 research outputs found
LaFeAsOF: A low carrier density superconductor near itinerant magnetism
Density functional studies of 26K superconducting LaFeAs(O,F) are reported.
We find a low carrier density, high density of states, and modest
phonon frequencies relative to . The high leads to proximity to
itinerant magnetism, with competing ferromagnetic and antiferromagnetic
fluctuations and the balance between these controlled by doping level. Thus
LaFeAs(O,F) is in a unique class of high superconductors: high
ionic metals near magnetism.Comment: Shortened published form. Typos correcte
Crystallization of Simple Fluids: Relative Stability of f.c.c. and b.c.c Structures
A free-energy functional for a crystal that contains both the symmetry
conserved and symmetry broken parts of the direct pair correlation function is
developed. The free-energy functional is used to investigate the
crystallization of fluids interacting via the inverse power potential ;
. In agreement with simulation results we find
that for the freezing is into close packed f.c.c structure while for
soft repulsions b.c.c phase is more stable.Comment: 9 pages, 2 figure
Competitions in layered ruthenates: ferro- vs. antiferromagnetism and triplet vs. singlet pairing
Ru based perovskites demonstrate an amazing richness in their magnetic
properties, including 3D and quasi-2D ferromagnetism, antiferromagnetism, and
unconventional superconductivity. Tendency to ferromagnetism, stemming from the
unusually large involvement of O in magnetism in ruthenates, leads to
ferromagnetic spin fluctuations in Sr2RuO4 and eventually to p-wave
superconductivity. A related compound Ca2RuO4 was measured to be
antiferromagnetic, suggesting a possibility of antiferromagnetic fluctuations
in Sr2RuO4 as well. Here we report first principles calculations that
demonstrate that in both compounds the ferro- and antiferromagnetic
fluctuations coexist, leading to an actual instability in Ca2RuO4 and to a
close competition between p-wave and d-wave superconducting symmetries in
Sr2RuO4. The antiferromagnetism in this system appears to be mostly related
with the nesting, which is the strongest at Q=(2pi/3,2pi/3,0). Surprisingly,
for the Fermiology of Sr2RuO4 the p-wave state wins over the d-wave one
everywhere except in close vicinity of the antiferromagnetic instability. The
most stable state within the d-wave channel has vanishing order parameter at
one out of three Fermi surfaces in Sr2RuO4, while in the p channel its
amplitude is comparable at all three of them.Comment: 4 Revtex pages with 4 embedded postscript figure. Some figures are
color, but should look OK in B&W as wel
Density functional study of FeS, FeSe and FeTe: Electronic structure, magnetism, phonons and superconductivity
We report density functional calculations of the electronic structure, Fermi
surface, phonon spectrum, magnetism and electron-phonon coupling for the
superconducting phase FeSe, as well as the related compounds FeS and FeTe. We
find that the Fermi surface structure of these compounds is very similar to
that of the Fe-As based superconductors, with cylindrical electron sections at
the zone corner, cylindrical hole surface sections, and depending on the
compound, other small hole sections at the zone center. As in the Fe-As based
materials, these surfaces are separated by a 2D nesting vector at
(,). The density of states, nesting and Fermi surface size increase
going from FeSe to FeTe. Both FeSe and FeTe show spin density wave ground
states, while FeS is close to an instability. In a scenario where
superconductivity is mediated by spin fluctuations at the SDW nesting vector,
the strongest superconductor in this series would be doped FeTe.Comment: Added note regarding recent experimental observations of
superconductivity under pressure. Some additional discussio
Thermoelectric properties of AgGaTe and related chalcopyrite structure materials
We present an analysis of the potential thermoelectric performance of p-type
AgGaTe, which has already shown a of 0.8 with partial optimization,
and observe that the same band structure features, such as a mixture of light
and heavy bands and isotropic transport, that lead to this good performance are
present in certain other ternary chalcopyrite structure semiconductors. We find
that optimal performance of AgGaTe will be found for hole concentrations
between 4 and 2 cm at 900 K, and 2
and 10 cm at 700 K, and that certain other
chalcopyrite semiconductors might show good thermoelectric performance at
similar doping ranges and temperatures if not for higher lattice thermal
conductivity
Investigation of the effects of cobalt ions on epoxy properties
The effects of Co(acac)sub x complexes on MY-720 epoxy properties have been investigated. It appears that Co2(+) ions form antibonding or nonbonding orbitals which increase the free volume and also reduce the cohesiveness of the host epoxy. The effects of Co2(+) ions, on the other hand, seem to result in increased Cohesiveness of the epoxy. The experimental values of magnetic moments of both types of ions in MY-720 suggest that the orbital momentum contributions of the (3d) electrons are partially conserved, though the effect is more pronounced for Co2(+) ions. The coordination environment of the cobalt ions in the host epoxy does not appear to be uniquely defined. These results indicate that the effects of metal ions on resin properties cannot be easily predicted on the basis of ligand field theory argument alone. Complex interactions between metal ions and host epoxy molecular structure suggest the desirability of parallel experimental investigations of electronic, magnetic, and mechanical properties of metal ion-containing epoxy samples for comparison with theory
Electronic Structure and Thermoelectric Prospects of Phosphide Skutterudites
The prospects for high thermoelectric performance in phosphide skutterudites
are investigated based on first principles calculations. We find that
stoichiometric CoP_3 differs from the corresponding arsenide and antimonide in
that it is metallic. As such the band structure must be modified if high
thermopowers are to be achieved. In analogy to the antimonides it is expected
that this may be done by filling with La. Calculations for LaFe_4P_12 show that
a gap can in fact be opened by La filling, but that the valence band is too
light to yield reasonable p-type thermopowers at appropriate carrier densities;
n-type La filled material may be more favorable.Comment: 3 pages, 3 figures, 1 tabl
A Tight-Binding Investigation of the NaxCoO2 Fermi Surface
We perform an orthogonal basis tight binding fit to an LAPW calculation of
paramagnetic NaCoO for several dopings. The optimal position of the
apical oxygen at each doping is resolved, revealing a non-trivial dependence of
the band structure and Fermi surface on oxygen height. We find that the small
e hole pockets are preserved throughout all investigated dopings and
discuss some possible reasons for the lack of experimental evidence for these
Fermi sheets
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