98 research outputs found
Spin-phonon interaction in doped high-T superconductors from density functional calculations
Effects of coupling between phonon distortions and stripe-like spin waves in
the CuO plane of HgBaCuO are studied by band calculations. Local
exchange enhancements depend sensitively on the local structure around Cu
sites. Interactions where spin waves have twice as long wave length as phonon
waves can lead to a 'dip' in the density of states (DOS) below the Fermi
energy, . This type of interaction is compatible with several seemingly
contradictory observations among high- cuprates, like the isotope effect,
anti-ferromagnetic fluctuations, stripes and pseudogaps. It can also account
for a large .Comment: (5 pages, 2 figures
Ferromagnetic and antiferromagnetic spin fluctuations and superconductivity in the hcp-phase of Fe
High purity iron, which transforms into the hcp phase under pressure, has
recently been reported to be superconducing in the pressure range 150-300 kBar
[shim]. The electronic structure and the electron-phonon coupling
() are calculated for hcp iron at different volumes. A
parameter-free theory for calculating the coupling constants
from ferromagnetic (FM) and antiferromagnetic (AFM) spin fluctuations is
developed. The calculated are sufficiently large to explain
superconductivity especially from FM fluctuations. The results indicate that
superconductivity mediated by spin fluctuations is more likely than from
electron-phonon interaction.Comment: (4 pages, 1 figure
Spin fluctuations, electron-phonon coupling and superconductivity in near-magnetic elementary metals; Fe,Co,Ni and Pd
An investigation of possibilities for superconductivity mediated by spin
fluctuations in some elementary metals is motivated by the recent discovery of
superconductivity in the hcp high-pressure phase of iron.
The electronic structure, the electron-phonon coupling () and
the coupling due to spin-fluctuations () are calculated for
different phases and different volumes for four elementary metals. The results
show that such possibilities are best for systems near, but on the non-magnetic
side of, a magnetic instability. Fcc Ni, which show stable magnetism over a
wide pressure range, is not interesting in this respect. Ferro- and
antiferro-magnetic fluctuations in hcp Fe contribute to a relatively strong
coupling in the pressure range where superconductivity is observed. The absence
of fluctuations at large q-vectors makes fcc Pd only moderately interesting
despite its large exchange enhancement for q=0. Fcc Co at high pressure (
0.5 Mbar) behaves as an improved version of Pd, where the fluctuations extend
to larger q. The estimations of T, which reproduce the experimental
situation in Fe quite well, suggest a measurable T for the high-pressure
phase of fcc Co, while the estimate is lower for the ambient-pressure phase of
fcc Pd.Comment: 9 pages, 4 figures, 2 table
Spin-phonon interaction and band effects in the high-T_C superconductor HgBa_2CuO_4
Band calculations show that a stripe-like anti-ferromagnetic spin wave is
enforced by a 'half-breathing' phonon distortion within the CuO plane of
HgBa_2CuO_4. This spin-phonon coupling is increased further by shear distortion
and by increased distance between Cu and apical oxygens. The effects from
spin-phonon coupling are consistent with many observations in high-T_C
materials. Spin-phonon coupling can be important for the mechanism of spin
fluctuations and superconductivity, although the effects are quantitatively
weak when using the local density potential.Comment: 4 pages, 1 figur
Magnetism, Critical Fluctuations and Susceptibility Renormalization in Pd
Some of the most popular ways to treat quantum critical materials, that is,
materials close to a magnetic instability, are based on the Landau functional.
The central quantity of such approaches is the average magnitude of spin
fluctuations, which is very difficult to measure experimentally or compute
directly from the first principles. We calculate the parameters of the Landau
functional for Pd and use these to connect the critical fluctuations beyond the
local-density approximation and the band structure.Comment: Replaced with the revised version accepted for publication.
References updated, errors corrected, other change
Spin-fluctuation exchange study of superconductivity in two- and three-dimensional single-band Hubbard models
In order to identify the most favorable situation for superconductivity in
the repulsive single-band Hubbard model, we have studied instabilities for
d-wave pairing mediated by antiferromagnetic spin fluctuations and p-pairing
mediated by ferromagnetic fluctuations with the fluctuation exchange
approximation in both two dimensions and three dimensions. By systematically
varying the band filling and band structure we have shown that (i) d-pairing is
stronger in two dimensions than in three dimensions, and (ii) p-pairing is much
weaker than the d-pairing.Comment: RevTex, 5 figures in Postscript, to be published in Phys. Rev.
Spin fluctuations in nearly magnetic metals from ab-initio dynamical spin susceptibility calculations:application to Pd and Cr95V5
We describe our theoretical formalism and computational scheme for making
ab-initio calculations of the dynamic paramagnetic spin susceptibilities of
metals and alloys at finite temperatures. Its basis is Time-Dependent Density
Functional Theory within an electronic multiple scattering, imaginary time
Green function formalism. Results receive a natural interpretation in terms of
overdamped oscillator systems making them suitable for incorporation into spin
fluctuation theories. For illustration we apply our method to the nearly
ferromagnetic metal Pd and the nearly antiferromagnetic chromium alloy Cr95V5.
We compare and contrast the spin dynamics of these two metals and in each case
identify those fluctuations with relaxation times much longer than typical
electronic `hopping times'Comment: 21 pages, 9 figures. To appear in Physical Review B (July 2000
A Catalog of Neutral and Deleterious Polymorphism in Yeast
The abundance and identity of functional variation segregating in natural populations is paramount to dissecting the molecular basis of quantitative traits as well as human genetic diseases. Genome sequencing of multiple organisms of the same species provides an efficient means of cataloging rearrangements, insertion, or deletion polymorphisms (InDels) and single-nucleotide polymorphisms (SNPs). While inbreeding depression and heterosis imply that a substantial amount of polymorphism is deleterious, distinguishing deleterious from neutral polymorphism remains a significant challenge. To identify deleterious and neutral DNA sequence variation within Saccharomyces cerevisiae, we sequenced the genome of a vineyard and oak tree strain and compared them to a reference genome. Among these three strains, 6% of the genome is variable, mostly attributable to variation in genome content that results from large InDels. Out of the 88,000 polymorphisms identified, 93% are SNPs and a small but significant fraction can be attributed to recent interspecific introgression and ectopic gene conversion. In comparison to the reference genome, there is substantial evidence for functional variation in gene content and structure that results from large InDels, frame-shifts, and polymorphic start and stop codons. Comparison of polymorphism to divergence reveals scant evidence for positive selection but an abundance of evidence for deleterious SNPs. We estimate that 12% of coding and 7% of noncoding SNPs are deleterious. Based on divergence among 11 yeast species, we identified 1,666 nonsynonymous SNPs that disrupt conserved amino acids and 1,863 noncoding SNPs that disrupt conserved noncoding motifs. The deleterious coding SNPs include those known to affect quantitative traits, and a subset of the deleterious noncoding SNPs occurs in the promoters of genes that show allele-specific expression, implying that some cis-regulatory SNPs are deleterious. Our results show that the genome sequences of both closely and distantly related species provide a means of identifying deleterious polymorphisms that disrupt functionally conserved coding and noncoding sequences
Triplet p-wave superconductivity in low-density extended Hubbard model with Coulomb repulsion
We analyze superconducting instabilities in 3D and 2D extended Hubbard model
with Coulomb repulsion between electrons on neighboring sites in the limit of
low electron density () on simple cubic (square) lattice.
We show that in a realistic strong-coupling case ( and are
the onsite and the intersite Coulomb repulsions, the bandwidth) the main SC
instability corresponds to the p-wave pairing and in the leading order is
correctly described by the equations obtained earlier in the absence of the
intersite Coulomb interaction V=0.Comment: 6 pages, 4 figures, accepted for JETP Letter
Pervasive Hitchhiking at Coding and Regulatory Sites in Humans
Much effort and interest have focused on assessing the importance of natural
selection, particularly positive natural selection, in shaping the human genome.
Although scans for positive selection have identified candidate loci that may be
associated with positive selection in humans, such scans do not indicate whether
adaptation is frequent in general in humans. Studies based on the reasoning of
the MacDonaldβKreitman test, which, in principle, can be used to
evaluate the extent of positive selection, suggested that adaptation is
detectable in the human genome but that it is less common than in Drosophila or
Escherichia coli. Both positive and purifying natural
selection at functional sites should affect levels and patterns of polymorphism
at linked nonfunctional sites. Here, we search for these effects by analyzing
patterns of neutral polymorphism in humans in relation to the rates of
recombination, functional density, and functional divergence with chimpanzees.
We find that the levels of neutral polymorphism are lower in the regions of
lower recombination and in the regions of higher functional density or
divergence. These correlations persist after controlling for the variation in GC
content, density of simple repeats, selective constraint, mutation rate, and
depth of sequencing coverage. We argue that these results are most plausibly
explained by the effects of natural selection at functional
sitesβeither recurrent selective sweeps or background
selectionβon the levels of linked neutral polymorphism. Natural
selection at both coding and regulatory sites appears to affect linked neutral
polymorphism, reducing neutral polymorphism by 6% genome-wide and by
11% in the gene-rich half of the human genome. These findings suggest
that the effects of natural selection at linked sites cannot be ignored in the
study of neutral human polymorphism
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