937 research outputs found
Phonon Assisted Multimagnon Optical Absorption and Long Lived Two-Magnon States in Undoped Lamellar Copper Oxides
We calculate the effective charge for multimagnon infrared (IR) absorption
assisted by phonons in the parent insulating compounds of cuprate
superconductors and the spectra for two-magnon absorption using interacting
spin-wave theory. Recent measured bands in the mid IR [Perkins et al. Phys.
Rev. Lett. {\bf 71} 1621 (1993)] are interpreted as involving one phonon plus a
two-magnon virtual bound state, and one phonon plus higher multimagnon
absorption processes. The virtual bound state consists of a narrow resonance
occurring when the magnon pair has total momentum close to .Comment: 4 page
Role of Van Hove Singularities and Momentum Space Structure in High-Temperature Superconductivity
There is a great deal of interest in attributing the high critical
temperatures of the cuprates to either the proximity of the Fermi level to a
van Hove singularity or to structure of the superconducting pairing potential
in momentum space far from the Fermi surface. We examine these ideas by
calculating the critical temperature Tc for model Einstein-phonon- and
spin-fluctuation-mediated superconductors within both the standard,
Fermi-surface-restricted Eliashberg theory and the exact mean field theory,
which accounts for the full momentum structure of the pairing potential and the
energy dependence of the density of states. By using two models of
spin-fluctuation-mediated pairing in the cuprates, we demonstrate that our
results are independent of the details of the dynamical susceptibility, which
is taken to be the pairing potential. We also compare these two models against
available neutron scattering data, since these data provide the most direct
constraints on the susceptibility. We conclude that the van Hove singularity
does not drastically alter Tc from its value when the density of states is
constant and that the effect of momentum structure is significant but secondary
in importance to that of the energy dependence in the density of states.Comment: 23 pages, 6 figures upon request, revtex version 2, vHs-
The Electron-Phonon Interaction in the Presence of Strong Correlations
We investigate the effect of strong electron-electron repulsion on the
electron-phonon interaction from a Fermi-liquid point of view: the strong
interaction is responsible for vertex corrections, which are strongly dependent
on the ratio. These corrections generically lead to a strong
suppression of the effective coupling between quasiparticles mediated by a
single phonon exchange in the limit. However, such effect
is not present when . Analyzing the Landau stability
criterion, we show that a sizable electron-phonon interaction can push the
system towards a phase-separation instability. A detailed analysis is then
carried out using a slave-boson approach for the infinite-U three-band Hubbard
model. In the presence of a coupling between the local hole density and a
dispersionless optical phonon, we explicitly confirm the strong dependence of
the hole-phonon coupling on the transferred momentum versus frequency ratio. We
also find that the exchange of phonons leads to an unstable phase with negative
compressibility already at small values of the bare hole-phonon coupling. Close
to the unstable region, we detect Cooper instabilities both in s- and d-wave
channels supporting a possible connection between phase separation and
superconductivity in strongly correlated systems.Comment: LateX 3.14, 04.11.1994 Preprint no.101
Polarons and bipolarons in strongly interacting electron-phonon systems
The Holstein Hubbard and Holstein t--J models are studied for a wide range of
phonon frequencies, electron--electron and electron--phonon interaction
strengths on finite lattices with up to ten sites by means of direct Lanczos
diagonalization. Previously the necessary truncation of the phononic Hilbert
space caused serious limitations to either very small systems (four or even two
sites) or to weak electron--phonon coupling, in particular in the adiabatic
regime. Using parallel computers we were able to investigate the transition
from `large' to `small' polarons in detail. By resolving the low--lying
eigenstates of the Hamiltonian and by calculating the spectral function we can
identify a polaron band in the strong--coupling case, whose dispersion deviates
from the free--particle dispersion at low and intermediate phonon frequencies.
For two electrons (holes) we establish the existence of bipolaronic states and
discuss the formation of a bipolaron band. For the 2D Holstein t--J model we
demonstrate that the formation of hole--polarons is favoured by strong Coulomb
correlations. Analyzing the hole--hole correlation functions we find that hole
binding is enhanced as a dynamical effect of the electron--phonon interaction.Comment: 23 pages (Revtex) with 13 figures (ps, uuencoded
Characterization of the interactions of a polycationic, amphiphilic, terminally branched oligopeptide with lipid A and lipopolysaccharide from the deep rough mutant of salmonella minnesota
The lipid A and lipopolysaccharide (LPS) binding and neutralizing activities of a synthetic, polycationic, amphiphilic peptide were studied. The branched peptide, designed as a functional analog of polymyxin B, has a six residue hydrophobic sequence, bearing at its N-terminus a penultimate lysine residue whose α- and E-amino groups are coupled to two terminal lysine residues. In fluorescence spectroscopic studies designed to examine relative affinities of binding to the toxin, neutralization of surface charge and fluidization of the acyl domains, the peptide was active, closely resembling the effects of polymyxin B and its nonapeptide derivative; however, the synthetic peptide does not induce phase transitions in LPS aggregates as do polymyxin B and polymyxin B nonapeptide. The peptide was also comparable with polymyxin B in its ability to inhibit LPS-mediated IL-1 and IL-6 release from human peripheral blood mononuclear cells. The synthetic compound is devoid of antibacterial activities and did not induce conductance fluxes in LPS-containing asymmetric planar membranes. These results strengthen the premise that basicity and amphiphilicity are necessary and sufficient physical properties that ascribe endotoxin binding and neutralizing activities, and further suggest that antibacterial/membrane perturbant and LPS neutralizing activities are dissociable, which may be of value in designing LPS-sequestering agents of low toxicity
Superconductivity in Fullerides
Experimental studies of superconductivity properties of fullerides are
briefly reviewed. Theoretical calculations of the electron-phonon coupling, in
particular for the intramolecular phonons, are discussed extensively. The
calculations are compared with coupling constants deduced from a number of
different experimental techniques. It is discussed why the A_3 C_60 are not
Mott-Hubbard insulators, in spite of the large Coulomb interaction. Estimates
of the Coulomb pseudopotential , describing the effect of the Coulomb
repulsion on the superconductivity, as well as possible electronic mechanisms
for the superconductivity are reviewed. The calculation of various properties
within the Migdal-Eliashberg theory and attempts to go beyond this theory are
described.Comment: 33 pages, latex2e, revtex using rmp style, 15 figures, submitted to
Review of Modern Physics, more information at
http://radix2.mpi-stuttgart.mpg.de/fullerene/fullerene.htm
[Erratum to:] Does Childhood Trauma Moderate Polygenic Risk for Depression? A Meta-analysis of 5765 Subjects From the Psychiatric Genomics Consortium (vol 84, pg 138, 2018)
Impact of a cis-associated gene expression SNP on chromosome 20q11.22 on bipolar disorder susceptibility, hippocampal structure and cognitive performance.
BackgroundBipolar disorder is a highly heritable polygenic disorder. Recent enrichment analyses suggest that there may be true risk variants for bipolar disorder in the expression quantitative trait loci (eQTL) in the brain.AimsWe sought to assess the impact of eQTL variants on bipolar disorder risk by combining data from both bipolar disorder genome-wide association studies (GWAS) and brain eQTL.MethodTo detect single nucleotide polymorphisms (SNPs) that influence expression levels of genes associated with bipolar disorder, we jointly analysed data from a bipolar disorder GWAS (7481 cases and 9250 controls) and a genome-wide brain (cortical) eQTL (193 healthy controls) using a Bayesian statistical method, with independent follow-up replications. The identified risk SNP was then further tested for association with hippocampal volume (n = 5775) and cognitive performance (n = 342) among healthy individuals.ResultsIntegrative analysis revealed a significant association between a brain eQTL rs6088662 on chromosome 20q11.22 and bipolar disorder (log Bayes factor = 5.48; bipolar disorder P = 5.85×10(-5)). Follow-up studies across multiple independent samples confirmed the association of the risk SNP (rs6088662) with gene expression and bipolar disorder susceptibility (P = 3.54×10(-8)). Further exploratory analysis revealed that rs6088662 is also associated with hippocampal volume and cognitive performance in healthy individuals.ConclusionsOur findings suggest that 20q11.22 is likely a risk region for bipolar disorder; they also highlight the informative value of integrating functional annotation of genetic variants for gene expression in advancing our understanding of the biological basis underlying complex disorders, such as bipolar disorder
Lack of guidelines and translational knowledge is hindering the implementation of psychiatric genetic counseling and testing within Europe - A multi-professional survey study
Genetic research has identified a large number of genetic variants, both rare and common, underlying neurodevelopmental disorders (NDD) and major psychiatric disorders. Currently, these findings are being translated into clinical practice. However, there is a lack of knowledge and guidelines for psychiatric genetic testing (PsychGT) and genetic counseling (PsychGC). The European Union-funded COST action EnGagE (CA17130) network was started to investigate the current implementation status of PsychGT and PsychGC across 35 participating European countries. Here, we present the results of a pan-European online survey in which we gathered the opinions, knowledge, and practices of a self-selected sample of professionals involved/interested in the field. We received answers from 181 respondents. The three main occupational categories were genetic counselor (21.0%), clinical geneticist (24.9%), and researcher (25.4%). Of all 181 respondents, 106 provide GC for any psychiatric disorder or NDD, corresponding to 58.6% of the whole group ranging from 43.2% in Central Eastern Europe to 66.1% in Western Europe. Overall, 65.2% of the respondents reported that genetic testing is offered to individuals with NDD, and 26.5% indicated the same for individuals with major psychiatric disorders. Only 22.1% of the respondents indicated that they have guidelines for PsychGT. Pharmacogenetic testing actionable for psychiatric disorders was offered by 15%. Interestingly, when genetic tests are fully covered by national health insurance, more genetic testing is provided for individuals with NDD but not those with major psychiatric disorders. Our qualitative analyses of responses highlight the lack of guidelines and knowledge on utilizing and using genetic tests and education and training as the major obstacles to implementation. Indeed, the existence of psychiatric genetic training courses was confirmed by only 11.6% of respondents. The question on the relevance of up-to-date education and training in psychiatric genetics on everyday related practice was highly relevant. We provide evidence that PsychGC and PsychGT are already in use across European countries, but there is a lack of guidelines and education. Harmonization of practice and development of guidelines for genetic counseling, testing, and training professionals would improve equality and access to quality care for individuals with psychiatric disorders within Europe
Resilience and corpus callosum microstructure in adolescence
Background. Resilience is the capacity of individuals to resist mental disorders despite exposure to stress. Little is known about its neural underpinnings. The putative variation of white-matter microstructure with resilience in adolescence, a critical period for brain maturation and onset of high-prevalence mental disorders, has not been assessed by diffusion tensor imaging (DTI). Lower fractional anisotropy (FA) though, has been reported in the corpus callosum (CC), the brain’s largest white-matter structure, in psychiatric and stress-related conditions. We hypothesized that higher FA in the CC would characterize stress-resilient adolescents.
Method. Three groups of adolescents recruited from the community were compared: resilient with low risk of mental disorder despite high exposure to lifetime stress (n = 55), at-risk of mental disorder exposed to the same level of stress (n = 68), and controls (n = 123). Personality was assessed by the NEO-Five Factor Inventory (NEO-FFI). Voxelwise statistics of DTI values in CC were obtained using tract-based spatial statistics. Regional projections were identified by probabilistic tractography.
Results. Higher FA values were detected in the anterior CC of resilient compared to both non-resilient and control adolescents. FA values varied according to resilience capacity. Seed regional changes in anterior CC projected onto anterior cingulate and frontal cortex. Neuroticism and three other NEO-FFI factor scores differentiated non-resilient participants from the other two groups.
Conclusion. High FA was detected in resilient adolescents in an anterior CC region projecting to frontal areas subserving cognitive resources. Psychiatric risk was associated with personality characteristics. Resilience in adolescence may be related to white-matter microstructure
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