1,363 research outputs found
Concepts of flywheels for energy storage using autostable high-T(sub c) superconducting magnetic bearings
A flywheel for energy storage using autostable high-T(sub c) superconducting magnetic bearings has been built. The rotating disk has a total weight of 2.8 kg. The maximum speed is 9240 rpm. A process that allows accelerated, reliable and reproducible production of melt-textured superconducting material used for the bearings has been developed. In order to define optimum configurations for radial and axial bearings, interaction forces in three dimensions and vertical and horizontal stiffness have been measured between superconductors and permanent magnets in different geometries and various shapes. Static as well as dynamic measurements have been performed. Results are being reported and compared to theoretical models
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
Solution of the Multi-Channel Anderson Impurity Model: Ground state and thermodynamics
We present the solution of the SU(N) x SU(M) Anderson impurity model using
the Bethe-Ansatz. We first explain what extensions to the formalism were
required for the solution. Subsequently we determine the ground state and
derive the thermodynamics over the full range of temperature and fields. We
identify the different regimes of valence fluctuation at high temperatures,
followed by moment formation or intrinsic mixed valence at intermediate
temperatures and a low temperature non-Fermi liquid phase. Among other things
we obtain the impurity entropy, charge valence and specific heat over the full
range of temperature. We show that the low-energy physics is governed by a line
of fixed points. This describes non-Fermi-liquid behavior in the integral
valence regime, associated with moment formation, as well as in the mixed
valence regime where no moment forms.Comment: 28 pages, 8 figures, 1 tabl
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
Quasiparticle Effective Mass for the Two- and Three-Dimensional Electron Gas
We calculate the quasiparticle effective mass for the electron gas in two and
three dimensions in the metallic region. We employ the single particle
scattering potential coming from the Sj\"{o}lander-Stott theory and enforce the
Friedel sum rule by adjusting the effective electron mass in a scattering
calculation. In 3D our effective mass is a monotonically decreasing function of
throughout the whole metallic domain, as implied by the most recent
numerical results. In 2D we obtain reasonable agreement with the experimental
data, as well as with other calculations based on the Fermi liquid theory. We
also present results of a variety of different treatments for the effective
mass in 2D and 3D.Comment: 12 pages, 2 figure
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-
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
The Depression Network (DeNT) Study: methodology and sociodemographic characteristics of the first 470 affected sibling pairs from a large multi-site linkage genetic study
Glaxo Wellcome Research and Development
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Polygenic risk for circulating reproductive hormone levels and their influence on hippocampal volume and depression susceptibility
Altered reproductive hormone levels have been associated with the pathophysiology of depressive disorders and this risk may be imparted by their modulatory effect upon hippocampal structure and function. Currently it is unclear whether altered levels of reproductive hormones are causally associated with hippocampal volume reductions and the risk of depressive disorders. Here, we utilize genome-wide association study (GWAS) summary statistics from a GWAS focusing on reproductive hormones, consisting of 2913 individuals. Using this data, we generated polygenic risk scores (PRS) for estradiol, progesterone, prolactin and testosterone in the European RADIANT cohort consisting of 176 postpartum depression (PPD) cases (100% female, mean age: 41.6 years old), 2772 major depressive disorder (MDD) cases (68.6% female, mean age: 46.9 years old) and 1588 control participants (62.5% female, mean age: 42.4 years old), for which there was also a neuroimaging subset of 111 individuals (60.4% female, mean age: 50.0 years old). Only the best-fit PRS for estradiol showed a significant negative association with hippocampal volume, as well as many of its individual subfields; including the molecular layer and granule cell layer of the dentate gyrus, subiculum, CA1, CA2/3 and CA4 regions. Interestingly, several of these subfields are implicated in adult hippocampal neurogenesis. When we tested the same estradiol PRS for association with case-control status for PPD or MDD there was no significant relationship observed. Here, we provide evidence that genetic risk for higher plasma estradiol is negatively associated with hippocampal volume, but this does not translate into an increased risk of MDD or PPD. This work suggests that the relationship between reproductive hormones, the hippocampus, and depression is complex, and that there may not be a clear-cut pathway for etiology or risk moderation
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