1,254 research outputs found
Fermi-liquid based theory for the in-plane magnetic anisotropy in untwinned high-T superconductors
Using a generalized RPA-type theory we calculate the in-plane anisotropy of
the magnetic excitations in hole-doped high- superconductors. Extending
our earlier Fermi-liquid based studies on the resonance peak by inclusion of
orthorhombicity we still find two-dimensional spin excitations, however, being
strongly anisotropic. This reflects the underlying anisotropy of the hopping
matrix elements and of the resultant superconducting gap function. We compare
our calculations with new experimental data on {\it fully untwinned}
and find good agreement. Our results are in contrast
to earlier interpretations on the in-plane anisotropy in terms of stripes (H.
Mook {\it et al.}, Nature {\bf 404}, 729 (2000)), but reveal a conventional
solution to this important problem.Comment: 5 pages, 6 figure
Theory for ultrafast nonequilibrium dynamics in d-wave superconductors
We use density-matrix theory to calculate the ultrafast dynamics of
unconventional superconductors from a microscopic viewpoint. We calculate the
time evolution of the optical conductivity as well as pump-probe spectra for a
d-wave order parameter. Three regimes can be distinguished in the spectra. The
Drude response at low photon energies is the only one of those which has been
measured experimentally so far. At higher energies, we predict two more
regimes: the pair-breaking peak, which is reduced as Cooper-pairs are broken up
by the exciting pulse; and a suppression above the pair-breaking peak due to
nonequilibrium quasiparticles. Furthermore, we consider the influence of the
electron-phonon coupling, and derive rate equations which have been widely used
so far.Comment: 4 pages, 4 figure
Electronic Raman response in anisotropic metals
Using a generalized response theory we derive the electronic Raman response
function for metals with anisotropic relaxation rates. The calculations account
for the long--range Coulomb interaction and treat the collision operator within
a charge conserving relaxation time approximation. We extend earlier treatments
to finite wavenumbers () and incorporate inelastic
electron--electron scattering besides elastic impurity scattering. Moreover we
generalize the Lindhard density response function to the Raman case. Numerical
results for the quasiparticle scattering rate and the Raman response function
for cuprate superconductors are presented.Comment: 5 pages, 4figures. accepted in PRB (Brief Report), in pres
Properties of the phonon-induced pairing interaction in YBaCuO within the local density approximation
The properties of the phonon-induced interaction between electrons are
studied using the local density approximation (LDA). Restricting the electron
momenta to the Fermi surface we find generally that this interaction has a
pronounced peak for large momentum transfers and that the interband
contributions between bonding and antibonding band are of the same magnitude as
the intraband ones. Results are given for various symmetry averages of this
interaction over the Fermi surface. In particular, we find that the
dimensionless coupling constant in the d-wave channel , relevant for
superconductivity, is only 0.022, i.e., even about ten times smaller than the
small value of the s-wave channel. Similarly, the LDA contribution to the
resistivity is about a factor 10 times smaller than the observed resistivity
suggesting that phonons are not the important low-energy excitations in
high-T oxides.Comment: 6 pages, 7 figure
Unconventional superconductivity and magnetism in SrRuO and related materials
We review the normal and superconducting state properties of the
unconventional triplet superconductor SrRuO with an emphasis on the
analysis of the magnetic susceptibility and the role played by strong
electronic correlations. In particular, we show that the magnetic activity
arises from the itinerant electrons in the Ru -orbitals and a strong
magnetic anisotropy occurs () due to spin-orbit
coupling. The latter results mainly from different values of the -factor for
the transverse and longitudinal components of the spin susceptibility (i.e. the
matrix elements differ). Most importantly, this anisotropy and the presence of
incommensurate antiferromagnetic and ferromagnetic fluctuations have strong
consequences for the symmetry of the superconducting order parameter. In
particular, reviewing spin fluctuation-induced Cooper-pairing scenario in
application to SrRuO we show how p-wave Cooper-pairing with line nodes
between neighboring RuO-planes may occur.
We also discuss the open issues in SrRuO like the influence of
magnetic and non-magnetic impurities on the superconducting and normal state of
SrRuO. It is clear that the physics of triplet superconductivity in
SrRuO is still far from being understood completely and remains to be
analyzed more in more detail. It is of interest to apply the theory also to
superconductivity in heavy-fermion systems exhibiting spin fluctuations.Comment: short review article. Annalen der Physik, vol. 13 (2004), to be
publishe
Exchange Enhancement of the Electron-Phonon Pair Interaction
The critical temperature of high- superconductors is determined, at
least in part, by the electron-phonon coupling. We include the effect of an
exchange interaction between the electrons and calculate the renormalization of
the bare phonon frequencies and the electron-phonon verticies in a random phase
approximation and obtain a strongly enhanced attractive phonon-induced
electron-electron interaction. Using Fast Fourier Transform techniques, the
weak-coupling selfconsistency equation for the order parameter is solved in the
2D first Brillouin zone for the Emery tight-binding band with different band
fillings. The enhancement of arises primarily from the softening of the
phonon frequencies rather than the vertex renormalization.Comment: (2 pages, postscript file, hardcopies available from the authors
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