1,158 research outputs found
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
Resonant generation of coherent phonons in a superconductor by ultrafast optical pump pulses
We study the generation of coherent phonons in a superconductor by ultrafast
optical pump pulses. The nonequilibrium dynamics of the coupled Bogoliubov
quasiparticle-phonon system after excitation with the pump pulse is analyzed by
means of the density-matrix formalism with the phonons treated at a full
quantum kinetic level. For ultrashort excitation pulses, the superconductor
exhibits a nonadiabatic behavior in which the superconducting order parameter
oscillates. We find that in this nonadiabatic regime the generation of coherent
phonons is resonantly enhanced when the frequency of the order-parameter
oscillation is tuned to the phonon energy, a condition that can be achieved in
experiments by varying the integrated pump pulse intensity.Comment: 8 pages, 4 figures, typos correcte
Andreev spectroscopy and surface density of states for a three-dimensional time-reversal invariant topological superconductor
A topological superconductor is a fully gapped superconductor that exhibits
exotic zero-energy Andreev surface states at interfaces with a normal metal. In
this paper we investigate the properties of a three-dimensional time reversal
invariant topological superconductor by means of a two-band model with
unconventional pairing in both the inter- and intraband channels. Due to the
bulk-boundary correspondence the presence of Andreev surface states in this
system is directly related to the topological structure of the bulk
wavefunctions, which is characterized by a winding number. Using quasiclassical
scattering theory we construct the spectrum of the Andreev bound states that
appear near the surface and compute the surface density of states for various
surface orientations. Furthermore, we consider the effects of band splitting,
i.e., the breaking of an inversion-type symmetry, and demonstrate that in the
absence of band splitting there is a direct transition between the fully gapped
topologically trivial phase and the nontrivial phase, whereas in the presence
of band splitting there exists a finite region of a gapless nodal
superconducting phase between the fully gapped topologically trivial and
nontrivial phases.Comment: 7 pages, 4 figures, typos corrected, two footnotes adde
Theory of nonequilibrium dynamics of multiband superconductors
We study the nonequilibrium dynamics of multiband BCS superconductors
subjected to ultrashort pump pulses. Using density-matrix theory, the time
evolution of the Bogoliubov quasiparticle densities and the superconducting
order parameters are computed as a function of pump pulse frequency, duration,
and intensity. Focusing on two-band superconductors, we consider two different
model systems. The first one, relevant for iron-based superconductors,
describes two-band superconductors with a repulsive interband interaction
which is much larger than the intraband pairing terms. The second
model, relevant for MgB, deals with the opposite limit where the intraband
interactions are dominant and the interband pair scattering is weak
but attractive. For ultrashort pump pulses, both of these models exhibit a
nonadiabatic behavior which is characterized by oscillations of the
superconducting order parameters. We find that for nonvanishing , the
superconducting gap on each band exhibits two oscillatory frequencies which are
determined by the long-time asymptotic values of the gaps. The relative
strength of these two frequency components depends sensitively on the magnitude
of the interband interaction .Comment: 6 pages, 3 figure
Screening in (d+s)-wave superconductors: Application to Raman scattering
We study the polarization-dependent electronic Raman response of untwinned
YBaCuO superconductors employing a tight-binding band
structure with anisotropic hopping matrix parameters and a superconducting gap
with a mixing of - and s-wave symmetry. Using general arguments we find
screening terms in the B^{\}_{1g} scattering channel which are required by
gauge invariance. As a result, we obtain a small but measurable softening of
the pair-breaking peak, whose position has been attributed for a long time to
twice the superconducting gap maximum. Furthermore, we predict
superconductivity-induced changes in the phonon line shapes that could provide
a way to detect the isotropic s-wave admixture to the superconducting gap.Comment: typos corrected, 6 pages, 3 figure
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
The First Detailed Look at a Brown Dwarf Disk
The combination of mid-infrared and recent submm/mm measurements allows us to
set up the first comprehensive spectral energy distribution (SED) of the
circumstellar material around a young Brown Dwarf. Simple arguments suggest
that the dust is distributed in the form of a disk. We compare basic models to
explore the disk parameters. The modeling shows that a flat disk geometry fits
well the observations. A flared disk explains the SED only if it has a
puffed-up inner rim and an inner gap much larger than the dust sublimation
radius. Similarities and differences with disks around T Tauri stars are
discussed.Comment: 11 pages, 1 figur
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