296 research outputs found
Impurity effects on optical response in a finite band electronic system coupled to phonons
The concepts, which have traditionally been useful in understanding the
effects of the electron--phonon interaction in optical spectroscopy, are based
on insights obtained within the infinite electronic band approximation and no
longer apply in finite band metals. Impurity and phonon contributions to
electron scattering are not additive and the apparent strength of the coupling
to the phonon degrees of freedom is substantially reduced with increased
elastic scattering. The optical mass renormalization changes sign with
increasing frequency and the optical scattering rate never reaches its high
frequency quasiparticle value which itself is also reduced below its infinite
band value
Effect of disorder on the NMR relaxation rate in two-band superconductors
We calculate the effect of nonmagnetic impurity scattering on the
spin-lattice relaxation rate in two-band superconductors with the s-wave
pairing symmetry. It is found that for the interaction parameters appropriate
for MgB2 the Hebel-Slichter peak is suppressed by disorder in the limit of
small interband impurity scattering rate. In the limit of strong impurity
scattering, when the gap functions in the two bands become nearly equal, the
single-band behavior is recovered with a well-defined coherence peak just below
the transition temperature.Comment: 6 pages, 4 figure
Finite band inversion of ARPES in BiSrCaCuO in comparison with optics
Using a maximum entropy technique within a finite band Eliashberg formalism
we extract from recent high accuracy nodal direction angular resolved
photo-emission spectroscopy (ARPES) data in optimally doped
BiSrCaCuO (Bi2212) a quasiparticle electron-boson
spectral density. Both normal and superconducting state with d-wave gap
symmetry are treated. Finite and infinite band results are considered and
contrasted. We compare with results obtained for the related transport spectral
density which follows from a similar inversion of optical data. We discuss the
implication of our results for quasiparticle renormalizations in the antinodal
direction.Comment: 9 pages, 7 figures submitted to Physical Review
Effect of electron-phonon interaction on spectroscopies in graphene
We calculate the effect of the electron-phonon interaction on the electronic
density of states (DOS), the quasiparticle properties and on the optical
conductivity of graphene. In metals with DOS constant on the scale of phonon
energies, the electron-phonon renormalizations drop out of the dressed DOS,
however, due to the Dirac nature of the electron dynamics in graphene, the band
DOS is linear in energy and phonon structures remain, which can be emphasized
by taking an energy derivative. There is a shift in the chemical potential and
in the position in energy of the Dirac point. Also, the DOS can be changed from
a linear dependence out of value zero at the Dirac point to quadratic out of a
finite value. The optical scattering rate sets the energy scale for
the rise of the optical conductivity from its universal DC value
(expected in the simplest theory when chemical potential and temperature are
both ) to its universal AC background value . As in ordinary metals the DC conductivity remains unrenormalized
while its AC value is changed. The optical spectral weight under the intraband
Drude is reduced by a mass renormalization factor as is the effective
scattering rate. Optical weight is transferred to an Holstein phonon-assisted
side band. Due to Pauli blocking the interband transitions are sharply
suppressed, but also nearly constant, below twice the value of renormalized
chemical potential and also exhibit a phonon-assisted contribution. The
universal background conductivity is reduced below at large
energies.Comment: 22 pages, 19 figures, submitted to PR
Classical phase fluctuations in d-wave superconductors
We study the effects of low-energy nodal quasiparticles on the classical
phase fluctuations in a two-dimensional d-wave superconductor. The
singularities of the phase-only action at T\to 0 are removed in the presence of
disorder, which justifies using an extended classical XY-model to describe
phase fluctuations at low temperatures.Comment: 14 pages, brief review for Mod. Phys. Lett.
Properties of the superconducting state in a two-band model
Eliashberg theory is used to investigate the range of thermodynamic
properties possible within a two-band model for s-wave superconductivity and to
identify signatures of its two-band nature. We emphasize dimensionless BCS
ratios (those for the energy gaps, the specific heat jump and the negative of
its slope near Tc, the thermodynamic critical field Hc(0), and the normalized
slopes of the critical field and the penetration depth near Tc), which are no
longer universal even in weak coupling. We also give results for
temperature-dependent quantities, such as the penetration depth and the energy
gap. Results are presented both for microscopic parameters appropriate to MgB2
and for variations away from these. Strong coupling corrections are identified
and found to be significant. Analytic formulas are provided which show the role
played by the anisotropy in coupling in some special limits. Particular
emphasis is placed on small interband coupling and on the opposite limit of no
diagonal coupling. The effect of impurity scattering is considered,
particularly for the interband case.Comment: 20 pages, 14 figures, final version accepted in PR
Phonon spectroscopy through the electronic density of states in graphene
We study how phonon structure manifests itself in the electronic density of
states of graphene. A procedure for extracting the value of the electron-phonon
renormalization is developed. In addition, we identify direct phonon
structures. With increasing doping, these structures, along with ,
grow in amplitude and no longer display particle-hole symmetry.Comment: 5 page
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