64 research outputs found
Optical conductivity of unconventional charge density wave systems: Role of vertex corrections
The optical conductivity of a d-CDW conductor is calculated for electrons on
a square lattice and a nearest-neighbor charge-charge interaction using the
lowest-order conserving approximation. The spectral properties of the
Drude-like peak at low frequencies and the broad hump due to transitions across
the gap at large frequencies are discussed, also as a function of temperature
and of the second-nearest neighbor hopping term t'. We find that vertex
corrections enhance the d.c. conductivity, make the Drude peak narrower and
provide a smooth transition from a renormalized regime at low to the bare
theory at high frequencies. It is also shown that vertex corrections enhance
the temperature dependence of the restricted optical sum leading to a
non-negligible violation of the sum rule in the d-CDW state.Comment: 10 pages, 6 figure
The Hall conductivity in unconventional charge density wave systems
Charge density waves with unconventional order parameters, for instance, with
d-wave symmetry (DDW), may be relevant in the underdoped regime of high-T_c
cuprates or other quasi-one or two dimensional metals. A DDW state is
characterized by two branches of low-lying electronic excitations. The
resulting quantum mechanical current has an inter-branch component which leads
to an additional mass term in the expression for the Hall conductivity. This
extra mass term is parametrically enhanced near the ``hot spots'' of fermionic
dispersion and is non-neglegible as is shown by numerical calculations of the
Hall number in the DDW state.Comment: 4 pages, 4 figure
Influence of spin fluctuations on the superconducting transition temperature and resistivity in the t-J model at large N
Spin fluctuations enter the calculation of the superconducting transition
temperature T only in the next-to-leading order (i.e., in O(1/N) of the
1/N expansion of the t-J model. We have calculated these terms and show that
they have only little influence on the value of T obtained in the leading
order O(1/N) in the optimal and overdoped region, i.e., for dopings larger than
the instability towards a flux phase. This result disagrees with recent
spin-fluctuation mediated pairing theories. The discrepancies can be traced
back to the fact that in our case the coupling between electrons and spins is
determined by the t-J model and not adjusted and that the spin susceptibility
is rather broad and structureless and not strongly peaked at low energies as in
spin-fluctuation models. Relating T and transport we show that the
effective interactions in the particle-particle and particle-hole channels are
not simply related within the 1/N expansion by different Fermi surface averages
of the same interactin as in the case of phonons or spin fluctuations. As a
result, we find that large values for T and rather small scattering rates
in the normal state as found in the experiments can easily be reconciled with
each other. We also show that correlation effects heavily suppress transport
relaxation rates relative to quasiparticle relaxation rates in the case of
phonons but not in the case of spin fluctuations.Comment: 16 pages, 10 figures, will appear in Phys. Rev.
Large-N expansion based on the Hubbard-operator path integral representation and its application to the model
In the present work we have developed a large-N expansion for the model
based on the path integral formulation for Hubbard-operators. Our large-N
expansion formulation contains diagrammatic rules, in which the propagators and
vertex are written in term of Hubbard operators. Using our large-N formulation
we have calculated, for J=0, the renormalized boson propagator. We
also have calculated the spin-spin and charge-charge correlation functions to
leading order 1/N. We have compared our diagram technique and results with the
existing ones in the literature.Comment: 6 pages, 3 figures, Phys.Rev.B (in press
d_{x^2-y^2} Symmetry and the Pairing Mechanism
An important question is if the gap in the high temperature cuprates has
d_{x^2-y^2} symmetry, what does that tell us about the underlying interaction
responsible for pairing. Here we explore this by determining how three
different types of electron-phonon interactions affect the d_{x^2-y^2} pairing
found within an RPA treatment of the 2D Hubbard model. These results imply that
interactions which become more positive as the momentum transfer increases
favor d_{x^2-y^2} pairing in a nearly half-filled band.Comment: 9 pages and 2 eps figs, uses revtex with epsf, in press, PR
Influence of the pseudogap on the superconductivity-induced phonon renormalization in high-T superconductors
We investigate the influence of a d-density wave (DDW) gap on the
superconductivity-induced renormalization of phonon frequency and linewidth.
The results are discussed with respect to Raman and inelastic neutron
scattering experiments. It turns out that the DDW gap can enhance the range of
frequencies for phonon softening depending on the underlying band
structure. Moreover we show that an anisotropic 'd-wave' pseudogap can also
contribute to the q-dependent linewidth broadening of the 340cm phonon
in YBaCuO.Comment: 4 page
Beyond Eliashberg superconductivity in MgB2: anharmonicity, two-phonon scattering, and multiple gaps
Density-functional calculations of the phonon spectrum and electron-phonon
coupling in MgB are presented. The phonons, which involve in-plane
B displacements, couple strongly to the electronic bands. The
isotropic electron-phonon coupling constant is calculated to be about 0.8.
Allowing for different order parameters in different bands, the superconducting
in the clean limit is calculated to be significantly larger. The
phonons are strongly anharmonic, and the non-linear contribution to
the coupling between the modes and the p bands is significant.Comment: 4 pages, 3 figure
Phonon `notches' in a-b -plane optical conductivity of high-Tc superconductors
It is shown that a correlation between the positions of the -axis
longitudinal optic () phonons and ``notch''-like structures in the
- plane conductivity of high- superconductors results from
phonon-mediated interaction between electrons in different layers. It is found
that the relative size of the notches depends on
, where ,
and are the effective coupling strength, the frequency and the
width of the optical phonon which is responsible for the notch. Even for
the effect can be large if the phonon is very sharp.Comment: 5 pages, REVTeX, 4 uuencoded figure
Effect of an Electron-phonon Interaction on the One-electron Spectral Weight of a d-wave Superconductor
We analyze the effects of an electron-phonon interaction on the one-electron
spectral weight A(k,omega) of a d_{x^2-y^2} superconductor. We study the case
of an Einstein phonon mode with various momentum-dependent electron-phonon
couplings and compare the structure produced in A(k,omega) with that obtained
from coupling to the magnetic pi-resonant mode. We find that if the strength of
the interactions are adjusted to give the same renormalization at the nodal
point, the differences in A(k,omega) are generally small but possibly
observable near k=(pi,0).Comment: 10 pages, 14 figures (color versions of Figs. 2,4,10,11,12 available
upon request
Electron-phonon vertex in the two-dimensional one-band Hubbard model
Using quantum Monte Carlo techniques, we study the effects of electronic
correlations on the effective electron-phonon (el-ph) coupling in a
two-dimensional one-band Hubbard model. We consider a momentum-independent bare
ionic el-ph coupling. In the weak- and intermediate-correlation regimes, we
find that the on-site Coulomb interaction acts to effectively suppress the
ionic el-ph coupling at all electron- and phonon- momenta. In this regime, our
numerical simulations are in good agreement with the results of perturbation
theory to order . However, entering the strong-correlation regime, we find
that the forward scattering process stops decreasing and begins to
substantially increase as a function of , leading to an effective el-ph
coupling which is peaked in the forward direction. Whereas at weak and
intermediate Coulomb interactions, screening is the dominant correlation effect
suppressing the el-ph coupling, at larger values irreducible vertex
corrections become more important and give rise to this increase. These vertex
corrections depend crucially on the renormalized electronic structure of the
strongly correlated system.Comment: 5 pages, 4 eps-figures, minor change
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