217 research outputs found
Optical Response for the d-density wave model
We have calculated the optical conductivity and the Raman response for the
d-density wave model, proposed as a possible explanation for the pseudogap seen
in high Tc cuprates. The total optical spectral weight remains approximately
constant on opening of the pseudogap for fixed temperature. This occurs because
there is a transfer of weight from the Drude peak to interband transitions
across the pseudogap. The interband peak in the optical conductivity is
prominent but becomes progressively reduced with increasing temperature, with
impurity scattering, which distributes it over a larger energy range, and with
ineleastic scattering which can also shift its position, making it difficult to
have a direct determination of the value of the pseudogap. Corresponding
structure is seen in the optical scattering rate, but not necessarily at the
same energies as in the conductivity.Comment: 14 pages, 15 figures, final revised version published in PR
Anomalous optical absorption in overdoped cuprates near the charge-ordering instability
We propose an interpretation for the hump observed in the optical
conductivity at or below a few hundreds of cm, in overdoped cuprates
like the electron-doped Nd_{2-x}Ce_xCuO_{4-y} at x\gtrsim 0.15 and the
hole-doped Bi_2Sr_2CuO_6 and La_{2-x}Sr_xCuO_4. This interpretation is based on
the direct excitation of charge collective modes, which become nearly critical
in the proximity to a charge-ordering instability. The nearly critical
character of these excitations entails a peculiar temperature dependence and a
pseudo-scaling form of the lineshapes, which are in agreement with the
experimental data.Comment: 5 pages, 3 figure
Theory of the Optical Conductivity in the Cuprate Superconductors
We present a study of the normal state optical conductivity in the cuprate
superconductors using the nearly antiferromagnetic Fermi liquid (NAFL)
description of the magnetic interaction between their planar quasiparticles. We
find that the highly anisotropic scattering rate in different regions of the
Brillouin zone, both as a function of frequency and temperature, a benchmark of
NAFL theory, leads to an average relaxation rate of the Marginal Fermi Liquid
form for overdoped and optimally doped systems, as well as for underdoped
systems at high temperatures. We carry out numerical calculations of the
optical conductivity for several compounds for which the input spin fluctuation
parameters are known. Our results, which are in agreement with experiment on
both overdoped and optimally doped systems, show that NAFL theory explains the
anomalous optical behavior found in these cuprate superconductors.Comment: REVTEX file, 8 PostScript figure
Временные рубежи крупных изверженных провинций России: предварительный обзор
This is an initial step toward a full-scale review paper and map of Large Igneous Provinces (LIPs) for Russia
Sum rules and electrodynamics of high-Tc cuprates in the pseudogap state
We explore connections between the electronic density of states (DOS) in a
conducting system and the frequency dependence of the scattering rate
inferred from infrared spectroscopy. We show that changes in
the DOS upon the development of energy gaps can be reliably tracked through the
examination of the spectra using the sum rules discussed in
the text. Applying this analysis to the charge dynamics in high- cuprates
we found radically different trends in the evolution of the DOS in the
pseudogap state and in the superconducting state.Comment: 4 pages, 3 figure
Implications of reflectance measurements on the mechanism for superconductivity in MgB
Recent optical studies in c-axis oriented superconducting MgB films
indicate that the electron-phonon coupling is weak [tu01]. We reinforce this
conclusion by examining the raw reflectance data; its frequency dependence is
incompatible with strong electron-phonon scattering. This is further
strengthened by analysis of the real part of the conductivity, and by the
temperature dependence of the effective Drude scattering rate. Using a
realistic electron-phonon spectral shape [kong01], we find , in agreement with Tu et al. [tu01]. To the extent that
, this disagrees sharply with model
calculations [kong01,kortus01,an01], and is far too low to provide the means
for K. A simple model is constructed with coupling to a high
frequency excitation, which is consistent with both the low frequency optical
data and the high .Comment: 4 pages, 4 figure
The optical response of Ba_{1-x}K_xBiO_3: Evidence for an unusual coupling mechanism of superconductivity?
We have analysed optical reflectivity data for Ba_{1-x}K_xBiO_3 in the
far-infrared region using Migdal-Eliashberg theory and found it inconsistent
with standard electron-phonon coupling: Whereas the superconducting state data
could be explained using moderate coupling, \lambda=0.7, the normal state
properties indicate \lambda \le 0.2. We have found that such behaviour could be
understood using a simple model consisting of weak standard electron-phonon
coupling plus weak coupling to an unspecified high energy excitation near 0.4
eV. This model is found to be in general agreement with the reflectivity data,
except for the predicted superconducting gap size. The additional high energy
excitation suggests that the dominant coupling mechanism in Ba_{1-x}K_xBiO_3 is
not standard electron-phonon.Comment: 5 pages REVTex, 5 figures, 32 refs, accepted for publication in Phys.
Rev.
Two Gaps Make a High Temperature Superconductor?
One of the keys to the high-temperature superconductivity puzzle is the
identification of the energy scales associated with the emergence of a coherent
condensate of superconducting electron pairs. These might provide a measure of
the pairing strength and of the coherence of the superfluid, and ultimately
reveal the nature of the elusive pairing mechanism in the superconducting
cuprates. To this end, a great deal of effort has been devoted to investigating
the connection between the superconducting transition temperature Tc and the
normal-state pseudogap crossover temperature T*. Here we present a review of a
large body of experimental data that suggests a coexisting two-gap scenario,
i.e. superconducting gap and pseudogap, over the whole superconducting dome.Comment: Related material can be found at
http://www.physics.ubc.ca/~quantmat/ARPES/PUBLICATIONS/articles.htm
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