382 research outputs found
Optical sum in Nearly Antiferromagnetic Fermi Liquid Model
We calculate the optical sum (OS) and the kinetic energy (KE) for a tight
binding band in the Nearly Antiferromagnetic Fermi Liquid (NAFFL) model which
has had some success in describing the electronic structure of the high
cuprates. The interactions among electrons due to the exchange of spin
fluctuations profoundly change the probability of occupation of states of momentum {\bf k} and spin which is the
central quantity in the calculations of OS and KE. Normal and superconducting
states are considered as a function of temperature. Both integrals are found to
depend importantly on interactions and an independent electron model is
inadequate.Comment: 9 Pages, 5 Figures Accepted for publication in Phys. Rev.
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
Highly anisotropic energy gap in superconducting Ba(FeCo)As from optical conductivity measurements
We have measured the complex dynamical conductivity, , of superconducting Ba(FeCo)As ( K) at terahertz frequencies and temperatures 2 - 30 K. In the frequency
dependence of below , we observe clear signatures of the
superconducting energy gap opening. The temperature dependence of
demonstrates a pronounced coherence peak at frequencies below 15 cm (1.8
meV). The temperature dependence of the penetration depth, calculated from
, shows power-law behavior at the lowest temperatures. Analysis of
the conductivity data with a two-gap model, gives the smaller isotropic s-wave
gap of meV, while the larger gap is highly anisotropic with
possible nodes and its rms amplitude is meV. Overall, our
results are consistent with a two-band superconductor with an gap
symmetry.Comment: 6 pages, 4 figures, discussion on pair-barking scattering and
possible lifting of the nodes is adde
Optical properties of the pseudogap state in underdoped cuprates
Recent optical measurements of deeply underdoped cuprates have revealed that
a coherent Drude response persists well below the end of the superconducting
dome. In addition, no large increase in optical effective mass has been
observed, even at dopings as low as 1%. We show that this behavior is
consistent with the resonating valence bond spin-liquid model proposed by Yang,
Rice, and Zhang. In this model, the overall reduction in optical conductivity
in the approach to the Mott insulating state is caused not by an increase in
effective mass, but by a Gutzwiller factor, which describes decreased coherence
due to correlations, and by a shrinking of the Fermi surface, which decreases
the number of available charge carriers. We also show that in this model, the
pseudogap does not modify the low-temperature, low-frequency behavior, though
the magnitude of the conductivity is greatly reduced by the Gutzwiller factor.
Similarly, the profile of the temperature dependence of the microwave
conductivity is largely unchanged in shape, but the Gutzwiller factor is
essential in understanding the observed difference in magnitude between ortho-I
and -II YBaCuO.Comment: 9 pages, 6 figures, submitted to Eur. Phys. J.
Optical Sum Rule in Finite Bands
In a single finite electronic band the total optical spectral weight or
optical sum carries information on the interactions involved between the charge
carriers as well as on their band structure. It varies with temperature as well
as with impurity scattering. The single band optical sum also bears some
relationship to the charge carrier kinetic energy and, thus, can potentially
provide useful information, particularly on its change as the charge carriers
go from normal to superconducting state. Here we review the considerable
advances that have recently been made in the context of high oxides, both
theoretical and experimental.Comment: Review article accepted for publication in J. Low Temp. Phys. 29
pages, 33 figure
Electron-phonon renormalization in small Fermi energy systems
The puzzling features of recent photoemission data in cuprates have been
object of several analysis in order to identity the nature of the underlying
electron-boson interaction. In this paper we point out that many basilar
assumptions of the conventional analysis as expected to fail in small Fermi
energy systems when, as the cuprates, the Fermi energy is
comparable with the boson energy scale. We discuss in details the novel
features appearing in the self-energy of small Fermi energy systems and the
possible implications on the ARPES data in cuprates.Comment: 4 pages, 5 eps figures include
A possible cooling effect in high temperature superconductors
We show that an adiabatic increase of the supercurrent along a superconductor
with lines of nodes of the order parameter on the Fermi surface can result in a
cooling effect. The maximum cooling occurs if the supercurrent increases up to
its critical value. The effect can also be observed in a mixed state of a bulk
sample. An estimate of the energy dissipation shows that substantial cooling
can be performed during a reasonable time even in the microkelvin regime.Comment: 5 pages, to appear in Phys. Rev.
Bosons in high temperature superconductors: an experimental survey
We review a number of experimental techniques that are beginning to reveal
fine details of the bosonic spectrum \alpha^2F(\Omega) that dominates the
interaction between the quasiparticles in high temperature superconductors.
Angle-resolved photo emission (ARPES) shows kinks in electronic dispersion
curves at characteristic energies that agree with similar structures in the
optical conductivity and tunnelling spectra. Each technique has its advantages.
ARPES is momentum resolved and offers independent measurements of the real and
imaginary part of the contribution of the bosons to the self energy of the
quasiparticles. The optical conductivity can be used on a larger variety of
materials and with the use of maximum entropy techniques reveals rich details
of the spectra including their evolution with temperature and doping. Scanning
tunnelling spectroscopy offers spacial resolution on the unit cell level. We
find that together the various spectroscopies, including recent Raman results,
are pointing to a unified picture of a broad spectrum of bosonic excitations at
high temperature which evolves, as the temperature is lowered into a peak in
the 30 to 60 meV region and a featureless high frequency background in most of
the materials studied. This behaviour is consistent with the spectrum of spin
fluctuations as measured by magnetic neutron scattering. However, there is
evidence for a phonon contribution to the bosonic spectrum as well.Comment: 71 pages, 52 figure
- …