1,568 research outputs found
Microwave conductivity of d-wave superconductors with extended impurities
We investigate the influence of extended scatterers on the finite temperature
and finite frequency microwave conductivity of d-wave superconductors. For this
purpose we generalize a previous treatment by Durst and Lee, which is based on
a nodal approximation of the quasiparticle excitations and scattering
processes, and apply it to the analysis of experimental spectra of YBCO-123 and
BSCCO-2212. For YBCO, we find that accounting for a slight spatial extension of
the strong scattering in-plane defects improves the fit of the low temperature
microwave conductivity to experiment. With respect to BSCCO we conclude that it
is necessary to include a large concentration of weak-to-intermediate strength
extended scatterers, which we attribute to the out-of plane disorder introduced
by doping. These findings for BSCCO are consistent with similar analyses of the
normal state ARPES spectra and of STM spectra in the superconducting state,
where an enhanced forward scattering has been inferred as well.Comment: 10 pages, 11 figure
The Cypro-Minoan Corpus Project Takes an Archaeological Approach
It has now been more than a century since the first studies of a still undeciphered script, one used both on Cyprus and in Syria between 1550 and 1050 BCE, began to appear. Based upon visual similarity to the linear scripts he found at Knossos on Crete, Sir Arthur Evans, who had been writing about the inscriptions since 1895, coined the name Cypro-Minoan for them (1909: 69). Since that time a host of international scholars have attempted to unravel the meanings of the inscriptions. Our effort, one begun in 1996 and titled The Cypro-Minoan Corpus Project, builds on that work while emphasizing an archaeological perspective
The Cypro-Minoan Corpus Project Wins Best of Show Award
The tum of the millennium also marks a century of study of the undeciphered Late Bronze Age script of Cyprus, Cypro-Minoan. In 1909, Sir Arthur Evans labeled it Cypro-Minoan based on its visual similarity to the linear scripts he found at Knossos on Crete. We began to discuss the need for a detailed corpus of Cypro-Minoan a decade ago when we both attended a seminar on ancient Cypriot writing conducted by Thomas G. Palaima of the Program in Aegean Scripts and Prehistory (PASP) at the University of Texas at Austin. We went on separately to pursue specific problems in the publication and study of Cypro-Minoan. It was not until 1996 at CAARI that we had the opportunity to renew our collaborative effort. In the invigorating surrounds of the Institute we started to plan for a formal corpus. Since that time we have sought funds and worked towards the goal of combining our individual efforts. Receipt of the 1998 Best of Show Poster Award at the Annual Meeting of the Archaeological Institute of America (AlA) is an honor for us and our project. It goes a long way toward making our project and its goals visible within the community of Mediterranean archaeology and beyond
Residual absorption at zero temperature in d-wave superconductors
In a d-wave superconductor with elastic impurity scattering, not all the
available optical spectral weight goes into the condensate at zero temperature,
and this leads to residual absorption. We find that for a range of impurity
parameters in the intermediate coupling regime between Born (weak) and unitary
(strong) limit, significant oscillator strength remains which exhibits a cusp
like behavior of the real part of the optical conductivity with upward
curvature as a function of frequency, as well as a quasilinear temperature
dependence of the superfluid density. The calculations offer an explanation of
recent data on ortho-II YBaCuO which has been considered
anomalous.Comment: Accepted for publication by Phys. Rev. B 7 Pages and 4 Figure
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.
Infrared conductivity of a d_{x^2-y^2}-wave superconductor with impurity and spin-fluctuation scattering
Calculations are presented of the in-plane far-infrared conductivity of a
d_{x^2-y^2}-wave superconductor, incorporating elastic scattering due to
impurities and inelastic scattering due to spin fluctuations. The impurity
scattering is modeled by short-range potential scattering with arbitrary phase
shift, while scattering due to spin fluctuations is calculated within a
weak-coupling Hubbard model picture. The conductivity is characterized by a
low-temperature residual Drude feature whose height and weight are controlled
by impurity scattering, as well as a broad peak centered at 4 Delta_0 arising
from clean-limit inelastic processes. Results are in qualitative agreement with
experiment despite missing spectral weight at high energies.Comment: 29 pages (11 tar-compressed-uuencoded Postscript figures), REVTeX 3.0
with epsf macro
Anisotropic Optical Conductivity of Nd2-xCexCuO4 Thin Films
Opticcal conductivity spectra of Nd2-xCexCuO4 thin films,
measured by the reflectance-transmittance method (R-T method) which has been
proposed to investigate far-infrared spectroscopy, are investigated based on
the anisotropic pairing model. Precise measurements of the frequency-dependent
conductivity enable us to examine quantitatively the nature of the
superconducting gap through infrared properties in the electron-doped high-Tc
superconductors. We show that the behavior of optical conductivity
is consistent with the anisotropic superconducting gap and is well explained by
the formula for d-wave pairing in the low-energy regime of the far-infrared
region. Our results suggest that the electron-doped cuprate superconductors
Nd2-xCexCuO4 have nodes in the superconducting gap.Comment: 4 pages, 3 figure
Superfluid density and competing orders in d-wave superconductors
We derive expressions for the superfluid density in the
low-temperature limit in d-wave superconductors, taking into account
the presence of competing orders such as spin-density waves, -pairing, etc. Recent experimental data for the thermal conductivity and
for elastic neutron scattering in LaSrCuO suggest there are
magnetic field induced anomalies that can be interpreted in terms of competing
orders. We consider the implications of these results for the superfluid
density and show in the case of competing spin-density wave order that the
usual Volovik-like depletion of is replaced by a slower
dependence on applied magnetic field. We find that it is crucial to include the
competing order parameter in the self-consistent equation for the impurity
scattering rate.Comment: 17 pages, RevTeX4, 6 EPS figures; final version published in PR
Ab Initio Calculation of Impurity Effects in Copper Oxide Materials
We describe a method for calculating, within density functional theory, the
electronic structure associated with typical defects which substitute for Cu in
the CuO2 planes of high-Tc superconducting materials. The focus is primarily on
Bi2Sr2CaCu2O8, the material on which most STM measurements of impurity
resonances in the superconducting state have been performed. The magnitudes of
the effective potentials found for Zn, Ni and vacancies on the in-plane Cu
sites in this host material are remarkably consistent with phenomenological
fits of potential scattering models to STM resonance energies. The effective
potential ranges are quite short, of order 1 A with weak long range tails, in
contrast to some current models of extended potentials which attempt to fit STM
data. For the case of Zn and Cu vacancies, the effective potentials are
strongly repulsive, and states on the impurity site near the Fermi level are
simply removed. The local density of states (LDOS) just above the impurity is
nevertheless found to be a maximum in the case of Zn and a local minimum in
case of the vacancy, in agreement with experiment. The Zn and Cu vacancy
patterns are explained as due to the long-range tails of the effective impurity
potential at the sample surface. The case of Ni is richer due to the Ni atom's
strong hybridization with states near the Fermi level; in particular, the short
range part of the potential is attractive, and the LDOS is found to vary
rapidly with distance from the surface and from the impurity site. We propose
that the current controversy surrounding the observed STM patterns can be
resolved by properly accounting for the effective impurity potentials and
wave-functions near the cuprate surface. Other aspects of the impurity states
for all three species are discussed.Comment: 37 pp. pdf including figures, submitted to Phys. Rev.
Transport Properties of d-Wave Superconductors in the Vortex State
We calculate the magnetic field dependence of quasiparticle transport
properties in the vortex state of a d-wave superconductor arising solely from
the quasiparticle's Doppler shift in the superflow field surrounding the
vortex. Qualitative features agree well with experiments on cuprate and heavy
fermion superconductors at low fields and temperatures. We derive scaling
relations in the variable valid at sufficiently low temperatures
and fields , but show that these relations depend on the scattering
phase shift, and are in general fulfilled only approximately even in the clean
limit, due to the energy dependence of the quasiparticle relaxation time.Comment: 5 pages, 2 Postscript figure
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