1,317 research outputs found
Photoinduced melting of superconductivity in the high-Tc superconductor La2-xSrxCuO4 probed by time-resolved optical and THz techniques
Dynamics of depletion and recovery of superconducting state in La2-xSrxCuO_4
thin films is investigated utilizing optical pump-probe and optical pump - THz
probe techniques as a function of temperature and excitation fluence. The
absorbed energy density required to suppress superconductivity is found to be
about 8 times higher than the thermodynamically determined condensation energy
density and nearly temperature independent between 4 and 25 K. These findings
indicate that during the time when superconducting state suppression takes
place (~0.7 ps), a large part (nearly 90%) of the energy is transferred to the
phonons with energy lower than twice the maximum value of of the SC gap and
only 10% is spent on Cooper pair breaking.Comment: 8 pages, 5 figure
Modulated electronic configurations in selectively doped multilayered nanostructures
A simple theoretical model is proposed to describe the recent experimental
results on formation of induced superconducting state and anomalous tunneling
characteristics in selectively doped multilayered nanostructures based on
LaCuO perovskite. In particular, it is shown that the structure
composed from the nominally non-superconducting (undoped and overdoped) layers
turns to be superconducting with superconductivity confined to narrow regions
near the interfaces, in agreement with the experimental observations.Comment: 4 pages, 4 figures, revte
Probing Pseudogap by Josephson Tunneling
We propose here an experiment aimed to determine whether there are
superconducting pairing fluctuations in the pseudogap regime of the high-
materials. In the experimental setup, two samples above are brought into
contact at a single point and the differential AC conductivity in the presence
of a constant applied bias voltage between the samples, , should be
measured. We argue the the pairing fluctuations will produce randomly
fluctuating Josephson current with zero mean, however the current-current
correlator will have a characteristic frequency given by Josephson frequency
. We predict that the differential AC conductivity
should have a peak at the Josephson frequency with the width determined by the
phase fluctuations time.Comment: 4 pages, 2 eps figure
Pairing interactions and pairing mechanism in high temperature copper oxide superconductors
The polaron binding energy E_{p} in undoped parent cuprates has been
determined to be about 1.0 eV from the unconventional oxygen-isotope effect on
the antiferromagnetic ordering temperature. The deduced value of E_{p} is in
quantitative agreement with that estimated from independent optical data and
that estimated theoretically from the measured dielectric constants. The
substantial oxygen-isotope effect on the in-plane supercarrier mass observed in
optimally doped cuprates suggests that polarons are bound into the Cooper
pairs. We also identify the phonon modes that are strongly coupled to
conduction electrons from the angle-resolved photoemission spectroscopy,
tunneling spectra, and optical data. We consistently show that there is a very
strong electron-phonon coupling feature at a phonon energy of about 20 meV
along the antinodal direction and that this coupling becomes weaker towards the
diagonal direction. We further show that high-temperature superconductivity in
cuprates is caused by strong electron-phonon coupling, polaronic effect, and
significant coupling with 2 eV Cu-O charge transfer fluctuation.Comment: 11 pages, 7 figure
Vanishing of phase coherence in underdoped Bi_2Sr_2CaCu_2O_8+d
Coherent time-domain spectroscopy is used to measure the screening and
dissipation of high-frequency electromagnetic fields in a set of underdoped
Bi_2Sr_2CaCu_2O_8+d thin films. The measurements provide direct evidence for a
phase-fluctuation driven transition from the superconductor to normal state,
with dynamics described well by the Berezinskii-Kosterlitz-Thouless theory of
vortex-pair unbinding.Comment: Nature, Vol. 398, 18 March 1999, pg. 221 4 pages with 4 included
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.
Evidence for Magnetic Pseudoscaling in Overdoped La(2-x)Sr(x)CuO(4)
We report the results of electronic Raman scattering experiments on an
overdoped La(1.78)Sr(0.22)CuO(4) single crystal as a function of temperature.
The scattering rate Gamma(w->0,T) has been determined from the normal state
B(1g) spectra in the range 50 K < T < 300 K. Gamma(T) decreases linearly from
300 K to about 175 K and then undergoes a reduction with respect to the
expected mean-field behavior. This trend suggests a crossover to pseudoscaling
regime at about T(cr)=160 K. The results are in good agreement with the
prediction of the nearly antiferromagnetic Fermi liquid model. There is no
evidence of a pseudogap in the spectra obtained from this overdoped sample.Comment: RevTex, 4 pages with 3 PS figures included, replaced with minor
changes in the text and reference
Anisotropic optical properties of single-crystal GdBa2Cu3O7-delta
The optical spectrum of reduced-T(c) GdBa2Cu3O7-delta has been measured for polarizations parallel and perpendicular to the ab plane. The sample was an oxygen-deficient single crystal with a large face containing the c axis. The polarized reflectance from this face was measured from 20-300 K in the spectral region from 30-3000 cm-1, with 300 K data to 30 000 cm-1. Kramers-Kronig analysis was used to determine the spectral dependence of the ab and the c components of the dielectric tensor. The optical properties are strongly anisotropic. The ab-plane response resembles that of other reduced-T(c) materials whereas the c axis, in contrast, shows only the presence of several phonons. There is a complete absence of charge carrier response along c above and below T(c). This observation allows us to set an upper limit to the free-carrier spectral weight for transport perpendicular to the CuO2 planes
Non-Fermi liquid behavior of SrRuO_3 -- evidence from infrared conductivity
The reflectivity of the itinerant ferromagnet SrRuO_3 has been measured
between 50 and 25,000 cm-1 at temperatures ranging from 40 to 300 K, and used
to obtain conductivity, scattering rate, and effective mass as a function of
frequency and temperature. We find that at low temperatures the conductivity
falls unusually slowly as a function of frequency (proportional to
\omega^{-1/2}), and at high temperatures it even appears to increase as a
function of frequency in the far-infrared limit. The data suggest that the
charge dynamics of SrRuO_3 are substantially different from those of
Fermi-liquid metals.Comment: 4 pages, 3 postscript figure
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