48 research outputs found
Observation of out-of-phase bilayer plasmons in YBa_2Cu_3O_7-delta
The temperature dependence of the c-axis optical conductivity \sigma(\omega)
of optimally and overdoped YBa_2Cu_3O_x (x=6.93 and 7) is reported in the far-
(FIR) and mid-infrared (MIR) range. Below T_c we observe a transfer of spectral
weight from the FIR not only to the condensate at \omega = 0, but also to a new
peak in the MIR. This peak is naturally explained as a transverse out-of-phase
bilayer plasmon by a model for \sigma(\omega) which takes the layered crystal
structure into account. With decreasing doping the plasmon shifts to lower
frequencies and can be identified with the surprising and so far not understood
FIR feature reported in underdoped bilayer cuprates.Comment: 7 pages, 3 eps figures, Revtex, epsfi
Hole depletion and localization due to disorder in insulating PrBa2Cu3O7-d: a Compton scattering study
The (mostly) insulating behaviour of PrBa2Cu3O7-d is still unexplained and
even more interesting since the occasional appearance of superconductivity in
this material. Since YBa2Cu3O7-d is nominally iso-structural and always
superconducting, we have measured the electron momentum density in these
materials. We find that they differ in a striking way, the wavefunction
coherence length in PrBa2Cu3O7-d being strongly suppressed. We conclude that Pr
on Ba-site substitution disorder is responsible for the metal-insulator
transition. Preliminary efforts at growth with a method to prevent disorder
yield 90K superconducting PrBa2Cu3O7-d crystallites.Comment: 4 pages, 3 figures, revised version submitted to PR
Experimental evidence for fast cluster formation of chain oxygen vacancies in YBa2Cu3O7-d being at the origin of the fishtail anomaly
We report on three different and complementary measurements, namely
magnetisation measurements, positron annihilation spectroscopy and NMR
measurements, which give evidence that the formation of oxygen vacancy clusters
is on the origin of the fishtail anomaly in YBa2Cu3O7-d. While in the case of
YBa2Cu3O7.0 the anomaly is intrinsically absent, it can be suppressed in the
optimally doped state where vacancies are present. We therefore conclude that
the single vacancies or point defects can not be responsible for this anomaly
but that clusters of oxygen vacancies are on its origin.Comment: 10 pages, 4 figures, submitted to PR
Phase fluctuations and the pseudogap in YBa2Cu3Ox
The thermodynamics of the superconducting transition is studied as a function
of doping using high-resolution expansivity data of YBa2Cu3Ox single crystals
and Monte-Carlo simulations of the anisotropic 3D-XY model. We directly show
that Tc of underdoped YBa2Cu3Ox is strongly suppressed from its mean-field
value (Tc-MF) by phase fluctuations of the superconducting order parameter. For
overdoped YBa2Cu3Ox fluctuation effects are greatly reduced and Tc ~ Tc-MF . We
find that Tc-MF exhibits a similar doping dependence as the pseudogap energy,
naturally suggesting that the pseudogap arises from phase-incoherent Cooper
pairing.Comment: 9 pages, 3 Figure
Physical origin of the buckling in CuO: Electron-phonon coupling and Raman spectra
It is shown theoretically that the buckling of the CuO planes in
certain cuprate systems can be explained in terms of an electric field across
the planes which originates from different valences of atoms above and below
the plane. This field results also in a strong coupling of the Raman-active
out-of-phase vibration of the oxygen atoms ( mode) to the electronic
charge transfer between the two oxygens in the CuO plane. Consequently,
the electric field can be deduced from the Fano-type line shape of the
phonon. Using the electric field estimated from the electron-phonon coupling
the amplitude of the buckling is calculated and found to be in good agreement
with the structural data. Direct experimental support for the idea proposed is
obtained in studies of YBaCuO and
BiSr(CaY)CuO with different oxygen and
yttrium doping, respectively, including antiferromagnetic samples. In the
latter compound, symmetry breaking by replacing Ca partially by Y leads to an
enhancement of the electron-phonon coupling by an order of magnitude.Comment: 12 pages, 4 figures, and 1 tabl
Evolution of the resonance and incommensurate spin fluctuations in superconducting YBaCuO
Polarized and unpolarized neutron triple-axis spectrometry was used to study
the dynamical magnetic susceptibility as
a function of energy () and wave vector () in a wide
temperature range for the bilayer superconductor YBaCuO with
oxygen concentrations, , of 0.45, 0.5, 0.6, 0.7, 0.8, 0.93, and 0.95. The
most prominent features in the magnetic spectra include a spin gap in the
superconducting state, a pseudogap in the normal state, the much-discussed
resonance, and incommensurate spin fluctuations below the resonance. We
establish the doping dependence of the spin gap in the superconducting state,
the resonance energy, and the incommensurability of the spin fluctuations. We
discuss in detail the procedure used for separating the magnetic scattering
from phonon and other spurious effects. In the comparison of our experimental
results with various microscopic theoretical models, particular emphasis was
made to address the similarities and differences in the spin fluctuations of
YBaCuO and LaSrCuO.Comment: 23 pages with 30 figures, Phys. Rev. B (in press). If necessary,
contact me for higher resolution figure
Magnetic Dynamics in Underdoped YBCO: Direct Observation of a Superconducting Gap
Polarized and unpolarized triple-axis spectrometry has been used to study an
underdoped YBa_2Cu3O6.6. Our results indicate that spin excitations in the
superconducting state are essentially the same as those in the fully doped
material except that the unusual 41 meV resonance is observed at 34.8 meV. The
normal state spin excitations are characterized by a weakly energy-dependent
continuum whose temperature dependence shows the clear signature of a
superconducting gap at Tc. The enhancement at the resonance is accompanied by a
suppression of magnetic fluctuations at both higher and lower energies.Comment: 12 pages, 4 Postscript figures, Physical Review Letters, to be
publishe
Carrier relaxation, pseudogap, and superconducting gap in high-Tc cuprates: A Raman scattering study
We describe results of electronic Raman-scattering experiments in differently
doped single crystals of Y-123 and Bi-2212. The comparison of AF insulating and
metallic samples suggests that at least the low-energy part of the spectra
originates predominantly from excitations of free carriers. We therefore
propose an analysis of the data in terms of a memory function approach.
Dynamical scattering rates and mass-enhancement factors for the carriers are
obtained. In B2g symmetry the Raman data compare well to the results obtained
from ordinary and optical transport. For underdoped materials the dc scattering
rates in B1g symmetry become temperature independent and considerably larger
than in B2g symmetry. This increasing anisotropy is accompanied by a loss of
spectral weight in B2g symmetry in the range between the superconducting
transition at Tc and a characteristic temperature T* of order room temperature
which compares well with the pseudogap temperature found in other experiments.
The energy range affected by the pseudogap is doping and temperature
independent. The integrated spectral loss is approximately 25% in underdoped
samples and becomes much weaker towards higher carrier concentration. In
underdoped samples, superconductivity related features in the spectra can be
observed only in B2g symmetry. The peak frequencies scale with Tc. We do not
find a direct relation between the pseudogap and the superconducting gap.Comment: RevTeX, 21 pages, 24 gif figures. For PostScript with embedded eps
figures, see http://www.wmi.badw-muenchen.de/~opel/k2.htm
One-dimensional nature of the magnetic fluctuations in YBaCuO
There is increasing evidence that inhomogeneous distributions of charge and
spin--so-called "striped phases"--play an important role in determining the
properties of the high-temperature superconductors. For example, recent
neutron-scattering measurements on the YBaCuO family of
materials show both spin and charge fluctuations that are consistent with the
striped-phase picture. But the fluctuations associated with a striped phase are
expected to be one-dimensional, whereas the magnetic fluctuations observed to
date appear to display two-dimensional symmetry. We show here that this
apparent two-dimensionality results from measurements on twinned crystals, and
that similar measurements on substantially detwinned crystals of
YBaCuO reveal the one-dimensional character of the magnetic
fluctuations, thus greatly strengthening the striped-phase interpretation.
Moreover, our results also suggest that superconductivity originates in charge
stripes that extend along the b crystal axis, where the superfluid density is
found to be substantially larger than for the a direction.Comment: 3 pages, PDF onl