11 research outputs found
Doping dependence of the superconducting gap in Bi2Sr2CaCu2O{8 + delta}
Bi2Sr2CaCu2O{8 + \delta} crystals with varying hole concentrations (0.12 < p
< 0.23) were studied to investigate the effects of doping on the symmetry and
magnitude of the superconducting gap. Electronic Raman scattering experiments
that sample regions of the Fermi surface near the diagonal (B_{2g}) and
principal axes (B_{1g}) of the Brillouin Zone have been utilized. The frequency
dependence of the Raman response function at low energies is found to be linear
for B_{2g} and cubic for B_{1g} (T< T_c). The latter observations have led us
to conclude that the doping dependence of the superconducting gap is consistent
with d_{x^2-y^2} symmetry, for slightly underdoped and overdoped crystals.
Studies of the pair-breaking peak found in the B_{1g} spectra demonstrate that
the magnitude of the maximum gap decreases monotonically with increasing hole
doping, for p > 0.12. Based on the magnitude of the B_{1g} renormalization, it
is found that the number of quasiparticles participating in pairing increases
monotonically with increased doping. On the other hand, the B_{2g} spectra show
a weak "pair-breaking peak" that follows a parabolic-like dependence on hole
concentration, for 0.12 < p < 0.23.Comment: 9 pages REvTex document including 8 eps figures; new table II;
changes to Fig. 5 and tex
Nonresonant inelastic light scattering in the Hubbard model
Inelastic light scattering from electrons is a symmetry-selective probe of
the charge dynamics within correlated materials. Many measurements have been
made on correlated insulators, and recent exact solutions in large dimensions
explain a number of anomalous features found in experiments. Here we focus on
the correlated metal, as described by the Hubbard model away from half filling.
We can determine the B1g Raman response and the inelastic X-ray scattering
along the Brillouin zone diagonal exactly in the large dimensional limit. We
find a number of interesting features in the light scattering response which
should be able to be seen in correlated metals such as the heavy fermions.Comment: 9 pages, 7 figures, typeset with ReVTe
Raman Scattering versus Infrared Conductivity: Evidence for one-dimensional Conduction in La_{2-x}Sr_{x}CuO_{4}
Raman and Infrared (IR) spectra of an underdoped La_{1.90}Sr_{0.10}CuO_{4}
single crystal have been measured as a function of temperature. Both techniques
provide unconventional low-energy spectra. The IR conductivity exhibits
features peaked at finite frequencies which do not have a counterpart in the
Raman response. Below approximately 100 K a transfer of both Raman and IR
spectral weight towards lower energies is found and a new component in the
Raman response builds up being characterized by a very long lifetime of
electrons propagating along the Cu-O bonds.Comment: 4 pages, 3 eps figure
Hole concentration and phonon renormalization in Ca-doped YBa_2Cu_3O_y (6.76 < y < 7.00)
In order to access the overdoped regime of the YBa_2Cu_3O_y phase diagram, 2%
Ca is substituted for Y in YBa_2Cu_3O_y (y = 7.00,6.93,6.88,6.76). Raman
scattering studies have been carried out on these four single crystals.
Measurements of the superconductivity-induced renormalization in frequency
(Delta \omega) and linewidth (\Delta 2\gamma) of the 340 cm^{-1} B_{1g} phonon
demonstrate that the magnitude of the renormalization is directly related to
the hole concentration (p), and not simply the oxygen content. The changes in
\Delta \omega with p imply that the superconducting gap (\Delta_{max})
decreases monotonically with increasing hole concentration in the overdoped
regime, and \Delta \omega falls to zero in the underdoped regime. The linewidth
renormalization \Delta 2\gamma is negative in the underdoped regime, crossing
over at optimal doping to a positive value in the overdoped state.Comment: 18 pages; 5 figures; submitted to Phys. Rev. B Oct. 24, 2002 (BX8292
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