We study the effects of Ni doping on the infrared reflectance of YBa2(Cu1-xNix)3O7-delta films with up to 4% substitution of Cu by Ni. Normal-state spectra indicate that the primary effect of Ni on the conduction carriers is to increase their elastic scattering rate. At 4 at. % Ni, the elastic-scattering rate is calculated to be large enough to generate a large absorption onset at the gap edge in the superconducting state, if a gap exists. The superconducting-state spectra show no evidence for such a feature. Comparisons with phenomenological models for the optical conductivity show that the conductivity, and hence the superconducting density of states, is very gapless.

In this paper, we report the infrared reflectance R(nu,T) of a YBa2Cu3O7-delta film in which 6% of the Cu atoms have been substituted by Ni. These spectra are taken with better resolution than in our earlier report of films with up to 4% Ni. The reflectance spectra for the 6% Ni-doped film are very similar to those for a 4% Ni-doped film, thus confirming our earlier conclusions about the very non-BCS gapless nature of the optical conductivity. The new spectra make evident a plateau at R = 96% for nu < 150 cm-1 and a sharp decrease in R for nu > 150cm-1. These features imply an interesting non-Drude structure in the superconducting density of states in the infrared.

Dielectric ceramics such as BaTi4O9 have been widely used as microwave resonators in communication systems. In order to study the effect of doping on dielectric properties, infrared reflection spectra of BaTi4O9 doped with Mn, Sn, Zr, Ca, Sr, and Pb were studied from 20 to 6000 cm−1 at temperatures ranging from 15 to 297 K. The reflectance data was converted to the dielectric data using Kramers-Kronig analysis. The dielectric parameters k, Q, and τk calculated from the reflectance data were in agreement with resonant cavity measurements at 4 GHz.

We report Raman-scattering measurements on superconducting ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{4}$${\mathrm{O}}_{8\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\delta}}}$ (1:2:4) films (${\mathit{T}}_{\mathit{c}}$=80 K) grown on (110) and (001) surfaces of ${\mathrm{SrTiO}}_{3}$. In contrast to ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\delta}}}$, phonons associated with CuO chain atoms are Raman active in ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{4}$${\mathrm{O}}_{8\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\delta}}}$. Four Raman-active phonons are identified with the CuO chains in addition to five phonons corresponding to the five well-studied ${\mathit{A}}_{\mathit{g}}$ modes in ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\delta}}}$. The 337-${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ mode associated with out-of-phase vibrations of the ${\mathrm{CuO}}_{2}$ planar oxygens softens by about 2 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ below ${\mathit{T}}_{\mathit{c}}$ just as is observed in ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\delta}}}$ films. A new result is that the phonon associated with in-phase vibrations of the ${\mathrm{CuO}}_{2}$ planar oxygens at 435 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ hardens about 2 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ below ${\mathit{T}}_{\mathit{c}}$. This nonthermal hardening has also been reported very recently in ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\delta}}}$. Interpreted with a recent theory for phonon energies that includes coupling to superconducting electrons, the hardening and softening of phonons with relatively close frequencies (337 and 435 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$) indicates that the energy gap is 2\ensuremath{\Delta}=280 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$\ifmmode\pm\else\textpm\fi{}10% so 2\ensuremath{\Delta}/${\mathit{kT}}_{\mathit{c}}$=5\ifmmode\pm\else\textpm\fi{}0.5. The 150-${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ mode associated with ${\mathrm{CuO}}_{2}$ planar Cu motion and the four new chain modes behave normally with temperature. We attempt to deplete oxygen from some of the ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{4}$${\mathrm{O}}_{8\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\delta}}}$ films by annealing in Ar up to 700 \ifmmode^\circ\else\textdegree\fi{}C. The films change crystal structure to ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\delta}}}$ before significant amounts of oxygen are removed.

We report Raman scattering and resistance measurements on a YBa2Cu3O7−δ film in which δ is varied by repeated annealing in oxygen or argon. As oxygen is removed, the resistance rises, the transition moves to lower temperature and broadens, and ‘‘forbidden’’ Raman modes are observed. Group theory, the Raman activity of another oxygen rich (δ∼0) film of different orientation, and comparison with results on single crystals allow us to assign each Raman phonon. We hence demonstrate that several ‘‘forbidden’’ Raman peaks are associated with depletion of oxygen chain sites. Our results establish that the oxygen concentration δ, and hence the superconducting properties, can be varied reversibly by reannealing in oxygen or argon with the Raman spectra providing a useful microscopic probe of the lattice.