Infrared-active phonons and the superconducting gap ofTc-reduced double-chainYBa2Cu4O8superconductors

We report a far-infrared reflectivity study in the temperature range 10–300 K of the double-chain ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_4$ ${\mathrm{O}}_8$ superconductor with Zn and Sr substituting for Cu and Ba, respectively. Superconductivity-induced self-energies of the transverse optical ${\mathit{B}}_{1\mathit{u}}$(z) phonon were used to estimate the position of the superconducting gap. Evidence for a shift of the gap to lower energies upon decreasing the superconducting transition temperature ${\mathit{T}}_{\mathit{c}}$ was found for Zn- as well as for Pr-doped materials. A comparison of the linewidth changes of the plane-oxygen phonon (at ∼300 ${\mathrm{cm}}^{\mathrm{-}1}$) with the results of a strong-coupling calculation yields the gap-to-${\mathit{T}}_{\mathit{c}}$ ratio 2${\mathrm{\Delta }}_0$/${\mathit{kT}}_{\mathit{c}}$=(8.2±1.0) for ${\mathrm{YBa}}_2$(${\mathrm{Cu}}_{3.96}$ ${\mathrm{Zn}}_{0.04}$)${\mathrm{O}}_8$ which is close to 2${\mathrm{\Delta }}_0$/${\mathit{kT}}_{\mathit{c}}$=(6.6±0.2) reported for (${\mathrm{Pr}}_{0.37}$ ${\mathrm{Y}}_{0.63}$)${\mathrm{Ba}}_2$ ${\mathrm{Cu}}_4$ ${\mathrm{O}}_8$. For Sr in place of Ba in Y(${\mathrm{Ba}}_{2\mathrm{-}\mathit{z}}$ ${\mathrm{Sr}}_{\mathit{z}}$)${\mathrm{Cu}}_4$ ${\mathrm{O}}_8$, for which ${\mathit{T}}_{\mathit{c}}$ remains essentially the same within a broad range of compositions (0≤z≤0.6), no change of the temperature behavior of the phonon and thus no variation of the position of the superconducting gap was observed. These experimental findings confirm that the picture of superconductivity-induced changes in the phonon parameters holds in the double-chain compound, similarly to the single-chain material. We also report the effects of these dopants on the phonon anomaly which has been attributed to a spin gap.