Infrared-active phonons and the superconducting gap of Tc-reduced double-chain YBa2Cu4O8 superconductors

Abstract

We report a far-infrared reflectivity study in the temperature range 10–300 K of the double-chain YBa2Cu4O8 superconductor with Zn and Sr substituting for Cu and Ba, respectively. Superconductivity-induced self-energies of the transverse optical B1u(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 Tc was found for Zn- as well as for Pr-doped materials. A comparison of the linewidth changes of the plane-oxygen phonon (at ∼300 cm−1) with the results of a strong-coupling calculation yields the gap-to-Tc ratio 2Δ0/kTc=(8.2±1.0) for YBa2(Cu3.96Zn0.04)O8 which is close to 2Δ0/kTc=(6.6±0.2) reported for (Pr0.37Y0.63)Ba2Cu4O8. For Sr in place of Ba in Y(Ba2−zSrz)Cu4O8, for which Tc 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.

Source-reported materials — not catalogue approval

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