INFRARED-ACTIVE PHONONS AND THE SUPERCONDUCTING GAP OF T(C)-REDUCED DOUBLE-CHAIN YBA2CU4O8 SUPERCONDUCTORS

We report a far-infrared reflectivity study in the temperature range 1 0-300 K of the double-chain YBa2Cu4O8 superconductor with Zn and Sr su bstituting for Cu and Ba, respectively. Superconductivity-induced self -energies of the transverse optical B1u(z) phonon were used to estimat e the position of the superconducting gap. Evidence for a shift of the gap to lower energies upon decreasing the superconducting transition temperature T(c) was found for Zn- as well as for Pr-doped materials. A comparison of the linewidth changes of the plane-oxygen phonon (at a pproximately 300 cm-1) with the results of a strong-coupling calculati on yields the gap-to-T(c) ratio 2DELTA0/kT(c) = (8.2 +/- 1.0) for YBa2 (Cu3.96Zn0.04)O8 Which is close to 2DELTA0/kT(c) = (6.6+/-0.2) reporte d for (Pro.37Y0.63)Ba2Cu4O8. For Sr in place of Ba in Y(Ba2-zSrz)Cu4O8 , for which T(c) remains essentially the same within a broad range of compositions (0 less-than-or-equal-to z less-than-or-equal-to 0.6), no change of the temperature behavior of the phonon and thus no variatio n of the position of the superconducting gap was observed. These exper imental 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 spi n gap.