Distinct effects of Al~(3+) substitution at Cu-site and Al_2O_3 addition on step-like elastic anomalies and electron-phonon coupling constant in EuBa_2Cu_3O_(7-delta) superconductors

摘要

Ultrasonic longitudinal and shear velocities at 9 MHz were measured in temperature ranges of 80-280 K and 80-220 K, respectively, in EuBa_2(Cu_(1-x)Al_x)_3O_(7-delta) (x = 0,0.06 and 0.10) and EuBa_2Cu_3O_(7-delta) +y wt.%Al_2O_3 (y = 0.2 and 0.4) superconductors. A step-like elastic anomaly indicating sudden lattice stiffening was observed for x = 0 at around 260K. Partial substitution of Al~(3+) in place of Cu at x = 0.06 and 0.10 sup-pressed the step-like velocity anomaly and displayed monotonous velocity change with temperature. In contrast however, addition of Al_2O_3 only shifted the anomaly to slightly lower temperatures. Inter-estingly, maximum T_c was observed at x = 0.06 and this coincides with enhanced value of the computed BCS electron-phonon coupling constant, X. The step-like elastic anomaly was discussed in terms of oxy-gen ordering involving Cu-O chains. Substitution of Al~(3+) is suggested to go into Cu-O chain sites and effectively destroys oxygen ordering which in turn caused suppression of the elastic anomaly.