Distinct effects of Al3+ substitution at Cu-site and Al2O3 addition on step-like elastic anomalies and electron–phonon coupling constant in EuBa2Cu3O7−δ superconductors

Ultrasonic longitudinal and shear velocities at 9 MHz were measured in temperature ranges of 80–280 K and 80–220 K, respectively, in EuBa2(Cu1−xAlx)3O7−δ (x = 0, 0.06 and 0.10) and EuBa2Cu3O7−δ + y wt.% Al2O3 (y = 0.2 and 0.4) superconductors. A step-like elastic anomaly indicating sudden lattice stiffening was observed for x = 0 at around 260 K. Partial substitution of Al3+ in place of Cu at x = 0.06 and 0.10 suppressed the step-like velocity anomaly and displayed monotonous velocity change with temperature. In contrast however, addition of Al2O3 only shifted the anomaly to slightly lower temperatures. Interestingly, maximum Tc was observed at x = 0.06 and this coincides with enhanced value of the computed BCS electron–phonon coupling constant, λ. The step-like elastic anomaly was discussed in terms of oxygen ordering involving Cu–O chains. Substitution of Al3+ is suggested to go into Cu–O chain sites and effectively destroys oxygen ordering which in turn caused suppression of the elastic anomaly.