Effect of Ba-site substitution by Sr on ultrasonic velocity and electron–phonon coupling constant of DyBa 2− xSr xCu 3O 7−δ superconductors

Abstract

Ultrasonic longitudinal velocity measurements between 80 K and 280 K and shear velocity measurements between 80 K and 230 K have been performed in polycrystalline superconducting DyBa 2− x Sr x Cu 3O 7−δ ( x = 0, 0.3, 0.6) samples utilizing the pulsed-echo-overlap technique. Substitution of Sr caused the percentage change in shear velocity to gradually decrease with x from 1.3% ( x = 0) to 0.8% ( x = 0.6). Longitudinal velocity showed initial increase from 1.7% ( x = 0) to 1.9% ( x = 0.3) before decreasing to 1.1% ( x = 0.6). A step-like longitudinal anomaly was observed at 240 K for x = 0 and x = 0.3 and at 260 K for x = 0.6. Sr substitution however, reduced the slope change of the step-like anomaly from 0.0078% K −1 ( x = 0) to 0.0043% K −1 ( x = 0.3) and 0.0023% K −1 ( x = 0.6). A comparison between experimental data and calculated lattice anharmonicity curve based on the model by Lakkad [S.C. Lakkad, J. Appl. Phys. 42 (1971) 4277–4281] showed that the large deviation of the experimental velocity curves for x = 0, 0.3 from the calculated curves is strongly influenced by existence of the step-like longitudinal anomalies. The longitudinal step-like anomalies are suggested to be due to oxygen ordering taking place in Cu–O chains during a phase transition. The reduction in the slope change of the step-like anomalies indicates some degree of weakening of oxygen ordering due to the Sr substitution. Sr substitution also has the effect of lowering the calculated electron–phonon coupling constant ( λ).