Influence of divalent Sr2+, Zn2+ AND Ca2+ substitutions on ultrasonic velocity and elastic properties of DyBa2Cu3O7-δ high temperature superconductors / Norashikin Abdul Rasih

Ultrasonic longitudinal velocity measurements between 80 K and 280 K and shear velocity measurements between 80 K and 220 K have been performed in polycrystalline superconducting DyBa2-xSrxCu3O7-δ (x = 0, 0.3, 0.6), DyBa2Cu3-xZnxO7-δ (x = 0.01 and 0.03) and DyBa2Cao.iBa2Cu3-xZnxO7-δ (x = 0.01) samples utilizing the pulsed-echo-overlap technique. A step-like longitudinal anomaly characterized by a step-like slope change was observed at 240 K for x = 0 and x = 0.3 and at 260 K for x = 0.6 in Dy0.9Ba2-xSrxCu3O7-δ samples. In addition, a longitudinal anomaly was also observed at around 240 K for DyBa2Cu3-xZn1-xO7-δ (x = 0) and at 230 K for x = 0.01 and 0.03. Substitution of Zn2+ which caused suppression of critical temperature, Tc however, did not suppress the step-like anomaly. On the other hand, substitution of Ca2+ in Dy0.9Ca0.1Ba2Cu3-yZn1-yO7-δ (y = 0.01) caused the step-like anomaly to disappear. The step-like anomalies observed in samples were suggested due to oxygen ordering process in Cu-O chains during a phase transition process at low temperatures. For DyBa2-xSrxCu3O7-δ (x = 0, 0.3, 0.6), the reduction in the slope change of the step-like anomalies indicates some degree of weakening of oxygen ordering due to the Sr substitution. Substitution of Zn2+ ions in place of Cu in Dy0.9Ca0.1Ba2Cu3-xZnxO7-δ (x = 0.01 and x = 0.03) was suggested to go into Cu-O planes and did not interfere with oxygen ordering in Cu-O chains and cause non-suppressed step-like anomalies in DyBa2Cu3-xZn1-xO7-δ (x = 0.01 and 0.03) samples. But, disappearance of the step-like anomaly due to Ca2+ substitution in Dy0.9Ca0.1Ba2Cu3-xZn1-xO7-δ (x = 0.01) is probably related to reduction in oxygen content which depletes at Cu-O chain sites. On the other hand, it was found that Sr substitution in DyBa2-xSrxCu3O7-δ (x = 0, 0.3 and 0.6), Zn substitution in DyBa2Cu3-xZn1-xO7-δ (x = 0.01 and 0.03) and Ca2+ substitution in Dy0.9Ca0.1Ba2Cu3-yZn1-yO7-δ (y= 0.01) has the effect of lowering the calculated Deb ye temperature and BCS electron-phonon coupling constant, signifying that the step-like anomaly may not be related to superconductivity. A comparison between experimental data and calculated lattice anharmonicity curve based on the model by Lakkad (1971) showed that the large deviation of the experimental velocity curves for DyBa2-xSrxCu3-xZn1-xO7-δ (x = 0, 0.3) and DyBa2Cu3-xZn1-xO7-δ (x = 0, 0.01 and 0.03) from the calculated curves is strongly influenced by the existence of the step-like longitudinal anomalies.