ULTRASONIC VELOCITY AND OXYGEN-RELATED ELASTIC ANOMALIES IN Zn2+-SUBSTITUTED (Dy,Ca)Ba2Cu3-yZnyO7-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 Cu 3-x Zn x O 7-δ (x = 0, 0.01 and 0.03) and Dy 0.9 Ca 0.1 Ba 2 Cu 3-y Zn y O 7-δ (y = 0.01) samples utilizing the pulsed-echo-overlap technique. A longitudinal anomaly characterized by a step-like slope change was observed at around 240 K for DyBa 2 Cu 3-x Zn x O 7-δ (x = 0) and at 230 K for x=0.01 and 0.03. Zn 2+ substitution suppressed critical temperature, Tc but did not suppress the step-like anomaly. On the other hand, substitution of Ca 2+ in Dy 0.9 Ca 0.1 Ba 2 Cu 3-y Zn y O 7-δ (y = 0.01) caused the step-like anomaly to disappear. The step-like anomaly was suggested due to the oxygen-ordering process in Cu – O chains during a phase transition process at low temperatures. Substitution of Zn 2+ ions in place of Cu was suggested to go into CuO 2 planes and did not interfere with oxygen-ordering in Cu – O chains. This is suggested as the reason why the step-like anomalies were not suppressed in DyBa 2 Cu 3-x Zn x O 7-δ (x = 0.01 and 0.03) samples. Disappearance of the step-like anomaly due to Ca 2+ substitution is probably related to reduction in oxygen content which depletes Cu – O chain sites.