ULTRASONIC SHEAR VELOCITY ANOMALIES IN BULK THALLIUM-1212 HIGH TEMPERATURE SUPERCONDUCTOR

摘要

Ultrasonic shear velocity of bulk TlSr2(Ca, Cr)Cu2O7 and (Tl, Bi)Sr-2(Ca, Cr)Cu2O7 high temperature superconductors has been measured between 80 Ei and above 200 It at 8 MHz. A hardening trend nas observed in both samples as the temperature was lowered. A large thermal hysteresis was observed in the TlSr2(Ca, Cr)Cu2O7 (density 3.74 g/cm(3)) but very much suppressed in the (Tl, Bi)Sr-2(Ca, Cr)Cu2O7 (density 5.40 g/cm(3)) sample. Shear velocity hysteresis in these materials is very much determined by the density and microstructure and may not be directly related to high temperature superconductivity. A pronounced change in the shear velocity at about 160-190 K signifying a softening tendency was observed in the (Tl, Bi)Sr-2(Ca, Cr)Cu2O7 material. Some possible mechanisms of the origin of these anomalies are discussed. [References: 20]