Ultrasonic study of the low temperature structural phase transition in YBa₃Cu₃O_7-δ and its dependence on oxygen concentration

Abstract

Elastic and structural instabilities have been reported in YBa₂Cu₃O_7-δ between 210 and 270 K. We have carried out an ultrasonic study of these instabilities as a function of both oxygen concentration and annealing temperature. Measurements of the sound velocity and ultrasonic attenuation have correspondingly been performed on two series of polycrystalline samples. The electrical resistance has also been measured simultaneously. The results strongly suggest that the observed elastic instabilities are associated with an athermal (martensitic-type) transition starting at a temperature, T_s , which decreases with decreasing oxygen concentration. A linear relationship is obtained between T_s and the superconducting transition temperature, T_c , suggesting that the second phase formed is essential for the high T_c .

Key Words:

• Structural phase transition
• high T superconductors
• ultrasonic.