Ultrasonic nonlinearities of high temperature superconductor YBa 2Cu3O7-δ from 300 to 77 K

The ultrasonic nonlinearity parameter β of the high-temperature superconductor YBa₂Cu₃O_7-δ was measured from 300 to 77 K by using the second-harmonic generation technique. The ultrasonic nonlinearity parameter β decreases with decreasing temperature very rapidly from room temperature to 200 K and vanishes around the transition temperature. The phenomenon can be explained by assuming a structural phase transition, a superconducting transition or a gain boundary change. The behavior is unique and different from that of single crystals