HYSTERESIS OF TEMPERATURE-DEPENDENCE AND SCALING LAW IN TIME-DEPENDENCE OF SOUND-VELOCITY IN SINTERED BI2SR2CACU2O8+DELTA

The temperature T dependence of the mechanical resonance frequency f(r ), of longitudinal sound waves in sintered bulk samples of Bi2Sr2CaCu2 O8+delta has been measured from room temperature down to liquid-nitrog en temperature. The f(r) increases steeply around 200 K with decreasin g T and on the other hand it decreases steeply around 230 K with incre asing T. This hysteresis of f(r) has a width depending on the change r ate of T. Keeping T at a point T-f between 230 K and 200 K, the time d ependence of f(r) has been measured. The f(r) increases toward a satur ated value f(rmax) on the hysteresis loop determined by T-f. When the time t is scaled by a characteristic time t(1/2) when f(r) reaches the middle point between the initial and the saturated ones, the plots of f(r)/f(rmax) versus t/t(1/2) at each T-f are almost reduced to one un iversal curve independent of T-f. These results suggest that the sampl e experiences a first-order phase transformation around those temperat ures.