ULTRASONIC STUDIES OF THE HIGH-TC SUPERCONDUCTOR Y2BA4CU6O14

Ultrasonic measurements were performed with the samples of Y//2Ba//4Cu//6O//1//4 with a high superconducting transition temperature T//c approximately equals 93 K by using the longitudinal sound waves of about 10 MHz frequency. In the curve of ultrasonic attenuation coefficient alpha (T) versus temperature T, there were observed two peaks at T approximately equals 160 K and 245 K, while there was only small change in the plots of sound velocity V//s(T) versus T. These attenuation peaks exhibit the existence of optical modes of the energy h omega //o//p/k//B approximately equals 160 K and 245 K at the B. Z. center, as were found in BaPb//1// minus //xBi//xO//3 (T//c approximately equals 12 K and h omega //o//p/k//B approximately equals 70, 145 and 220 K at x equals 0. 25) and (La//1// minus //xSr//x)//2CuO//4 (T// c approximately equals 38 K and h omega //o//p/k//B approximately equals 100 and 200 K at x equals 0. 07). On the basis of these experimental results, the relation between the magnitudes of T//cs and the phonon energy spectra in these materials is discussed.