Ultrasonic studies of the high Tsub(c) superconductor (Lasub(1-x)Srsub(x))2CuOsub(4-delta)

We synthesized (Lasub(1-x)Srsub(x))2CuOsub(4-delta) crystals with O <= x <= 0.4 by the sintering method, and obtained several samples with good homogeneity, high onset superconducting transition temperature Tsub(c) < approx. 38 K and narrow transition width ΔTsub(c). For the sample with the maximum Tsub(c) among them, ultrasonic measurements were performed. In the curve of the sound velocity change ΔVsub(s)(T) versus temperature T, there was observed an anomalously large decrease of sound velocity as T decreases in the range 150 K < T < 240 K. Furthermore, in the plots of the ultrasonic attenuation coefficient α(T) as a function of T, there were observed a broad and large peak located at T approx. 100 K and a small peak located at T approx. 200 K. These results show the existence of an optical mode of the energy h/2πωsub(op)/ksub(BETA) approx. 100 K and, probably, also of that of approx. 200 K. On the basis of these experimental results of elastic properties, we suggest the origin of high Tsub(c) of this material. (author)