Ultrasonic study of the soft phonon mode in Bi2Sr2Cu2O8 single crystals

Abstract Ultrasonic measurements on Bi 2 Sr 2 Ca 1 Cu 2 O 8 single crystals have been carried out. The in-plane shear moduli C 66 and C ' = 1 2 [ 1 2 ( C 11 + C 22 )- C 12 ] were derived from the sound velocity data. It was found that the C ' mode showed an overall trend of softening over a wide temperature range above T c and the C 66 mode exhibited three obvious softening minima around 240–250, 150 and 100 K. Implications of these results for a possible structural change are discussed.