ASA 126th Meeting Denver 1993 October 4-8

4aPA1. Acoustic studies of high-T[sub c] oxide superconductors.

H. Ledbetter

Natl. Inst. of Standards and Technology, 325 Broadway, Boulder, CO 80303

Various acoustic studies of high-T[sub c] oxide superconductors, mainly cuprates, will be reviewed. Transition temperatures T[sub c] range from about 20 K for the electron-conductor, (Pr-Ce)[sub 2] CuO[sub (Delta)], to about 125 K for thallium cuprate. Properties considered include sound velocities, elastic constants, attenuation, Debye characteristic temperature (Theta)[sub D], and Gruneisen parameter. Variables considered include composition, temperature, pressure, and magnetic field. Especially the polycrystals are complicated by defects such as twins, voids, microcracks, nonhomogeneities, texture, and impurity phases. Even monocrystals are seldom defect-free. Most studies focused on the YBa[sub 2]Cu[sub 3]O[sub x] compound, where x varies between 6 and 7. The bismuth cuprates behave differently from most other compounds, perhaps because of incommensurate structural modulations. No convincing evidence exists that the new superconductors show velocity-temperature or attenuation-temperature curves similar to conventional BCS superconductors. This suggests a mechanism different from the simple s-electron spherical-fermi-surface weak-coupling BCS model. However, the new mechanism involves phonons. A relationship between T[sub c] and (Theta)[sub D] has been established for four systems: La--O, Y--O, Bi--O, Tl--O. Interrelationships among acoustic properties and other physical properties such as thermal expansity and specific heat are emphasized. It is emphasized that some acoustic properties, especially the bulk modulus, can be estimated from simple models. Finally, topics are recommended for further study.