Acoustic studies of high‐Tc oxide superconductors

Abstract

Various acoustic studies of high‐Tc oxide superconductors, mainly cuprates, will be reviewed. Transition temperatures Tc range from about 20 K for the electron‐conductor, (Pr‐Ce)2 CuOΔ, to about 125 K for thallium cuprate. Properties considered include sound velocities, elastic constants, attenuation, Debye characteristic temperature ΘD, and Grüneisen 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 YBa2Cu3Ox 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 Tc and ΘD 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.