ULTRASONIC EVIDENCE OF STRONG VIBRATIONAL ANHARMONICITY IN HIGH-TC SUPERCONDUCTORS AND ITS EFFECT ON DETERMINATION OF THEIR ELASTIC PROPERTIES

To shed further light on outstanding problems associated with the magnitude of the bulk modulus and its pressure dependence in high-T(c) superconductors, a comparative study has been made of the effects of hydrostatic pressure on the elastic properties of ceramic specimens of the electron-doped Nd1.85Ce0.15CuO4-y and the hole-doped La1.8Sr0.2CuO4 superconductors and their nonsuperconducting parent compounds Nd2CuO4-y and La2CuO4. The effects of porosity on the elastic properties have been taken into account by use of wave-scattering theory in a porous medium. The ultrasonic-wave velocities and bulk modulus B0 measured in the polycrystalline La2CuO4 are in reasonable agreement with those determined from elastic-constant measurements in single-crystal La2CuO4. This observation lends strong support to the suggestion that for these mixed oxide ceramics the true bulk modulus at atmospheric pressure is the one B0 derived from ultrasonic measurements rather than that B(T)(P) obtained from x-ray experiments at very high pressures. The apparent discrepancy between B0 and B(T)(P) is found to be a common characteristic of all these materials and can be resolved by taking the contribution of (partial-B/partial-P)p = 0 to the high-pressure bulk modulus into account in B(T)(P). The hydrostatic-pressure derivatives (partial-B/partial-P)p = 0(= 15.6) and (partial-C11/partial-P)p = 0(= 18.3) at room temperature for the nonporous matrix of Nd1.85Ce0.15CuO4-y are found to be substantially smaller than those of other mixed oxide ceramics including YBa2Cu3O7-x and GdBa2Cu3O7-x. This reinforces the proposal that very large values of (partial-B/partial-P)p = 0 and (partial-C11/partial-P)p = 0 are intrinsic properties of YBa2Cu3O7-x and GdBa2Cu3O7-x rather than merely being a result of the application of pressure to a porous ceramic. Further confirmation comes from measurements in a very large-grained sample of YBa2Cu3O7-x (grown by the eutectic KCl-NaCl flux technique) of the longitudinal ultrasonic-wave velocity and its pressure dependence giving values of 108 GPa for C11 and 40 for (partial-C11/partial-P)p = 0 in the same range as those for ceramic material. The vibrational anharmonicity of the long-wavelength acoustic modes is discussed in terms of third-order elastic constants and Gruneisen parameters. In each of these materials, with the exception of Nd1.85Ce0.15CuO4-y, the anharmonicity of the longitudinal modes is large, particularly for YBa2Cu3O7-x.