Infra- to ultra-sound studies of lattice instability and carriers in high-Tc cuprate superconductors

Abstract

Acoustic measurements from 10-2 to 107 Hz show that phase-like transitions (PLTs) characterized by a small jump of lattice parameters in the range 100-300 K always exist in high-Tc (near 100 K) superconducting Y-Ba-Cu-O, Bi-Sr-Ca-Cu-O and Tl-Ba-Ca-Cu-O. Ferroelastic loops associated with the elastic softening of C'=((C11+C22)/2-C12)/2 and C66 invariably occur at the PLT temperatures, and in some cases the shape memory effect may be observed. For various samples of Bi-Sr-Ca-Cu-O and Tl-Ba-Ca-Cu-O with different Tc, the acoustic attenuation (Q-1, alpha ) from the kilohertz to megahertz range reveals a plateau (Qp-1, alpha p) above Tc and drops obviously below Tc with the turning point at Tc. This kind of anomaly has not been observed for non-superconducting samples. Qp-1 is found to be closely related to the carrier density for YBa2Cu3O7- delta , Y1-xPrxBa2Cu3O7 and Gd1-xPrxBa2Cu3O7, and the drop of Q-1 and a below Tc is considered to be caused by superconducting condensation. The energy dissipation (Qp-1 and alpha p) related to carriers can be explained reasonably by using a coupling model of carriers with local dynamic distortion. Furthermore, taking account of the smearing of the superconducting gap structure and by using the modified Bardeen-Cooper-Schrieffer (BCS) relative jump rate, S(E, E', Gamma )=Re(1- Delta 2/((E-i Gamma )(E'-i Gamma ))), the calculated results of acoustic attenuation below Tc are in fairly good agreement with the experimental data. The superconducting gap Delta and the damping rate Gamma for both Bi-Sr-Ca-Cu-O and TI-Ba-Ca-Cu-O have also been obtained.