Abstract
Temperature dependences of an ultrasonic attenuation coefficient α(T) and sound velocity Vs(T) have been measured for high-Tc superconductors (RE)Ba2Cu3O7 (RE=Y, Nd, Sm, Dy, Er). On increasing the ionic radius from Er to Nd, the following was found: (i) The increase of the Tc is found to be \lesssim1 K. (ii) The peak positions in α(T) versus T curves which correspond to the energies of optical phonons in a low-energy region show an increase by ∼10%. This change may be associated with the increase of the lattice constants by ∼1%. (iii) The sound velocity becomes larger, but the change is mainly caused by the difference in the degree of filling of the crystals. Using a comparison of the calculated results of phonon dispersion curves by Yasuda and Mase, a brief discussion is presented regarding the relation between the change of phonon spectra and that of Tc within the BCS-McMillan scheme.
