Anelastic relaxation in the high-$T_c$ superconductor $\mathrm{Y}{\mathrm{Ba}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{7-x}$

The first measurement of anelastic relaxation in a vibrating sample of $\mathrm{Y}{\mathrm{Ba}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{7-x}$ is reported. A marked drop in the dissipation is observed below $T_c$, from which a strong electron-phonon interaction is inferred. It is found that the Bardeen-Cooper-Schrieffer theory with a gap of $\frac{\Delta }{K_B}=240\mathrm{}$ K can explain this drop. The data also suggest the occurrence of a phase transition at 235 K.