Elastic response of polycrystalline and single-crystal Y${\mathrm{Ba}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_7$

The elastic response (Young's modulus and internal friction) of the high-$T_c$ superconductor Y${\mathrm{Ba}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_7$ has been measured in single-crystal and polycrystalline specimens. For the first time, we resolve the small expected lattice softening associated with the superconducting phase transition. The anomalous lattice stiffening below $T_c$ in polycrystalline samples is present also in single crystals.