Ion-channeling investigation of thermal vibrational amplitudes across the superconducting transition in $\mathrm{Y}{\mathrm{Ba}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{7-\delta }$

Ion-channeling measurements reveal that the thermal vibrational amplitude, $u_1$, decreases markedly from 0.008 to 0.0056 nm as the temperature of orthorhombic single crystals of $\mathrm{Y}{\mathrm{Ba}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{7-\delta }$ is lowered from 300 to 220 K. The rate of this decrease slows at lower temperatures, with $u_1$ reaching a value of 0.0042 nm at 100 K. Further cooling of the sample across the superconducting transition region (to 85 K) produces an abrupt, additional decrease in $u_1$ to 0.0035 nm. This abrupt lattice stiffening corresponds to an ∼140 K increase in Debye temperature across the superconducting transition.