Precision measurements of the temperature dependence of λ in ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{6.95}$: Strong evidence for nodes in the gap function

A miniature superconducting resonator operating at 1.3 K and 900 MHz has been used to measure the change in λ(T) from 1.3 K to ${\mathit{T}}_{\mathit{c}}$ in very high quality single crystals of ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{6.95}$. The data, which have a resolution of 1-2 Å, show a strong linear term extending from approximately 3 to 25 K. We believe the strong linear dependence to be characteristic of the pure system and that its apparent absence in thin films and some crystals is due to the presence of defects.