Precision measurements of the temperature dependence of λ in YBa2Cu3O6.95: 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 T_c in very high quality single crystals of YBa₂Cu₃O_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.