Low-temperature anomaly in the penetration depth of YBa2Cu3O7 films:: Evidence for Andreev bound states at surfaces

We report the observation of anomalous Meissner currents in thin films of superconducting YBCO with oriented internal surfaces introduced by heavy-ion bombardment. High-precision measurements of the penetration depth (lambda) reveal an upturn in the temperature dependence of lambda below T-x approximate to 15 K. The magnitude of the observed effect, its onset temperature, and its dependence on the orientation of the surfaces are in quantitative agreement with the theory of surface effects in d-wave superconductors. The anomaly is interpreted to arise from surface currents that are carried by surface-induced Andreev bound states.