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 (λ) reveal an upturn in the temperature dependence of λ below T*≈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.