Observation of paramagnetic Meissner currents—evidence for Andreev bound surface states

Abstract

We report the observation of anomalous Meissner currents (AMCs) in thin films of superconducting YBCO with oriented internal surfaces introduced by heavy-ion bombardment. High-precision measurements of the penetration depth λ( T) reveal an upturn in the temperature dependence of λ( T) below T≈15 K, which is strongly dependent on the orientation and density of the surfaces. 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 as a consequence of surface-induced Andreev bound states (ABSs) carrying paramagnetic currents.