Anomalous temperature dependence of the London penetration depth in hole doped high-T_c superconductors

摘要

The temperature dependence of the in-plane London penetration depth #lambda#_(ab)(T) has been determined with resolution of less than 1 nm by measuring the magnetic field dependence of the maximum Josephason current of high-temperature superconductor (HTS) Josephson junctions. We used YBa_2Cu_3O_(7-#delta#) (YBCO) bicrystal grain boundary Josephson junctions (GBJs) with different misorientation angles to measure the relative change triangle open#lambda#_(ab)(T)=#lambda#_(ab)(T)-#lambda#_(ab)(0) of the London penetration depth of hole doped HTS materials. For small misorientation angles the measured triangle open#lambda#_(ab)(T) was found in good agreement with the theoretical prediction for d-wave symmetry of the superconducting order parameter. In particular, a linear temperature dependence of trangle open#lambda#_(ab)(T) was found at low temperatures. However, for large misorientation angles deviations from this linear behavior were observed at T < 10 K, indicating an increase of #lambda#_(ab)(T) with decreasing temperature. This increase can be explained in terms of Andreev bound states at the grain boundary interface and anomalous Meissner currents resulting from a Doppler shift of these states in the presence of an applied magnetic field.