Observation of Andreev bound states in bicrystal grain-boundary Josephson junctions of the electron doped superconductor LaCeCuO

摘要

We observe a zero-bias conductance peak (ZBCP) in the ab-plane quasiparticletunneling spectra of thin film grain-boundary Josephson junctions made of theelectron doped cuprate superconductor LaCeCuO. An applied magnetic fieldreduces the spectral weight around zero energy and shifts it non-linearly tohigher energies consistent with a Doppler shift of the Andreev bound states(ABS) energy. For all magnetic fields the ZBCP appears simultaneously with theonset of superconductivity. These observations strongly suggest that the ZBCPresults from the formation of ABS at the junction interfaces, and,consequently, that there is a sign change in the symmetry of thesuperconducting order parameter of this compound consistent with a d-wavesymmetry.