Andreev Bound States in High Temperature Superconductors

TL;DR

This study observes Andreev bound states in high-temperature superconductors using tunneling spectroscopy, revealing zero bias conductance peaks in d-wave materials and their magnetic field dependence, advancing understanding of surface states in HTS.

Contribution

It provides experimental evidence of Andreev bound states in various high-temperature superconductors, highlighting their dependence on pairing symmetry and magnetic field effects.

Findings

Zero bias conductance peaks observed in YBCO, BSCCO, LSCO

No zero bias peak in NCCO, indicating different pairing symmetry

Magnetic field causes nonlinear shift of spectral weight from zero to finite energy

Abstract

Andreev bound states (ABS) at the surface of superconductors are expected for any pair potential showing a sign change in different k-directions with their spectral weight depending on the relative orientation of the surface and the pair potential. We report on the observation of ABS in HTS employing tunneling spectroscopy on bicrystal grain boundary Josephson junctions (GBJs). The tunneling spectra were studied as a function of temperature and applied magnetic field. The tunneling spectra of GBJ formed by YBCO, BSCCO, and LSCO show a pronounced zero bias conductance peak that can be interpreted in terms of Andreev bound states at zero energy that are expected at the surface of HTS having a d-wave symmetry of the order parameter. In contrast, for the most likely s-wave HTS NCCO no zero bias conductance peak was observed. Applying a magnetic field results in a shift of spectral weight from zero to finite energy. This shift is found to depend nonlinearly on the applied magnetic field. Further consequences of the Andreev bound states are discussed and experimental evidence for anomalous Meissner currents is presented.