Observation of Bound Surface States in Grain Boundary Junctions of High Temperature Superconductors

TL;DR

This study investigates tunneling spectra in grain boundary junctions of high-temperature superconductors, revealing d-wave symmetry with zero energy bound states in most materials, and s-wave symmetry in NCCO, supported by magnetic field effects.

Contribution

It provides experimental evidence of surface bound states in HTS grain boundary junctions and distinguishes pairing symmetries across different materials.

Findings

Zero bias conductance peak observed in YBCO, BSCCO, LSCO; absent in NCCO.

Peak diminishes with temperature and disappears at Tc.

Magnetic field shifts spectral weight, aligning with theoretical models.

Abstract

We have performed a detailed study of the tunneling spectra of bicrystal grain boundary junctions (GBJs) fabricated from the HTS YBCO, BSCCO, LSCO, and NCCO. In all experiments the tunneling direction was along the CuO planes. With the exception of NCCO, for all materials a pronounced zero bias conductance peak was observed which decreases with increasing temperature and disappears at the critical temperature. These results can be explained by the presence of a dominating d-wave symmetry of the order parameter resulting in the formation of zero energy Andreev bound states at surfaces and interfaces of HTS. The absence of a ZBCP for NCCO is consistent with a dominating s-wave symmetry of the pair potential in this material. The observed nonlinear shift of spectral weight to finite energies by applying a magnetic field is in qualitative agreement with recent theoretical predictions.