We investigate in-plane quasiparticle tunneling across thin film grain boundary junctions (GBJs) of the electron-doped cuprate La2−xCexCuO4 in magnetic fields up to B=16T, perpendicular to the CuO2 layers. The differential conductance in the superconducting state shows a zero bias conductance peak (ZBCP) due to zero energy surface Andreev bound states. With increasing temperature T, the ZBCP vanishes at the critical temperature Tc≈29K if B=0, and at T=12K for B=16 T. As the ZBCP is related to the macroscopic phase coherence of the superconducting state, we argue that the disappearance of the ZBCP at a field BZBCP(T) must occur below the upper critical field Bc2(T) of the superconductor. We find BZBCP(0)≈25T which is at least a factor of 2.5 higher than previous estimates of Bc2(0).
@article{arxiv.0709.2853,
title = {Andreev bound states at a cuprate grain boundary junction: A lower bound for the upper critical field},
author = {M. Wagenknecht and D. Koelle and R. Kleiner and S. Graser and N. Schopohl and B. Chesca and A. Tsukada and S. T. B. Goennenwein and R. Gross},
journal= {arXiv preprint arXiv:0709.2853},
year = {2008}
}