Andreev bound states at a cuprate grain boundary junction: A lower bound for the upper critical field

Authors
Wagenknecht, M.
Koelle, D.
Kleiner, R.
Graser, S.
Schopohl, N.
Chesca, B.
We investigate in-plane quasiparticle tunneling across thin film grain boundary junctions (GBJs) of the electron-doped cuprate La$_{2-x}$Ce$_{x}$CuO$_4$ in magnetic fields up to $B=16$T, perpendicular to the CuO$_2$ 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 $T_c\approx29$K if B=0, and at $T=12$K 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 $B_{ZBCP}(T)$ must occur below the upper critical field $B_{c2}(T)$ of the superconductor. We find $B_{ZBCP}(0) \approx 25$T which is at least a factor of 2.5 higher than previous estimates of $B_{c2}(0)$.