Phase diagram of the electron-doped La(2-x)Ce(x)CuO(4) cuprate superconductor from Andreev bound states at grain boundary junctions

We use quasiparticle tunneling across La2 xCexCuO4 grain boundary junctions to probe the superconducting state and its disappearance with increasing temperature and magnetic field. A zero bias conductance peak due to zero energy surface Andreev bound states is a clear signature of the phase coherence of the superconducting state. Hence, such a peak must disappear at or below the upper critical field Bc2 T . For La2 xCexCuO4 this approach sets a lower bound for Bc2 0 25 T which is substantially higher than reported previously. The method of probing the superconducting state via Andreev bound states should also be applicable to other cuprate superconductors.