Phase diagram of the electron-doped La2-xCexCuO4 cuprate superconductor from Andreev bound states at grain boundary junctions.

We use quasiparticle tunneling across La 2-x Ce x CuO 4 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 B c2 (T). For La 2-x Ce x CUO 4 this approach sets a lower bound for B c2 (0) as 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.