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)approximate to 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.