Zero Energy States at a Normal--Cuprate-Superconductor Interface Probed by Shot Noise

We report measurements of the current noise generated in the optimally doped, x=0.15, Au-La_2-xS_xCuO₄ junctions. For high transmission junctions on (110) surface, we observed a split zero-bias conductance peak (ZBCP), accompanied by enhanced shot noise. The ZBCP disappears and the noise decreases to the noise expected for the charge e with heating at temperatures well below T_c, and at voltages much smaller than the bulk superconducting gap, setting a new energy scale of 0.5 mV. We attribute this scale to a subdominant order parameter at the sample surface, which fascilitates Cooper pair transport through a disordered normal metal-superconductor interface.

Another option could be a spontanious breaking of the translational symmetry on (110) surface, and associated with it vortex formation. Motion of the vortices can lead to an excess noise.