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

Abstract

We report measurements of the current noise generated by the optimally doped, x=0.15, ${\rm Au- La_{2-x}Sr_xCuO_4}$ junctions. For high transmission junctions on (110) surface, we observed split zero-bias conductance peak (ZBCP), accompanied by enhanced shot noise. We attribute the enhanced noise to the Cooper pair transport through the junction. The ZBCP disappears and the noise decreases to the one expected for the charge $e$ with heating at temperatures well below ${\rm T_c}$, and at voltages much smaller than the bulk superconducting gap, setting a new energy scale of 0.5 mV. We attribute this scale to the existence of an $id_{xy}$ or $is$ order parameter at the sample surface.