Evidence for a distribution of discrete energy gaps in polycrystalline high-Tcsuperconductors

We present measurements of the differential conductance dI/dV versus V of normal-metal–high-${\mathit{T}}_{\mathit{c}}$ ceramic point contacts. We have found that the measured curves can be interpreted by considering two effects: an asymmetric tunneling barrier and a set of distinct superconducting energy gaps. We show that the BCS density of states, modified by introducing a Gaussian discrete distribution of gap values, yields a very good fit of the experimental curves.