Observation of large tunneling-conductance variations in direct mapping of the energy gap of single-crystal Bi₂Sr₂CaCu₂O_8-x

The tunneling conductance G(V,x,y) is mapped with 5 Å resolution on cleaved a-b planes of Bi₂Sr₂CaCu₂O_8-x crystals (T_c=90 K) using scanning (vacuum) tunneling spectroscopy at 4.2 K. A strong, spatially varying G(V,x,y), evidently arising from a superconducting gap Δ_c, is observed on the a-b plane. A more highly peaked G(V) with reduced peak spacing is seen near (∼50 Å) specific sites, and over broader areas. It is suggested that the striking variations in G(V,x,y) probably indicate local variations in BiO-layer metallicity, revealing an internal superconducting proximity effect between BiO and CuO₂ layers.