Effects of lead doping on T_c and energy gap of Bi₂Sr₂CaCu₂O₈ by tunneling spectroscopy

Doping the high-T_c superconductor Bi₂Sr₂CaCu₂O₈ (Bi-2212) with lead causes a partial substitution of the Bi atoms between the conducting CuO₂ planes, and modifies some of the electronic and structural properties of the material. We have studied Pb-doped Bi-2212 single crystals using a tunneling technique. We find that while Pb doping decreases T_c significantly, it has a minimal effect on the in-plane energy gap, leading to an increase in the reduced gap, 2Δ/kT_c. This result is consistent with other studies of the effects of in-plane dopants on Bi-2212, but is the first evidence of such an effect attributable to defects introduced out of the plane. Inspection of the physical evidence leads to the conclusion that the inter-layer coupling has been modified and indicates a connection between high-T_c superconductivity and the inter-layer coupling between the CuO₂ planes.