Electron tunneling spectroscopy of single-crystal Bi2Sr2CaCu2O8

The T-dependent conductance G(V,T) of evaporated Pb junctions on cleaved ab planes of Bi2Sr2CaCu2O8 (2:2:1:2) crystals is presented. Strong superconducting Gs(V,T) structures imply a large gap, approximately 0(0)=26 meV, or 20(0)/kBTc=6.7. An analysis of G(V,T) near Tc, assuming a lifetime-broadened BCS Ns(E), reveals a broadening (T) such that (Tc) (Tc) 2kBTc. The fitted (Tc), unlike typical BCS values, is 0.8(0). $G(V,T>T sub c) has a minimum at V=0, suggestive of known c-direction localization in 2:2:1:2 compounds. G(V,T) also reveals the expected fluctuation superconductivity above Tc. 4.2-K scanning-tunneling-microscope (STM) spectra are mentioned in comparison. A larger gap, about 40 meV, and dip at 80 meV, found in the STM G(V) are tentatively attributed to CuO2 planes.