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 G_s(V,T) structures imply a large gap, approximately Δ₀(0)=26 meV, or 2Δ₀(0)/k_BT_c=6.7. An analysis of G(V,T) near T_c, assuming a lifetime-broadened BCS N_s(E), reveals a broadening Γ(T) such that Δ(T_c)≊Γ(T_c)≊2k_BT_c. The fitted Δ(T_c), unlike typical BCS values, is ≊0.8Δ(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 T_c. 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.