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 DELTA-0(0) = 26 meV, or 2-DELTA-0(0)/k(B)T(c) = 6.7. An analysis of G(V,T) near T(c), assuming a lifetime-broadened BCS N(s)(E), reveals a broadening GAMMA(T) such that DELTA(T(c) almost-equal-to (T(c)) almost-equal-to 2k(B)T(c). The fitted DELTA(T(c)), unlike typical BCS values, is almost-equal-to 0.8-DELTA(0). G(V,T > T(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.