Electron tunneling spectroscopy of single-crystal ${\mathrm{Bi}}_2$${\mathrm{Sr}}_2$${\mathrm{CaCu}}_2$${\mathrm{O}}_8$

The T-dependent conductance G(V,T) of evaporated Pb junctions on cleaved ab planes of ${\mathrm{Bi}}_2$ ${\mathrm{Sr}}_2$ ${\mathrm{CaCu}}_2$ ${\mathrm{O}}_8$ (2:2:1:2) crystals is presented. Strong superconducting ${\mathit{G}}_{\mathit{s}}$(V,T) structures imply a large gap, approximately ${\mathrm{\Delta }}_0$(0)=26 meV, or 2${\mathrm{\Delta }}_0$(0)/${\mathit{k}}_{\mathit{B}}$ ${\mathit{T}}_{\mathit{c}}$=6.7. An analysis of G(V,T) near ${\mathit{T}}_{\mathit{c}}$, assuming a lifetime-broadened BCS ${\mathit{N}}_{\mathit{s}}$(E), reveals a broadening Γ(T) such that Δ(${\mathit{T}}_{\mathit{c}}$)≊Γ(${\mathit{T}}_{\mathit{c}}$)≊2${\mathit{k}}_{\mathit{B}}$ ${\mathit{T}}_{\mathit{c}}$. The fitted Δ(${\mathit{T}}_{\mathit{c}}$), 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 ${\mathit{T}}_{\mathit{c}}$. 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 ${\mathrm{CuO}}_2$ planes.