Temperature-dependent tunneling spectra ofBi2Sr2CaCu2O8single crystals with well-definedBi2Sr2CuO6epitaxial layers

The temperature dependence of the tunneling conductance G(V,T) along the c axis of ${\mathrm{Bi}}_2$ ${\mathrm{Sr}}_2$ ${\mathrm{CaCu}}_2$ ${\mathrm{O}}_8$ single crystals has been investigated using epitaxial junctions of the type Au/${\mathrm{Bi}}_2$ ${\mathrm{Sr}}_2$ ${\mathrm{CuO}}_6$ thin-film layer/${\mathrm{Bi}}_2$ ${\mathrm{Sr}}_2$ ${\mathrm{CaCu}}_2$ ${\mathrm{O}}_8$ single crystal. A linear background conductance is observed, which appears to originate from a normal density-of-states effect. The background can be adequately described by a simple first-order relation using only one fitting parameter. The shape of the superconducting gap agrees well with a convolution of this linear background and a BCS-like density of states that includes Andreev reflection. The fits yield values for the ratio 2Δ (T=11 K)/${\mathit{k}}_{\mathit{B}}$ ${\mathit{T}}_{\mathit{c}}$ (84 K)=5.4–6.0 and a very small lifetime width Γ (T=11 K)s-like character for the superconducting pairing function along the c axis in ${\mathrm{Bi}}_2$ ${\mathrm{Sr}}_2$ ${\mathrm{CaCu}}_2$ ${\mathrm{O}}_8$.