The Distribution of the Energy Gap and Josephson IcRn Product in Bi2Sr2CaCu2O8+x by Tunneling Spectroscopy

We present direct measurements of the density of states by tunneling spectroscopy on slightly overdoped Bi2Sr2CaCu2O8+x (Bi2212) single crystals at low temperature using break-junction and point-contact techniques. We find that (i) the variation of the gap magnitude, Δ, between 20 and 36 meV is likely to be intrinsic to Bi2212, and (ii) there is a correlation between the maximum value of the Josephson IcRn product and the gap magnitude: IcRn decreases with the increase of Δ. The maximum IcRn value of 26 mV is observed at Δ = 20.5 meV. For Δ = 36.5 meV, the maximum measured value of IcRn is 7.3 mV. We conclude that (i) the distribution of the Josephson IcRn product as a function of gap magnitude cannot be explained by the presence of a single energy gap in Bi2212, and (ii) the coherence energy scale in Bi2212 has the maximum Josephson strength.