The Distribution of the Energy Gap and Josephson IcRn Product in Bi2212 by Tunneling Spectroscopy

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We present direct measurements of the density of states by tunneling spectroscopy on slightly overdoped Bi2212 single crystals at low temperature using break-junction and point-contact techniques. We find that (i) the variation of the gap magnitude, Delta, between 20 and 36 meV is likely to be intrinsic to the 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 Delta. The maximum IcRn value of 26 mV is observed at Delta = 20.5 meV. For Delta = 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 can not 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.