Superconducting gap and pseudogap for overdopedBi2−xPbxSr2CaCu2O8+δusing60nstime-scale short-pulse interlayer tunneling spectroscopy

Abstract

We have measured the interlayer tunneling spectra for overdoped ${\mathrm{Bi}}_{2\ensuremath{-}x}{\mathrm{Pb}}_{x}{\mathrm{Sr}}_{2}\mathrm{Ca}{\mathrm{Cu}}_{2}{\mathrm{O}}_{8+\ensuremath{\delta}}$ single crystals with a doping level $p$ ranging from 0.20 to 0.22 and a ${T}_{c}$ from $80\phantom{\rule{0.3em}{0ex}}\text{to}\phantom{\rule{0.3em}{0ex}}67\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ using a small mesa structure and a $60\phantom{\rule{0.3em}{0ex}}\mathrm{ns}$ time-scale short-pulse technique. It is found that, with increasing doping, the superconducting gap decreases from $46\phantom{\rule{0.3em}{0ex}}\text{to}\phantom{\rule{0.3em}{0ex}}18\phantom{\rule{0.3em}{0ex}}\mathrm{meV}$, while the pseudogap decreases from $42\phantom{\rule{0.3em}{0ex}}\text{to}\phantom{\rule{0.3em}{0ex}}10\phantom{\rule{0.3em}{0ex}}\mathrm{meV}$. The result indicates the existence of the pseudogap in the overdoped region even at a doping level $p$ higher than 0.19, at which the pseudogap is expected to disappear in some generic phase diagram models. Furthermore, the values obtained for the superconducting gap, normal tunneling resistance, and the maximum Josephson current indicate the likeliness of the inhomogeneous superconducting state even in the overdoped region.