Increase of superfluid density with growth of quasiparticle density of states probed by intrinsic tunneling spectroscopy in Bi1.9Pb0.1Sr2CaCu2O8+δ

Abstract

Interlayer tunneling spectra, the Josephson critical current density ${J}_{c}$, and the normal tunneling resistance have been measured simultaneously for intrinsic Josephson junctions of Bi${}_{1.9}$Pb${}_{0.1}$Sr${}_{2}$CaCu${}_{2}$O${}_{8+\ensuremath{\delta}}$ (BiPb2212) in the underdoped region using a small mesa structure ($S=1\ifmmode\times\else\texttimes\fi{}1\phantom{\rule{0.28em}{0ex}}{\ensuremath{\mu}\mathrm{m}}^{2}$) and a short-pulse technique. ${J}_{c}$ was significantly enhanced when Bi was partially replaced with Pb. By comparing ${J}_{c}$ with the value estimated by the Ambegaokar-Baratoff formula, it was found that superfluid density doubles in the Pb-containing sample. In addition, Pb raises the superconducting coherence peak without changing the value of the superconducting gap $\ensuremath{\Delta}$. Based on a model that accounts for the inhomogeneity of $\ensuremath{\Delta}$ in $\mathbit{k}$ space, we argue that the superfluid density increases because Pb substitution increases the homogeneity of $\ensuremath{\Delta}(\mathbit{k})$.