Increase of superfluid density with growth of quasiparticle density of states probed by intrinsic tunneling spectroscopy in Bi_{1.9}Pb_{0.1}Sr_{2}CaCu_{2}O_{8+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+δ} (BiPb2212) in the underdoped region using a small mesa structure (S=1×1 μm2) and a short-pulse technique. J_{c} was significantly enhanced when Bi was partially replaced with Pb. By comparing Jc 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 Δ. Based on a model that accounts for the inhomogeneity of Δ in k space, we argue that the superfluid density increases because Pb substitution increases the homogeneity of Δ(k).