Specific-heat anomaly nearTcof the (Bi,Pb)-Sr-Ca-Cu-O superconductor (Tc=107 K)

Abstract

Specific-heat measurements of the 107-K phase of the (Bi,Pb)-Sr-Ca-Cu-O system were made in the temperature range 80–250 K by an ac calorimetry method. A finite specific-heat jump at ${\mathit{T}}_{\mathit{c}}$ (Δ${\mathit{C}}_{\mathit{p}}$/${\mathit{T}}_{\mathit{c}}$=16 mJ ${\mathrm{mol}}^{\mathrm{-}1}$ ${\mathrm{K}}^{\mathrm{-}2}$) was first detected, from which the electronic specific-heat coefficient was calculated through the BCS relationship, i.e., 11 mJ ${\mathrm{mol}}^{\mathrm{-}1}$ ${\mathrm{K}}^{\mathrm{-}2}$, corresponding to the density of states N(0)=0.8 states ${\mathrm{eV}}^{\mathrm{-}1}$ (Cu atom${)}^{\mathrm{-}1}$ ${\mathrm{spin}}^{\mathrm{-}1}$. This N(0) value was rather smaller than that of (${\mathrm{La}}_{1\mathrm{-}\mathit{x}}$ ${\mathrm{M}}_{\mathit{x}}$ ${)}_2$ ${\mathrm{CuO}}_4$ (M=1/rSr,Ca) or ${\mathrm{Ba}}_2$ ${\mathrm{YCu}}_3$ ${\mathrm{O}}_7$. This is in contrast with the usually observed positive correlation between ${\mathit{T}}_{\mathit{c}}$ and N(0) among the conventional metallic superconductors. A broadened transition width and the sharpened peak at ${\mathit{T}}_{\mathit{c}}$ observed in the ${\mathit{C}}_{\mathit{p}}$–T curve was attributed to the two-dimensional superconducting fluctuation effect with coherence length ξ(0)=13 i/rÅ in the plane.