Specific-heat anomaly near Tc of 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 ${\mathit{T}}_{\mathit{c}}$ (\ensuremath{\Delta}${\mathit{C}}_{\mathit{p}}$/${\mathit{T}}_{\mathit{c}}$=16 mJ ${\mathrm{mol}}^{\mathrm{\ensuremath{-}}1}$ ${\mathrm{K}}^{\mathrm{\ensuremath{-}}2}$) was first detected, from which the electronic specific-heat coefficient was calculated through the BCS relationship, i.e., 11 mJ ${\mathrm{mol}}^{\mathrm{\ensuremath{-}}1}$ ${\mathrm{K}}^{\mathrm{\ensuremath{-}}2}$, corresponding to the density of states N(0)=0.8 states ${\mathrm{eV}}^{\mathrm{\ensuremath{-}}1}$ (Cu atom${)}^{\mathrm{\ensuremath{-}}1}$ ${\mathrm{spin}}^{\mathrm{\ensuremath{-}}1}$. This N(0) value was rather smaller than that of (${\mathrm{La}}_{1\mathrm{\ensuremath{-}}\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 \ensuremath{\xi}(0)=13 i/rA\r{} in the plane.