Magnetic susceptibility in the normal state: A tool to optimizeTcwithin a given superconducting oxide system

We compare the static magnetic susceptibilities of ${\mathrm{La}}_{2\mathrm{-}\mathit{x}}$ ${\mathrm{Sr}}_{\mathit{x}}$ ${\mathrm{CuO}}_4$, ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{6+\mathit{y}}$, ${\mathrm{Bi}}_2$ ${\mathrm{Sr}}_{1.6}$ ${\mathrm{La}}_{0.4}$ ${\mathrm{CuO}}_{6+\mathit{y}}$, ${\mathrm{Bi}}_2$ ${\mathrm{Sr}}_2$ ${\mathrm{CaCu}}_2$ ${\mathrm{O}}_{8+\mathit{y}}$, ${\mathrm{Tl}}_2$ ${\mathrm{Ba}}_2$ ${\mathrm{CuO}}_{6+\mathit{y}}$, ${\mathrm{Tl}}_2$ ${\mathrm{Ba}}_2$ ${\mathrm{CaCu}}_2$ ${\mathrm{O}}_{8+\mathit{y}}$, and ${\mathrm{Tl}}_2$ ${\mathrm{Ba}}_2$ ${\mathrm{Ca}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{10+\mathit{y}}$ as a function of the oxygen content. The main results are that (1) the spin susceptibility in the normal state ${\mathrm{\chi }}_{\mathit{s}}$ is primarily determined by the ${\mathrm{CuO}}_2$ layers, (2) the Meissner fraction increases markedly with the transition temperature ${\mathit{T}}_{\mathit{c}}$, and (3) for samples with the optimum value of ${\mathit{T}}_{\mathit{c}}$, ${\mathrm{\chi }}_{\mathit{s}}$ correlates linearly with the calculated band-structure density of states at the Fermi energy. We also discuss how the value of the magnetic susceptibility in the normal state can be used to optimize ${\mathit{T}}_{\mathit{c}}$ within a given compound system.