Abstract
We observed a rare-earth ion-size effect on T c in R 1-x-y Pr x Ca y Ba 2 Cu 3 O 7-z (R = Er, Dy, Gd, Eu, Sm, and Nd) systems which is similar to that in R 1-x Pr x Ba 2 Cu 3 O 7-z systems in our previous reports. For fixed Pr and Ca concentration (fixed x and y), T c is linearly dependent on rare-earth ion radius r R 3+ . For a fixed Pr concentration x, there exists a maximum of T c (T c,max ) in T c vs Ca concentration y curves. T c,max shifts to higher Ca concentration region for samples with larger R ion radius. The enhancement of T c (ΔT c,max =T c,max -T c,y=0 ; T c,y=0 is the transition temperature T c without Ca doping) increases with increasing R ion radius. We proposed an empirical formula for T c (r R3+ ,x,y) to fit our experimental data: T c (x,y) = T c0 -Aβ 2 (α/x+y/β) 2 -Bx. All fitting parameters in this formula, T c0 , B, β, α/β, and Aβ 2 , are rare-earth ion-size dependent. © 1999 The American Physical Society.