Thermodynamic properties of superconductors containing impurities in a crystal-field singlet ground state

Abstract

Experimental results on superconducting and magnetic properties of ${\mathrm{La}}_{3}$In and ${\mathrm{La}}_{3}$Tl containing magnetic impurities are presented and discussed. From magnetic susceptibility and low-temperature specific heat we conclude that ${\mathrm{Pr}}^{3+}$ substituted for ${\mathrm{La}}^{3+}$ in these compounds is in a crystal-field singlet ground state; this also explains the characteristic behavior of the superconducting transition temperature as a function of the Pr concentration. The upper critical field of ${({\mathrm{La}}_{1\ensuremath{-}x}{\mathrm{Pr}}_{x})}_{3}$ In is limited by the mean exchange field due to the Pr ions which are polarized in the external magnetic field. At Pr concentrations higher than $x=0.90$ the system ${({\mathrm{La}}_{1\ensuremath{-}x}{\mathrm{Pr}}_{x})}_{3}$ In shows a transition to a self-polarized antiferromagnetic state at low temperatures.