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

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 ${\left({\mathrm{La}}_{1-x}{\mathrm{Pr}}_x\right)}_3$ 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 $x=0.90\mathrm{}$ the system ${\left({\mathrm{La}}_{1-x}{\mathrm{Pr}}_x\right)}_3$ In shows a transition to a self-polarized antiferromagnetic state at low temperatures.