First finding of reentrant superconductivity driven by hyperfine interaction

Compared to magnetically doped superconductors described by the theory of Abrikosov and Gorkov, electronic singlet ground state systems can have a much larger critical concentration of magnetic impurities, following the model of Keller and Fulde. The recent study of the superconducting Van Vleck paramagnet La_{1-x}Pr_xTe revealed a critical Pr^{3+} concentration x close above 0.50. Surprisingly, in La_{0.50}Pr_{0.50}Te the superconducting state with T_c = 0.20 K appeares not to be stable down to zero temperature. Instead, a reentrant transition to the normal state likely caused by the hyperfine enhanced magnetic moments of the ^{141}Pr nuclei occurs at about 0.02 K. Although these moments are not in a magnetically ordered ground state at T = 0.02 K, their contribution to Cooper pair breaking seems to be even stronger than of ferromagnetically ordered but non enhanced nuclear moments in type-I superconducting AuIn₂.