Anomalous pressure dependences of the superconducting transition temperature of dilute alloys of LaSn₃ containing light rare earth impurities

The initial depressions of the superconducting transition temperature T_c of LaSn₃ by rare earth (RE) impurities are known to be anomalous. In an attempt to clarify this behavior, we have measured the pressure dependence of the T_c of (LaRE)Sn₃ alloys (RE = Y, Ce, Pr, Nd, Sm, Eu, Gd, and Lu) to pressures up to 25 kbar. The magnitude of the pressure dependence of the T_c of (LaRE)Sn₃ alloys containing RE impurities with partially filled 4 f electron shells decreases monotonically with increasing RE atomic number. A general discussion of existing zero- and high-pressure data for various physical properties of (LaRE)Sn₃ alloys demonstrates that the association of large pressure dependences of T_c with the occurrence of unstable impurity 4 f shells (the Kondo effect) should be made with caution. The experimental high-pressure results are analyzed in terms of appropriate theoretical models for magnetic and nonmagnetic localized impurity states in superconductors.