| PrOs4Sb12 is the first discovered Pr-based heavy fermion metal and superconductor. Our high magnetic field specific heat measurements provided clear evidence for the non-magnetic singlet crystalline electric field (CEF) ground state. This CEF ground state precludes the conventional Kondo effect as the origin of the heavy fermion behavior. The superconductivity in PrOs4Sb12 is unconventional, as inferred from the double superconducting transition in the specific heat. Pr 1-xLaxOs 4Sb12 (0≤x≤1) crystals were synthesized and investigated in order to provide additional evidences for a postulated CEF configuration, to discriminate between different conduction electron mass enhancement (m*) mechanisms proposed, and to provide insight into the nature of the superconductivity. Lanthanum doping induces anomalously small increase of the lattice constant. The specific heat results in high magnetic fields indicated that CEF scheme is unaltered between x=0 and at least 0.2, followed by an abrupt (but small) change somewhere between x=0.2 and 0.4. Magnetoresistance measurements on La-doped samples were consistent with a singlet CEF ground state of Pr. Investigation of the specific heat discontinuity at Tc and of the upper critical field slope at Tc indicated that the electronic effective mass, m*, is strongly reduced with x, between x=0 and xcr≈0.2--0.3, followed by a weak dependence on x for x> xcr. Therefore, we have postulated that single-impurity type models cannot account for the heavy fermion behavior of PrOs4Sb 12. Investigation of the magnetic phase diagram and magnetoresistance provided strong correlations between a closeness to the long-range order (antiferroquadrupolar type) and m*, suggesting a possibility of fluctuations of the antiferroquadrupolar order parameter responsible for m* enhancement. Lanthanum has very weak effect on the superconducting transition temperature in a stark contrast to other known heavy fermion superconductors. The study of superconductivity provided constraints on proposed theoretical models, including the two band model. |
