Superconductivity, heavy fermion behavior, and crystalline electric field effects in the filled skutterudite series

A thorough investigation of the Pr(Os1- xRux)4Sb12 series of compounds is presented. The compound PrOs4Sb12 is the first known Pr-based heavy fermion superconductor, with an electronic specific heat coefficient gamma ∼ 500 mJ/mol K2 and a superconducting transition temperature Tc ≈ 1.8 K. PrOs4Sb12 displays unconventional superconductivity which may be driven by quadrupolar fluctuations, and also exhibits a unique phase in high magnetic fields. Multiple superconducting transitions, structure in the energy gap, and time reversal symmetry breaking have all been observed in the superconducting state of PrOs4Sb12. In contrast, PrRu4Sb12 is a more conventional superconductor with Tc ≈ 1.0 K. Substitution of Ru for Os to form Pr(Os 1-xRux) 4Sb12 has resulted in several very interesting findings.; Crystalline electric field (CEF) effects play a strong and important role in the physical properties of PrOs4Sb12. In Chapter III, we present CEF fits to zero-field electrical resistivity which, when extrapolated to higher magnetic fields, display a similar shape to that of the measured rho(H) data.; The Pr(Os1-xRu x)4Sb12 system has been extensively studied. In Chapter IV, we present x-ray, magnetic susceptibility, electrical resistivity, and specific heat data for the Pr(Os1-xRu x)4Sb12 series of compounds. A minimum in Tc is observed near x = 0.6, while the CEF splitting between the ground state and first excited state is found to increase monotonically between the end member compounds. The electronic specific heat coefficient gamma increases to a maximum near x ≈ 0.02, after which it drops rapidly and stabilizes at a more conventional value above x = 0.6. The jump in specific heat at the superconducting transition DeltaC/Tc, however, drops abruptly with Ru doping and stabilizes by x = 0.1. Structure in the specific heat jump at Tc appears to persist up to x ≈ 0.04, although fits to C(T) below Tc may indicate a change in the superconducting energy gap upon any Ru substitution.