Crystal fields, magnetoresistance, and superconductivity of Pr$_{1-x}$La$_{x}$Os$_{4}$Sb$_{12}$*

Investigation of Pr$_{1-x}$La$_{x}$Os$_{4}$Sb$_{12}$ in strong magnetic fields implies that crystalline electric field (CEF) energies of Pr are unchanged to at least x=0.2. CEF energies for x=0.4 are approximately 20 {\%} larger than for x=0.2 and increase further for x=0.67. Specific heat discontinuity at T$_{c}$ and the upeer critical field slope at T$_{c}$ indicate that the strongest suppression of m* takes place between x=0 and x$_{cr}\sim $0.3. High accuracy specific heat data obtained on a large crystal of x=0.67 exhibit significant deviations with respect to the Schottky specific heat corresponding to singlet-triplet excitations. Similar deviations are seen for other crystals with x$>$x$_{cr}$. On the other hand, magnetoresistance of moderately and strongly dilute alloys is consistent with predictions for a singlet-triplet CEF model. Correlations between these measurements of CEF's and superconductivity of Pr$_{1-x}$La$_{x}$Os$_{4}$Sb$_{12}$ will be discussed. Also, evidences against and for homogeneous coexistence of two superconducting transitions in the specific heat of pure PrOs$_{4}$Sb$_{12}$ will be presented and discussed.