Fermi-surface topology and field-dependent effective masses in the skuttuderite PrOs$_4$As$_{12}$*

Comprehensive magnetic-field-orientation dependent studies of the de Haas-van Alphen effect have been carried out on single crystals of the skuttuderites PrOs$_4$As$_{12}$ and LaOs$_4$Sb$_{12}$ using fields of up to 60 T. The Fermi- surface topologies of the two compounds are found to be very similar; in addition, they are in reasonable agreement with the predictions of bandstructure calculations for LaOs$_4$Sb$_{12}$. However, whilst the quasiparticle effective masses in LaOs$_4$Sb$_{12}$ are field- independent, those in PrOs$_4$As$_{12}$ are found to decrease with increasing field, possibly reflecting the gradual suppression of magnetic fluctuations associated with proximity to the low-temperature, low-field antiferromagnetic state.