Search for heavy quasiparticles in the resistivity of PrOs4Sb12 in magnetic fields: Comparison with Pr0.7La0.3Os4Sb12

We study the resistivity T of PrOs4Sb12 and Pr0.7La0.3Os4Sb12 in magnetic fields between 2.7 and 16 T and at temperatures 20 mK T 1 K. PrOs4Sb12 is known to exhibit field-induced antiferroquadrupolar AFQ order while there is no evidence for such order in the specific heat of Pr0.7La0.3Os4Sb12. We find the resistivity of Pr0.7La0.3Os4Sb12 to be consistent with the accepted crystalline electric field CEF model. The model predicts a low-temperature plateau in T in fields smaller or larger than the CEF level-crossing field of approximately 9 T, as is also observed in PrOs4Sb12. However, further analysis suggests that for PrOs4Sb12 in fields below 5 T, this plateau is not a CEF effect but is rather due either to a crossover to a state characterized by a small electronic effective mass m or to the existence of two resistivity contributions, one increasing and the other decreasing with temperature. In fields between 4 and 6 T, T shows a shallow minimum. A Kadowaki-Woods analysis of T of PrOs4Sb12 over a restricted temperature range implies that m depends strongly on magnetic field, increasing when the field approaches the boundary of the AFQ region. However, for all fields investigated, 2 / T2 for T→0 remains small compared with its value in canonical heavy fermions. In the AFQ regime only , the resistivity has different temperature dependences in fields parallel and perpendicular to the principal crystal direction of current flow, suggesting an increase in the orbital contribution to the resistivity in the ordered phase.