Search for heavy quasiparticles in the resistivity ofPrOs4Sb12in magnetic fields: Comparison withPr0.7La0.3Os4Sb12

Abstract

We study the resistivity $\ensuremath{\rho}(T)$ of ${\text{PrOs}}_{4}{\text{Sb}}_{12}$ and ${\text{Pr}}_{0.7}{\text{La}}_{0.3}{\text{Os}}_{4}{\text{Sb}}_{12}$ in magnetic fields between 2.7 and 16 T and at temperatures $20\text{ }\text{mK}\ensuremath{\le}T\ensuremath{\le}1\text{ }\text{K}$. ${\text{PrOs}}_{4}{\text{Sb}}_{12}$ is known to exhibit field-induced antiferroquadrupolar (AFQ) order while there is no evidence for such order in the specific heat of ${\text{Pr}}_{0.7}{\text{La}}_{0.3}{\text{Os}}_{4}{\text{Sb}}_{12}$. We find the resistivity of ${\text{Pr}}_{0.7}{\text{La}}_{0.3}{\text{Os}}_{4}{\text{Sb}}_{12}$ to be consistent with the accepted crystalline electric field (CEF) model. The model predicts a low-temperature plateau in $\ensuremath{\rho}(T)$ in fields smaller or larger than the CEF level-crossing field of approximately 9 T, as is also observed in ${\text{PrOs}}_{4}{\text{Sb}}_{12}$. However, further analysis suggests that for ${\text{PrOs}}_{4}{\text{Sb}}_{12}$ 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 ${m}^{\ensuremath{\ast}}$ or to the existence of two resistivity contributions, one increasing and the other decreasing with temperature. In fields between 4 and 6 T, $\ensuremath{\rho}(T)$ shows a shallow minimum. A Kadowaki-Woods analysis of $\ensuremath{\rho}(T)$ of ${\text{PrOs}}_{4}{\text{Sb}}_{12}$ over a restricted temperature range implies that ${m}^{\ensuremath{\ast}}$ depends strongly on magnetic field, increasing when the field approaches the boundary of the AFQ region. However, for all fields investigated, ${\ensuremath{\partial}}^{2}\ensuremath{\rho}/\ensuremath{\partial}{T}^{2}$ for $T\ensuremath{\rightarrow}0$ 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.