Field-dependent mass enhancement inPr1−xLaxOs4Sb12from aspherical Coulomb scattering

Abstract

The scattering of conduction electrons by crystalline electric field (CEF) excitations may enhance their effective quasiparticle mass similar to scattering from phonons. A well-known example is Pr metal where the isotropic exchange scattering from inelastic singlet-singlet excitations causes the mass enhancement. An analogous mechanism may be at work in the skutterudite compounds ${\text{Pr}}_{1\ensuremath{-}x}{\text{La}}_{x}{\text{Os}}_{4}{\text{Sb}}_{12}$ where close to $x=1$ the compound develops heavy quasiparticles with a large specific heat $\ensuremath{\gamma}$. There the low-lying CEF states are a singlet ground state and a triplet at $\ensuremath{\Delta}=8\text{ }\text{K}$. Due to the tetrahedral CEF the main scattering mechanism must be the aspherical Coulomb scattering. We derive the expression for mass enhancement in this model including also the case of dispersive excitations. We show that for small to moderate dispersion there is a strongly field-dependent mass enhancement due to the field-induced triplet splitting. It is suggested that this effect may be seen in ${\text{Pr}}_{1\ensuremath{-}x}{\text{La}}_{x}{\text{Os}}_{4}{\text{Sb}}_{12}$ with suitably large $x$ when the dispersion is small.