Fermi-surface geometry and pressure effects on the spin-fluctuation contributions to the specific heat: Anisotropic spin-fluctuation model for heavy-fermion UPt3.

Abstract

On the basis of an uniaxially anisotropic and nonparabolic band model, the effects of Fermi-surface (FS) anisotropy and topology and of hydrostatic pressure on the spin-fluctuation (SF) contributions to the low-temperature specific heat of exchange-enhanced paramagnetic metals are investigated in the framework of paramagnon theory. The model describes different types of SF; for the closed FS a parameter region is found in which there is a coexistence of anisotropic ferromagnetic and antiferrromagnetic SF. In this region and for the open FS a linear relation between the SF-mediated mass enhancement and the Stoner enhancement factor can be obtained which results in the pressure-dependent scaling of the SF parameters and the magnetic susceptibility in terms of only one parameter. For either FS topology the ${T}^{3}$ lnT law holds, where the amplitude is enlarged compared with the parabolic band model and the range of validity increases with FS anisotropy. Comparing theory with experiments, an anisotropic SF model for ${\mathrm{UPt}}_{3}$ is proposed which describes the coexistence of antiferromagnetic SF along the hexagonal axis and ferromagnetic SF in the basal planes and may explain the specific heat, magnetic susceptibility, and neutron scattering experiments.