Far-infrared absorptivity ofUPt3

The absorptivity of the heavy-fermion compound ${\mathrm{UPt}}_3$ is measured from 2 to 1000 ${\mathrm{cm}}^{\mathrm{-}1}$ (0.25–124 meV) at temperatures between 1.2 K and room temperature. Above 50 ${\mathrm{cm}}^{\mathrm{-}1}$ (6.2 meV) the absorptivity is relatively temperature independent, while below that frequency the absorptivity is very temperature dependent, in accord with the dc resistivity. By performing a Kramers-Kronig transformation of the data, augmented with recently published results at higher frequencies, the complex conductivity is obtained. The low-temperature conductivity may be characterized by free carriers which undergo frequency-dependent scattering and have a concomitant frequency-dependent mass enhancement, λ(ω), with λ(0)=65. The data indicate a bare free-carrier plasma frequency of 2.1×${10}^4$ ${\mathrm{cm}}^{\mathrm{-}1}$ (2.6 eV). Combining these results with the measured specific heat for ${\mathrm{UPt}}_3$ gives for the low-frequency effective mass $m^{\mathrm{*}}$=240m, and for the optical band mass $m_b$=3.7m. The carrier density is close to one electron per formula unit. The far-infrared absorptivity measurement indicates that the scattering rate begins to rise with an ${\omega }^2$ dependence, while the measured dc resistivity has a $T^2$ dependence at temperatures below 2 K. To account for the far-infrared data, the carrier scattering rate Γ(ω,T) can be written as Γ(ω,T)∼${\omega }^2$+(pπT${)}^2$, with an experimental upper limit of p=1. This is not consistent with electron-electron scattering, for which p=2.