The absorptivity of the heavy-fermion compound UPt3 is measured from 2 to 1000 cm-1 (0.25"124 meV) at temperatures between 1.2 K and room temperature. Above 50 cm-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, ≫(), with ≫(0)=65. The data indicate a bare free-carrier plasma frequency of 2.1×104 cm-1 (2.6 eV). Combining these results with the measured specific heat for UPt3 gives for the low-frequency effective mass m*=240m, and for the optical band mass mb=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 2 dependence, while the measured dc resistivity has a T2 dependence at temperatures below 2 K. To account for the far-infrared data, the carrier scattering rate "(,T) can be written as "(,T)1/42+(pT)2, with an experimental upper limit of p=1. This is not consistent with electron-electron scattering, for which p=2. © 1988 The American Physical Society.
