The optical properties of the heavy-fermion superconductor ${\mathrm{UNi}}_2$${\mathrm{Al}}_3$ have been investigated at temperatures between 10 and 300 K using reflectance spectroscopy. A characteristic energy scale (${\mathrm{\omega }}_{\mathit{c}}$=90 ${\mathrm{cm}}^{\mathrm{-}1}$), with almost the same value as the coherence temperature ${\mathit{T}}_0$=100 K derived from measurements of the dc resistivity and susceptibility, is obtained from the optical conductivity. At high temperatures (T≥${\mathit{T}}_0$), this scale represents the energy gap between the ground and excited level that results from the crystal-field splitting of the 5${\mathit{f}}^2$ (J=4) level of the tetravalent uranium ion. In the low-temperature coherent region (T<${\mathit{T}}_0$), a narrow, Drude-like, quasiparticle absorption mode develops. This mode is described using a frequency-dependent scattering rate Γ(ω) and mass enhancement factor λ(ω). This free-carrier mode may originate from a hybridization between the 3d conduction band of nickel and the 5f bands of uranium. Parameters such as the renormalized scattering rate ${\gamma }^{\mathrm{*}}$ and plasma frequency ${\mathrm{\omega }}_{\mathit{P}}^{\mathrm{*}}$ of the quasiparticle mode, as well as the quasiparticle bandwidth W at 10 K are derived using the model developed by Millis and Lee. © 1996 The American Physical Society.

• Received 6 September 1995

DOI:https://doi.org/10.1103/PhysRevB.53.2601