Optical investigations of the heavy-fermion superconducto

The optical properties of the heavy-fermion superconductor (Formula presented)(Formula presented) have been investigated at temperatures between 10 and 300 K using reflectance spectroscopy. A characteristic energy scale ((Formula presented)=90 (Formula presented)), with almost the same value as the coherence temperature (Formula presented)=100 K derived from measurements of the dc resistivity and susceptibility, is obtained from the optical conductivity. At high temperatures (T≥(Formula presented)), this scale represents the energy gap between the ground and excited level that results from the crystal-field splitting of the 5(Formula presented) (J=4) level of the tetravalent uranium ion. In the low-temperature coherent region (T<(Formula presented)), 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 (Formula presented) and plasma frequency (Formula presented) of the quasiparticle mode, as well as the quasiparticle bandwidth W at 10 K are derived using the model developed by Millis and Lee.