Optical investigations of the heavy-fermion superconductor UNi2Al3

The optical properties of the heavy-fermion superconductor UNi2Al3 have been investigated at temperatures between 10 and 300 K using reflectance spectroscopy. A characteristic energy scale (omega(c)=90 cm(-1)), with almost the same value as the coherence temperature T-0=100 K derived from measurements of the de resistivity and susceptibility, is obtained from the optical conductivity. At high temperatures (T greater than or equal to T-0), this scale represents the energy gap between the ground and excited level that results from the crystal-held splitting of the 5f(2) (J=4) level of the tetravalent uranium ion. In the low-temperature coherent region (T<T-0), a narrow Drude-like, quasiparticle absorption mode develops. This mode is described using a frequency-dependent scattering rate Gamma(omega) and mass enhancement factor lambda(omega). 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* and plasma frequency w(p)* of the quasiparticle mode, as well as the quasiparticle bandwidth W at 10 K are derived using the model developed by Millis and Lee.