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 (vc590 cm ), with almost the same value as the coherence temperature T05100 K derived from measurements of the dc resistivity and susceptibility, is obtained from the optical conductivity. At high temperatures (T>T0), this scale represents the energy gap between the ground and excited level that results from the crystal-field splitting of the 5f 2 (J54! level of the tetravalent uranium ion. In the low-temperature coherent region (T,T0), a narrow, Drude-like, quasiparticle absorption mode develops. This mode is described using a frequency-dependent scattering rate G(v) and mass enhancement factor l(v). 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 g* and plasma frequency vP * of the quasiparticle mode, as well as the quasiparticle bandwidth W at 10 K are derived using the model developed by Millis and Lee.