TEMPERATURE-DEPENDENCE OF THE OPTICAL CONDUCTIVITY OF THE HEAVY-FERMION SYSTEM CECU6

Among the heavy-fermion systems, CeCu6 shows one of the most enhanced coefficients of the electronic specific heat, indicating a very large density of quasiparticle states at the Fermi energy. Far-infrared-reflectivity measurements at low temperature reveal a sharp optical transition with 5 meV between narrow hybridized bands, and an effective plasma frequency of p*150 meV is obtained. The problem remains whether this behavior is basically independent of temperature, or whether it exists only at the lowest temperatures. We measured for the first time the far-infrared reflectivity of a single crystal of CeCu6 at several temperatures ranging from 5 to 50 K. The structure which is visible at low temperature disappears at 50 K. The plasma frequency remains almost constant (200 meV) up to 30 K; then it rises. In any case the low-temperature behavior is conserved up to a temperature much larger than the one expected (the Kondo temperature is 34 K). Therefore both hybridization and crystal-electric-field splitting of the f level must be considered for a proper understanding of this material. © 1990 The American Physical Society.