Inelastic neutron scattering study of spin gap formation in heavy fermion compounds

According to conventional theories of strongly correlated electron systems, the natural consequence of strong hybridization between f-electrons and conduction electrons is the opening of a gap in both charge and spin channels. Despite their importance, there are few experimental observations of such gaps in real materials. Recently the charge gap in d- and f-electron systems has been investigated using optical studies. However, due to the q=0 limitation of the technique, optical studies cannot give information on the wave vector dependence of the gap. Inelastic neutron scattering is a unique technique and provides direct information on the spin gap energy as well as its wave vector and temperature dependence. Further, knowledge of the wave vector dependence of the gap, in particular the spin gap, provides important information about the microscopic mechanism of the gap formation. Recently we have investigated the spin gap formation, and its relation to the charge gap, in several Ce, Yb and U based compounds using inelastic neutron scattering techniques. We review here the nature of the spin gap in CeT₄Sb₁₂ (T = Ru, Os and Fe), CeRhAs and U_2-xTh_xRu₂Sn compounds based on our recent neutron scattering studies. We compare the magnitude of the spin gaps we have measured in this way with that of the charge gaps measured in optical studies. We have found a universal scaling relation between the spin gap energy and the Kondo temperature (T_K) for many strongly correlated electron systems and the results are discussed.