Metal-insulator crossover accompanied by the dual nature of 5 electrons with localized and itinerant charancters in -US

Metoki, Naoto   ; Sakai, Hironori   ; Yamamoto, Etsuji  ; Haga, Yoshinori   ; Matsuda, Tatsuma; Ikeda, Shugo*

Magnetic excitation of a narrow gap semiconductor -USwas studied by means of neutron scattering. At low temperatures clear crystalline electric field (CEF) excitations were observed, indicating localized character of 5 electrons. The excitation energy 7 meV of the pseudo doublet state is consistent with the level scheme based on 5 configuration of U ions, which reproduces the magnetic susceptibility, magnetization, and specific heat. With elevating temperature the conductivity recovers with 5 carriers excited beyond the gap of 90 K. Simultaneously the CEF excitation becomes weak and broad, and is replaced by a quasi-elastic response of magnetic origin, indicating the crossover of 5 character from localized to itinerant.