Metal-insulator crossover accompanied by the dual nature of 5f electrons with localized and itinerant charancters in β-US2

The magnetic excitation of a narrow gap semiconductor β -US 2 was studied by means of neutron scattering. At low temperatures, clear crystalline electric-field (CEF) excitations were observed, indicating the localized character of 5 f electrons. The pseudo-doublet state at an excitation energy of ∼7 meV is consistent with the level scheme based on the 5 f 2 configuration of U 4+ ions, which reproduces the magnetic susceptibility, magnetization, and specific heat. At elevated temperatures, the conductivity recovers, with the excitation of 5 f carriers being beyond the conduction gap of 90 K. Simultaneously, the CEF excitation becomes weak and broad, and is replaced by a quasi-elastic response of magnetic origin, indicating a crossover of the 5 f character from localized to itinerant.