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-US2 was studied by means of neutron scattering. At low temperatures, clear crystalline electric-field (CEF) excitations were observed, indicating the localized character of 5f electrons. The pseudo-doublet state at an excitation energy of ∼7 meV is consistent with the level scheme based on the 5f 2 configuration of U4+ ions, which reproduces the magnetic susceptibility, magnetization, and specific heat. At elevated temperatures, the conductivity recovers, with the excitation of 5f 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 5f character from localized to itinerant.