Neutron scattering study on U-dichalcogenides

We will report the results of our recent inelastic neutron scattering study on beta-US2. This compound shows a semi-metallic or narrow gap semi-conducting behaviour at room temperature. A clear exponential up-turn of the resistivity in the order of similar to 10(6) Omega cm has been observed below 100 K. We found a sharp inelastic peak at the excitation energy of about 7 meV at 8 K. The Q-dependence of the peak intensity is in good agreement with the magnetic form factor of U4+ ion and no clear dispersion relation has been observed. Therefore we concluded that this is a crystalline electric field (CEF) excitation peak. The excitation energy is in good agreement with the CEF level scheme obtained from the susceptibility data. The CEF peak intensity decreases with increasing temperature and becomes much weaker than the calculated temperature factor expected from the CEF level scheme. Furthermore a quasi-elastic response appears, and coexists with a broadened CEF peak at higher temperatures. The quasi-elastic component is not due to phonon, because the temperature dependence of the intensity is inconsistent with calculation. We concluded that this quasi-elastic response is a hybridization effect of U-5f electrons with, most likely, p-electrons of sulfur. It is highly interesting that the energy scale of the CEF peak (similar to 7 meV) is very close to the conduction gap (90K), and the quasi-elastic component appears above the characteristic temperature of about 100 K. Our data strongly suggest that the crossover of 5f character plays an import role for the metal-insulating transition in beta-US2.