Two routes to metallic behavior for a Kondo insulator

Cold neutron spectroscopy was performed on CeNi_{1-x}Cu_xSn to investigate how the magnetic gap in the 'Kondo insulator' CeNiSn is destroyed by temperature (T), chemical substitution (x), and external magnetic field. Upon doping, the spin gap collapses and magnetic Bragg peaks occur initially (x = 0.13) at the commensurate wave vectors Q = (0,m/2,l) (m,l integer) where the higher-energy (4.1 meV) gap is located for x = 0. A magnetic field smooths the sharp spin gap structure in both momentum and energy, while leaving the associated static susceptibility unchanged, as does raising T.