Screened Moments in a Kondo insulator: extrinsic origin of in-gap states in SmB6

The small gap insulator SmB6 has long been known to have thermodynamic properties at odds with being an insulator, yet the extensive search for low-energy phenomena which could explain this has remained elusive and controversial even after 50 years of research into its unconventional nature. We have observed a Kondo impurity-like moment screening effect in the insulating state of nominally pure and Gd-doped SmB6 via heat capacity, magnetization, and resistivity measurements. High-resolution neutron scattering on the low energy regime confirms directly that no coherent magnetism is evident below the long-lived 13 meV spin exciton, restricting proposals of intrinsic neutral low-energy magnetic excitations.b We infer that the mixed valence, strongly-correlated nature of SmB6 elicits extreme sensitivity to impurities and defects which induce metal-like bulk properties.