The controversial ground-state properties of the Kondo insulator $\mathrm{Sm}{\mathrm{B}}_6$ have been investigated using ${}^{11}\mathrm{B}$ NMR in very high magnetic fields up to $37\mathrm{T}$. We find evidence that, following the development of a gap in the conduction-band density of states below $100\mathrm{K}$, the in-gap states dominate the nuclear relaxation at temperatures less than $10\mathrm{K}$. The Korringa product $1∕K^2{T}_{1}T$ exhibits anomalous behavior in this range and the application of high magnetic fields leads to suppression of nuclear relaxation. The hybridization gap, however, remains open up to $37\mathrm{T}$. The behavior of the relaxation at low temperatures suggests a strong field dependence of the in-gap states and rules out the possibility that bound states arise from ${\mathrm{B}}_6$ vacancies. A simple density-of-states model and a band scheme are introduced to account for these observations.

• Received 25 September 2006

DOI:https://doi.org/10.1103/PhysRevB.75.075106