Surface- and pressure-induced bulk Kondo breakdown in SmB6

Motivated by the prediction of surface Kondo breakdown in topological Kondo insulators, we investigated the valence and magnetism of ${\mathrm{SmB}}₆$ with x-ray absorption and magnetic circular dichroism at the Sm ${M}_4,5$ and ${L}₂$ edges with surface and bulk sensitivity, respectively. A higher Sm valence state at the surface and in the bulk under pressure indicates the surface- and pressure-induced bulk Kondo breakdown. We confirmed the different magnetization origin from the surface ($f$ electron) to the bulk ($d$ electron) and ascertained the direct relationship between the disappearance of the Sm ${L}₂$-edge x-ray magnetic circular dichroism signal and the closure of the $d\ensuremath{-}f$ hybridization gap in ${\mathrm{SmB}}₆$ bulk under pressure, paving the way for further investigations of the Kondo insulators. Above ${P}_c=8.5$ GPa, a more localized $4f$ state with higher valence and the disappearing of the $5d$ magnetism are observed, suggesting the closure of the hybridization gap in the bulk.