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 $d-f$ 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.

• Received 2 March 2018
• Revised 13 June 2018

DOI:https://doi.org/10.1103/PhysRevB.97.235153