High-pressure induced modifications in the hybridization gap of the intermediate-valence compound SmB 6

We have carried out the measurements of high-pressure $^{11}\mathrm{B}$-nuclear magnetic resonance on the intermediate-valence compound ${\mathrm{SmB}}_{6}$ to investigate the effects of pressure on Sm $4f$ states and the quasiparticle band. From the measurements of spin-lattice relaxation time, just below the critical pressure ${P}_{\mathrm{c}}$ of nonmagnetic-magnetic phase transition, we find that quasiparticle bandwidth clearly decreases with pressure, while the insulating gap is almost constant or slightly increases. The latter is consistent with the result of a band-structure calculation. These pressure induced modifications in the band structure indicate the enhancement of the density of states of the quasiparticles when approaching ${P}_{\mathrm{c}}$. The pressure dependence of the Sm $4f$ states and the origin of the insulating gap are well explained in terms of exchange interactions between conduction and $4f$ electrons.