Sm${\mathrm{B}}_6$: Kondo Insulator or Exotic Metal?

High pressure resistivity and Hall effect measurements on Sm${\mathrm{B}}_6$ show that the activation gap Δ vanishes discontinuously between 45 and 53 kbar, yielding at high pressures a mass-enhanced Fermi liquid phase. The low temperature transport is dominated by extended states in the gap with unusual superunitarity scattering properties, and a carrier density which grows almost exponentially with reduced Δ, saturating below 45 kbar near 0.15 electron per unit cell. Our results are inconsistent with hybridization gap models and suggest a striking parallel to Mott-Hubbard insulators.