We report the results of electrical resistivity measurements on ${\mathrm{SmB}}_6$ in magnetic fields as large as 18 T and pressures as large as 66 kbar. We find that the activation gap Δ extracted from these measurements has an anomalously weak field dependence, even when pressures of ∼50 kbar are used to tune Δ to instability. Our results imply the existence of strong exchange interactions in the low-pressure, gapped state, and argue that Δ originates with many-body correlations. Nonetheless, the small, negative magnetoresistance Δρ(H)/${\mathrm{\rho }}_{\mathit{H}=0\mathrm{}}$∝-${\mathit{H}}^2$ observed below 50 kbar is completely accounted for by the weak field variation of Δ. A sign change in the magnetoresistance accompanies the gap collapse, with the high-pressure, gapless metal having Δρ(H)/${\mathrm{\rho }}_{\mathit{H}=0\mathrm{}}$∝${\mathit{H}}^{3/2}$. The Fermi surface deduced from these measurements is similar to that of noninteracting, trivalent ${\mathrm{LaB}}_6$, but with a volume which increases rapidly with pressure.

• Received 6 June 1995

DOI:https://doi.org/10.1103/PhysRevB.52.7322