Closing the spin gap in the Kondo insulator Ce3Bi4Pt3 at high magnetic fields

Kondo insulator materials(1)-such as CeRhAs, CeRhSb, YbB12, Ce3Bi4Pt3 and S mB6-are 3d, 4f and 5f intermetallic compounds that have attracted considera ble interest in recent years(2-5). At high temperatures, they behave like m etals. But as temperature is reduced, an energy gap opens in the conduction band at the Fermi energy and the materials become insulating. This contras ts with other f-electron compounds, which are metallic at all temperatures. The formation of the gap in Kondo insulators has been proposed to be a con sequence of hybridization between the conduction band and the f-electron le vels(6,7), giving a 'spin' gap. If this is indeed the case, metallic behavi our should be recovered when the gap is closed by changing external paramet ers, such as magnetic field or pressure. Some experimental evidence suggest s that the gap can be closed in SmB6 (refs 5, 8) and YbB12 (ref. 9). Here w e present specific-heat measurements of Ce3Bi4Pt3 in d.c. and pulsed magnet ic fields up to 60 tesla. Numerical results and the analysis of our data us ing the Coqblin-Schrieffer model demonstrate unambiguously a field-induced insulator-to-metal transition.