Evidence for a conducting surface state in FeSi and comparison with SmB6

The recent discovery of a conducting surface state in FeSi suggests the possibility of topological behavior. We further explore this state using magnetoresistance M(H,T) measurements in high magnetic fields H to 60 T, electrical resistivity ρ(T) measurements at high pressures P to 7.6 GPa, and magnetic field modulated microwave spectroscopy (MFMMS). The two energy gaps Δ₁ and Δ₂ of FeSi from the T-dependence of ρ increased with P up to 7 GPa, followed by a drop coinciding with a sharp suppression on the conducting surface transition temperature. The closing of Δ₁ and Δ₂ with P was also seen in the suspected topological Kondo insulator SmB₆. The behavior of M(H,T) shows additional evidence for the conducting surface state by comparing the behavior perpendicular and parallel to H. Comparisons of M(H,T) of FeSi and SmB₆ show similar behavior. However, the MFMMS results suggests differences in the underlying physics of the surface state in these two compounds.

UCSD: DOE BES DEFG02-04-ER46105, NSF/DMR-1810310, and NNSA through DOE DENA0002909. The MFMMS: DOE BES Grant #DE FG02 87ER-45332. NHMFL: NSF/DMR-1157490/1644779 and the State of Florida. LANL: DOE 2019LANLE1FR HPSTAR: NSFC 51871054.