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The recent discovery of a conducting surface state in FeSi suggests the possibility of topological behavior [1]. 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 [2]. 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.

[1] Y. Fang et al., PNAS 117, 8558 (2019)
[2] S. Gabani et al., PRB 67,172406 (2003)