Experimental Signatures of Topological Transport in Polycrystalline FeSi Thin Films

Abstract

Iron monosilicide $\epsilon$-FeSi has recently been predicted to host nontrivial topological states, yet experimental evidence remains scarce. Here we report transport signatures of Weyl semimetal behavior in polycrystalline $\epsilon$-FeSi films (65 nm thick) grown by solid-state reaction of Fe on Si (100). Below 200 K, the anomalous Hall conductivity $\sigma_{xy}^{AHE}$ becomes temperature-independent ($\sigma_{xy}^{AHE}$$\approx$14 S/sq.$\sim$ const($\sigma_{xx})$), unequivocally demonstrating an intrinsic anomalous Hall effect driven by Berry curvature. By comparing our data with the large set of data available in the literature for FeSi single crystals and polycrystalline/amorphous/crystalline FeSi thin films of thicknesses between 10 nm and 1 mm, we discover a universal scaling of $\rho_{xy}^{AHE}$$\sim$$\rho_{xx}^{2}$, which confirms the existence of surface-dominated electron transport. In this regime, the chiral anomaly manifests in both anisotropic longitudinal magnetoresistance and the planar Hall effect as well. The above observations establish $\epsilon$-FeSi as a Weyl semimetal and allow to relate $\sigma_{xy}^{AHE}$ to a "quantized" Hall response estimating the effective Weyl-point separation as $(k_{+}^W$$-$$k_{-}^{W})/(2\pi)$$\approx$0.36. Our findings confirm the topological origin of electron transport in $\epsilon$-FeSi and discover its potential as a new high temperature and noble metal-free Weyl semimetal.