Evidence for a conducting surface ground state in high-quality single crystalline FeSi

Abstract

We report anomalous physical properties of high-quality single-crystalline FeSi over a wide temperature range of 1.8-400 K. The electrical resistivity ρ(T) can be described by activated behavior with an energy gap Δ = 57 meV between 150 and 67 K, below which the estimated energy gap is significantly smaller. The magneto-resistivity and Hall coefficient change sign in the vicinity of 67 K, suggesting a change of dominant charge carriers. At ∼19 K, ρ(T) undergoes a cross-over from semiconducting to metallic behavior which is very robust against external magnetic fields. The low-temperature metallic conductivity depends strongly on the width/thickness of the sample. In addition, no indication of a bulk-phase transition or onset of magnetic order is found down to 2 K from specific heat and magnetic susceptibility measurements. The measurements are consistent with one another and point to complex electronic transport behavior that apparently involves a conducting surface state in FeSi at low temperatures, suggesting the possibility that FeSi is a 3D topological insulator.

Electronic structure studies of FeSi: A chiral topological system

“…Thus, our resistivity measurements suggest that FeSi is a semiconductor following the activation formula within the temperature range of 75 to 143 K and gradually becomes a bad metal as we go away from this temperature range. These results are qualitatively in agreement with the existing reports, although the semiconducting temperature range is found to be different from different studies [58,65,73,[82][83][84][85].…”

“…The same is observed as the system goes above 143 K. And beyond 220 K, the resistivity increases with temperature in a metallic manner. This peculiar resistivity character below 75 K and above 143 K can be attributed to bad metallicity of FeSi [57,65]. Thus, our resistivity measurements suggest that FeSi is a semiconductor following the activation formula within the temperature range of 75 to 143 K and gradually becomes a bad metal as we go away from this temperature range.…”

“…Apart from these interesting physical properties, FeSi is further predicted to show the above-mentioned manyfold degenerate chiral fermions at the high symmetry points with a nonzero Berry phase [70]. Further, a recent transport study on FeSi shows anomalous temperature-dependent resistivity which they attribute it to the plausible topological surface states [65].…”

“…Earlier, the transition metal monosilicides were extensively studied for their low-energy electronic correlations [50][51][52][53][54][55][56][57][58][59]. Specifically, FeSi shows peculiar temperature-dependent electronic and magnetic properties [60][61][62][63][64][65]. Further, FeSi behaves as a semiconductor with an indirect band gap of 50 meV within the temperature range of 100-200 K [57,64], while is a bad metal [66] outside of this temperature range.…”

Metallic surface states in a correlated d-electron topological Kondo insulator candidate FeSb 2

“…Combined with more refined band-structure calculations, we may then be able to answer the question of whether the observed surface states are topological. We note that a recent transport work has suggested the presence of low-temperature surface conduction in the related compound FeSi (35), providing a hint of evidence for nontrivial conduction in a related d-electron-correlated insulator.…”