Probing FeSi, a d-electron topological Kondo insulator candidate, with magnetic field, pressure, and microwaves

TL;DR

This study investigates the surface and bulk properties of FeSi, a candidate topological Kondo insulator, under various conditions, revealing similarities to SmB6 and providing insights into its electronic and topological nature.

Contribution

It provides comprehensive experimental evidence on FeSi's surface states and phase behavior under magnetic field, pressure, and microwave spectroscopy, comparing it to SmB6 to explore its topological Kondo insulator characteristics.

Findings

FeSi exhibits a surface state below 19 K similar to SmB6.

Energy gaps in FeSi increase with pressure up to 7 GPa, then drop at T_S.

Microwave spectroscopy shows a sharp feature at T_S for FeSi, not observed in SmB6.

Abstract

Recently, evidence for a conducting surface state below 19 K was reported for the correlated d-electron small gap semiconductor FeSi. In the work reported herein, the conducting surface state and the bulk phase of FeSi were probed via electrical resistivity measurements as a function of temperature T, magnetic field B to 60 T and pressure P to 7.6 GPa, and by means of a magnetic field modulated microwave spectroscopy (MFMMS) technique. The properties of FeSi were also compared to those of the Kondo insulator SmB6 to address the question of whether FeSi is a d-electron analogue of an f-electron Kondo insulator and, in addition, a topological Kondo insulator. The overall behavior of the magnetoresistance MR of FeSi at temperatures above and below the onset temperature (T_S) 19 K of the conducting surface state is similar to that of SmB6. The two energy gaps, inferred from the resistivity data in the semiconducting regime, increase with pressure up to about 7 GPa, followed by a drop which coincides with a sharp suppression of T_S. This behavior is similar to that reported for SmB6, except that the two energy gaps in SmB6 decrease with pressure before dropping abruptly at T_S. The MFMMS measurements showed a sharp feature at T_S (19 K) for FeSi, but no such feature was observed at T_S 4.5 K for SmB6. The absence of a feature at T_S for SmB6 may be due to experimental issues and will be the subject of a future investigation.