Extraordinary Bulk Behavior in 3d-based Strongly Correlated Insulators FeSi and FeSb₂

Topological Kondo insulators, such as SmB₆, are a class of materials in which Kondo hybridization between conduction electrons and localized $f$-electrons opens a bulk band gap directly at the Fermi level. A consequence of this process can result in Dirac surface states and provides a platform for investigation of strongly correlated 2D electron physics on a bulk material. However, it remains an open question if simpler d-electron based correlated insulators could manifest similar physics. To this end we investigated electrical transport of the 3d-based correlated insulators FeSi and FeSb₂. By using a double-sided Corbino disk transport geometry, we show unambiguous evidence of a surface conductance in both of these materials, although their topological nature is called into question by our results which show evidence of localization. In particular, FeSi can exhibit sheet conductance in excess of the Mott-Ioffe-Regal limt (h/e²) of a 2D electron gas, which suggests the surface of FeSi is a 2D variable range hopping system distinct from the bulk. In addition, by using the inverted resistance technique with a 4-terminal Corbino disk, we extract the bulk resistivity as a function of temperature. Similar to SmB₆, the bulk resistivity of FeSi and FeSb₂ are confirmed to exponentially increase by 8-9 orders of magnitude from 300 K to 2 K, demonstrating the bulk of these materials are excellent insulators, which provides an ideal platform for studying correlated 2D physics, regardless of topology.