3D topological Kondo insulators (TKIs) have been studied in systems where a flat 4f band and a dispersive band with opposite parities participate in both the Kondo hybridization and topological band inversion. However, when it comes to 3d-based correlated insulators such as FeSi and FeSb2, the origin of the band structure becomes more elusive, and it is less clear whether they should even be classified as topological Kondo insulators. Nevertheless, their electrical transport properties exhibit striking similarities to the archetypal TKI, samarium hexaboride (SmB6). In this talk, I will focus on comparing our recent studies of FeSi and FeSb2 to our long-continued research on SmB6. Using the inverted resistance technique, we have identified unambiguous two-dimensional conducting states below a certain crossover temperature in both materials. Also, after peeling off the surface state contribution using the inverted resistance technique, we find the bulk resistivity continues to increase exponentially, with a total of 8-9 orders of magnitude exponential increase, consistent with a very robust band gap. In the concluding part of the talk, I will discuss future research directions, including the development of new transport methods to study these materials more effectively. Additionally, I will explore the potential for discovering other correlated topological insulators.

This colloquium is supported, in part, by the NMSU/UCSB Partnership for Research and Education in Materials (PREM), award no. DMR-2423992.