Kondo Insulators: Iconic forerunners of emergent quantum materials*

Over their history, Kondo insulators have played an iconic role in our understanding of strongly correlated electron systems.  In the 1970s they were fore-runners of heavy fermion metals and superconductors[1].  The traditional viewpoint, established during these times, was that Kondo insulators are highly renormalized band insulators[2]. In the 2000s, they entered a new role as topological Kondo insulators[3].  In the last five years, a spate of exciting new experiments, including the discovery of bulk quantum oscillations and T-linear thermal conductivity in the insulating state[4,5],  suggest that a possible role as an example of  fractionalized quantum matter. I will present an overview of some of the recent experimental and theoretical developments[6-9]contrasting the tension between the opposing world-views of these intriguing materials. 

[1]  N. F.  Mott, Phil. Mag. 30, 403 (1974).

[2] G. Aeppli, Z. Fisk, Comments Cond. Mat. Phys. 16, 155-170, (1992).

[3]  M. Dzero et al, Annu Rev. Cond. Mat. 7, 249-280 (2016).

[4]  B. S. Tan et al, Science 349, 287 (2015).

[5]  Y. Sato et al, Nature Physics 15, 954-959 (2019). 

[6]  Z. Xiang et al,  Nature Physics 17, 788 (2021)

[7]  Po-Yao Chang, O. Erten & P. Coleman, PRL 119, 057903 (2017)

[8]  I. Sodemann, D. Chowdhury & T. Senthil, Nat. Comm 9, 1766 (2018). 

[9]  B. Skinner, Phys. Rev. Materials 3, 104601 (2019).