Materials and Instrument advances in the study of emergent phenomena in Kondo insulators

Kondo insulators host a number of exotic emergent phenomena, including quantum oscillations in an insulating state, magnetoexcitons related to the mixed-valence ground state, and surface transport possibly arising from correlation-driven topological physics. However, experimental progress to study these phenomena is hampered by the limited model material families so far, as well as materials challenges. In the first part of this talk, I will describe a new class of Kondo insulators based on 3d electrons instead of 4f electrons, focusing on the compound FeSb₂, which we study with angle-resolved photoemission spectroscopy (ARPES). Sharing a number of similar transport and thermodynamic properties as SmB₆, we show that FeSb₂ also hosts unique electronic structure features that are distinct from the 4f compounds. The comparison across these two different families allows the identification of key ingredients that gives rise to the exotic emergent phenomena in Kondo insulators. In the second part of the talk, I will discuss band- and momentum-resolved investigation of the low energy bosonic excitations in SmB₆ with time-resolved ARPES. With improved time resolution, we discover coherent oscillations in the electronic spectra that surprisingly strongly couple to intensity at E_F but not the 4f intensities. I will discuss the relevance and possible mechanism of these oscillations with regards to the charge and magnetic excitations of the mixed valence ground state in Smb₆. Our results showcase a powerful technique to study the coupling of low energy bosonic excitations to the electronic structure.