Puzzle maker in SmB₆: accompany-type valence fluctuation state

In recent years, studying the Kondo insulator SmB₆, a strongly correlated electron material that has been puzzling the community for decades, has again become an attractive topic due to the discovery of its unusual metallic surface state coexisting with the bulk insulating state. Many efforts have been made to understand the microphysics in SmB₆, but some puzzles that have been hotly debated and argued have not been solved. In this article, based on the latest progress made in our high-pressure studies on SmB₆ and the accumulating results reported by other groups, we propose a notion named the 'accompany-type valence fluctuation state', which possibly coexists with the bulk Kondo insulating ground state of SmB₆. We expect that this notion could be taken as a common starting point for understanding in a unified way most of the low-temperature phenomena observed by different experimental investigations on SmB₆, thus promoting the deciphering of the puzzles. We also expect that this notion could attract rigorous theoretical interpretation and further experimental investigation, or stimulate better thinking on the physics in SmB₆. (key issues review)