Pressure-induced exotic states in mixed-valence rear earth hexaborides

Recently, a unique type of well-known compounds, mixed-valence rear earth hexaborides RB$_{\mathrm{6\thinspace }}$(R$=$ Sm and Yb), receive new interests due to the discovery of the coexistence of metallic surface state and insulating bulk state in SmB$_{\mathrm{6}}$. This encourages people to revisit the RB$_{\mathrm{6\thinspace }}$with an attempt to establish a new physics that links the correlated electron systems and topological insulators. Pressure is a way to help understanding the underlying mechanism, therefore it is specially needed in establishing this link because the valence state of RB$_{\mathrm{6}}$ is sensitive to pressure. In this talk, we will report some progress of high pressure studies on the RB$_{\mathrm{6}}$, mainly focusing on the phenomena of pressure-induced exotic states and corresponding quantum phase transitions. The connections between the related phenomena and the valence states are revealed.