Origin of negative thermal expansion in strongly correlated f− electron systems

We have investigated the origin of the negative-thermal-expansion (NTE) phenomena observed in various strongly correlated $\mathit{f}\text{\ensuremath{-}}$electron systems, including Ce, Sm, and Yb rare-earth compounds and actinide element Pu. We have thoroughly surveyed existing nonmagnetic $\mathit{f}\text{\ensuremath{-}}$electron NTE materials and identified that the temperature ($T$)-induced valence transition plays an essential role in realizing the NTE in Sm- and Yb-based mixed-valence (MV) systems. We have discussed the contrasting thermal-expansion behaviors between (Sm, Yb)-based MV systems and their electron-hole symmetric counterparts (Ce, Eu)-based MV systems that exhibit positive-thermal-expansion (PTE) phenomena. We have also clarified the origin of intriguing thermal expansion behavior of Pu, which, upon cooling, reveals not only the NTE, but also the strong PTE. Finally, we have predicted the possible existence of an additional NTE feature in topological-Kondo-insulator (TKI) candidates of ${\mathrm{SmB}}₆$, g-SmS, and ${\mathrm{YbB}}₁₂$ in the low-$T$ regime where the TKI nature is expected to emerge.