Superconductivity observed in the vicinity of Mott insulating states in A₃C₆₀

A similar Mott physics on unconventional superconductors, such as cuprates, Fe pnictides, and organic conductors is now claimed for expanded C₆₀ superconductors, and electron-electron (e-e) interactions are thought to be very important origin of its high Tc superconductivity. On the other hand, the highest superconductivity surpassing the cuprates recently found in H₂S under high pressure gives the discussion back to the electron-phonon (e-ph) mediated superconductivity for achieving high superconducting critical temperature (high T_c). The Fermi surface of A₃C₆₀ (A=alkali metals: K, Rb, Cs) superconductors with expanded cell (V_cell(C₆₀^3-)) provides an intriguing research platform for both e-e and e-ph interactions. However, being different from other unconventional superconductors, electrical transport measurements had been very difficult in expanded A₃C₆₀ and important scientific discussions have been made based on only limited magnetic and optical probes. Here, we give experimental results that accurate electrical resistivity (ρ(T,P)) can be achieved for a variety A₃C₆₀ with expanded V_cell(C₆₀^3-) near the Mott boundary under various temperature (T) and pressure (P). Electrical transport was carefully measured as a function of T and P, straddling the phase boundary between the Mott insulator and the metallic/superconducting phase. A new phase can be recognized to be located in the middle temperature range. The new phase is interpreted to be generated by e-ph interactions via dynamic Jahn Teller phonons and shows a strong T evolution of ρ(T), which can systematically be observed for A₃C₆₀ with a variety of cell volume controlled by chemical (stoichiometric composition of A₃C₆₀) and physical pressure. The new electronic phase observed unprecedentedly is considered to have common physics among many materials.