Electrical Conductivity of C₆₀ fullerides with expanded lattice in the Mott-insulator boundary

The Fermi surface of (A,A’)₃C₆₀ (where A, A’: alkali metals: K, Rb, and Cs) superconductors ranging from small to expanded cell volume (V_cell(C₆₀^3-)) provides an intriguing research platform for both e-e and e-ph interactions. However, being different from other unconventional superconductors like cuprates and Fe pnictides, electrical transport measurements had been very difficult in expanded and other series of binary alkali-metal (A,A’)₃C₆₀ fullerides, and are reported only for alkali-metal fullerides of K₃C₆₀ and Rb₃C₆₀ with small cell volume far apart from the Mott insulator boundary. Here, we give experimental results that accurate electrical resistivity (ρ(T,P)) can be achieved for a variety (A,A’)₃C₆₀ from small V_cell(C₆₀^3-) to the expanded ones in 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. An intriguing new phase can be recognized to be located in the middle temperature range. Based on the electrical conductivity of (A,A’)₃C₆₀ together with other static magnetic measurements as well as other physical properties, the electronic states of (A,A’)₃C₆₀ leading to superconductivity are discussed_.