Gradual metal-insulator transition in extended pressure region in doped fullerenes

Due to competition between strong electron-electron and electron-phonon interaction, the electron-doped fullerenes are close to metal-insulator transition and exhibit an unmatched variety of physical properties. To investigate the nature of the metal-insulator transition, we employed high pressures in transport and electron spin resonance (ESR) measurements. We found that the transition between the molecular (insulting) and delocalized (metallic) behavior is gradual, with a broad pressure region where both molecular and quasiparticle excitations coexist. We analyze this behavior within the recently proposed Mott-Jahn-Teller framework, and discuss fundamental connection to high superconducting transition temperature in fullerenes.