A Study of Interaction of Dopants in C60: Doping Alkali-Halogen-Alkali into C60 in Successive Steps

It is well established that for an alkali-doped C₆₀ sample to be superconducting, the oxidation state C ^3– ₆₀ must be satisfied. It has been found previously that T _c is proportional to crystal parameter a in a face-centered cubic (fcc) structure. It is thus desirable to intercalate the largest possible size of molecule that can contribute one electron into every interstitial space. The successful process of such doping depends on our understanding the interaction among the dopants and C₆₀. We introduce a fabrication scheme of three steps successive doping using alkali (A) and halogen (H) elements along this line. While A ₃C₆₀ is superconducting, A _3–x (AH) _x C₆₀ was found to be non-superconducting in the second step and the sample became superconducting again after doping with A, leading to stoichiometric ratio of A _3–x (AH) _x A _x C₆₀. We present an explanation based on interaction among dopants and C₆₀ and analyze the process of ionization of dopants, which is consistent with our experimental finding reported here.