Air stability of K 3C 60 superconductors: Low field microwave absorption and ESR study

Abstract

Using nonresonant microwave absorption in low magnetic field (so called LFS) combined with conventional ESR it is shown that degradation of the superconducting (SC) state of K XC 60 upon exposure to air is a surprisingly slow (e.g. compared to n-type doped conducting polymers) and partially reversible process. The annealing at 100–130°C in high vacuum of air exposed samples restores both LFS and ESR typical for the SC state, suggesting the formation of some unstable (K XC 60O 2) complex at a first stages of oxidation. At longer exposure times superconducting LFS can not be recovered meaning irreversible chemical bonding of 0 to C 60.