ESR AND LOW-FIELD MICROWAVE-ABSORPTION STUDIES OF POTASSIUM DOPED C-70 - OBSERVATION OF POSSIBLE METALLIC STATE IN KXC70

The magnetic property of KxC70 upon K doping process was investigated by electron spin resonance (ESR) and low-magnetic field microwave absorption (low-field signal: LFS). In the early stage of K doping (doping time t(d) < 40 h), ESR signal with a Curie-like behavior was observed, which means an insulating state of a doped phase in this stage. In t(d) > 40 h, ESR spectra broadened suddenly and the almost temperature-independent behavior of ESR intensity appeared in the high temperature region. This suggests a change from an insulating KxC70 phase to a metallic KxC70 phase. The chemical analysis has presented that the insulating phase corresponds to K3C70 and the metallic phase corresponds to K4C70. A very weak LFS below ca. 9 K was observed in the doping stage of t(d) greater-than-or-equal-to 10 h, indicating the coexistence of a small volume of some superconducting phase. A possible origin of the superconducting phase is described.