Superconductivity of Carbon Materials - Unstable Phases |
| W. Kempiński
Institute of Molecular Physics, Polish Academy of Sciences, Smoluchowskiego 17, 60-179 Poznań, Poland |
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| Different intercalated carbon systems: KxC60, RbxC60 as well as highly oriented pyrolytic graphite + potassium with unstable structural and superconducting behavior were investigated with the EPR and magnetically modulated microwave absorption techniques. Three characteristic, well-separated EPR signals were observed for AxC60 (A = K or Rb) systems at the early stages of doping process. We ascribed these signals to C60+, C601-, and C603- radicals. Evolution of the EPR spectrum characterizes different phases of KxC60 system including superconducting one. Two superconducting phases with Tc(1)=(21±0.5) K (unstable) and Tc(2)=(18.5±0.5) K (final, stable) were distinctly separated for KxC60 system. For C60 intercalated by rubidium a similar evolution of EPR spectrum is observed. Prior to well-defined superconducting Rb3C60 phase was created, strong instabilities of the magnetically modulated microwave absorption signal were observed at the narrow part of the doping process. For potassium intercalated highly oriented pyrolytic graphite, EPR oscillations below 100 K were observed as well as Josephson hysteresis loops were registered at 5 K. |
| DOI: 10.12693/APhysPolA.106.627 PACS numbers: 76.30.-v, 74.70.Wz |