Search for Superconductivity in Charge Doped Composites of Carbon Nanotubes and Fullerides

The motivation of this study is to develop a highly sensitive method of microwave absorption in low magnetic fields (LFMA), combined with SQUID magnetometry, for searching for superconducting phases in nanocomposites in which carrier concentration is changed by in-situ doping. The unzipped ribbons of multiwalled carbon nanotubes (MWCNTs) (which are stacks of graphene ribbons) are infiltrated by $\mbox{C}_{60}$ fullerenes and doped chemically (by alkali metals) and electrochemically to modulate the carrier concentration. It is demonstrated that hysteretic LFMA appearing below $T_c$ is indeed a highly sensitive test for negligibly small superconducting (SC) phases (\mbox{$\sim10^{-11}\ \mbox{g}$}), and multi-phase SC composites with several $T_c$ can be detected and separated. At the same time, non-SC signals (which are quite similar to SC LFMA) have been found from samples containing magnetic nanoclusters (Fe, Ni/Co, etc.) which are catalyst residuals in CNT synthesis. The strategies of differentiating SC LFMA from magnetic LFMA by complementary SQUID/transport studies are suggested.