Meissner effect in honeycomb arrays of multiwalled carbon nanotubes

We report a gradual magnetization drop with an onset temperature (Tc) of 18-23 K found in the honeycomb arrays of multiwalled CNTs (MWNTs) showing a slight resistance decrease due to superconductivity. Magnetic field dependence of the drop and temperature dependence of critical fields indicate that it is attributed to Meissner effect for type-II superconductors. The Tc value is the highest among those in new carbon-related superconductors. The weak magnetic anisotropy, superconductive coherence length (11-19 nm), and disappearance of the Meissner effect after destructing array structure suggest that intertube coupling of MWNTs in the honeycomb array is a dominant factor for the mechanism. Drastic reduction of ferromagnetic catalyst for synthesis of the MWNTs makes the finding possible.