Superconducting diamagnetic fluctuations in ropes of carbon nanotubes

Abstract

We report low-temperature magnetization measurements on a large number of purified ropes of single wall carbon nanotubes. In spite of a superparamagnetic contribution due to the small ferromagnetic catalytical particles still present in the sample, at low temperature $(T<0.5\phantom{\rule{0.3em}{0ex}}\mathrm{K})$ and low magnetic field $(H<80\phantom{\rule{0.3em}{0ex}}\mathrm{Oe})$, a diamagnetic signal is detectable with an amplitude equal to $\frac{1}{3}$ of the total magnetization signal. This low-temperature diamagnetism can be interpreted as the Meissner effect in ropes of carbon nanotubes which have previously been shown to exhibit superconductivity from transport measurements [M. Kociak, A. Y. Kasumov, S. Gu\'eron, B. Reulet, I. I. Khodos, Y. B. Gorbatov, V. T. Kolkov, L. Vaccarini, and H. Bouchiat, Phys. Rev. Lett. 86, 2416 (2001)]. Its amplitude corresponds to a field expulsion of the order ${10}^{\ensuremath{-}3}$ and is in agreement with simple expectations taking into account the intertube Josephson coupling.