Superconducting diamagnetic fluctuations in ropes of carbon nanotubes

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 K) and low magnetic field (H < 80 Oe), a diamagnetic signal is detectable with an amplitude equal to 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. Gueron, 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(-3) and is in agreement with simple expectations taking into account the intertube Josephson coupling.