A NEW PREPARATION METHOD FOR SUPERCONDUCTING ALKALI-METAL-DOPED FULLERENES - COMPARISON OF AC SUSCEPTIBILITY AND LOW-FIELD MICROWAVE-ABSORPTION CHARACTERIZATION

Alkali-metal-doped fullerenes are prepared by a rapid, new method employing alkali-metal azides. By means of ac susceptibility and low-field microwave absorption (LFMA), we have investigated the superconducting phases in K- and Rb-doped C60. With Rb doping, the transition to the superconducting state takes place at T(c) = 38 K when observed by LFMA. The LFMA signal shows significant hysteresis behavior. During a day following the doping process, the LFMA for nominal composition Rb3C60 is found to be drastically time dependent. Measurements by ac susceptibility, on the same sample, show an onset at 34 K which is lower than the T(c) obtained by LFMA but higher than the highest value reported for Rb3C60 by ac susceptibility. This could indicate the presence of another superconducting phase. At 12 K the superconducting fraction is found to be about 95% by ac susceptibility so the material formed by this new preparation method is almost entirely superconducting.