Alkali-Fulleride Superconductors: Synthesis, Composition, and Diamagnetic Shielding

The recent report ora superconductivity onset near the critical temperature T$_c$ = 18 K in potassium-doped C$_{60}$ raises questions concerning the composition and stability of the superconducting phase. The effects of mixing and heat treatment of K$_x$ C$_{60}$ samples prepared over a wide range of initial compositions on the superconducting transition was determined from shielding diamagnetism measurements. A single superconducting phase (T$_c$ = 19.3 K) occurs for which the composition is K$_3$C$_{60}$. The shielding reaches a maximum of greater than 40 percent of the perfect diamagnetism, a high value for a powder sample, in samples prepared from 3:1 mixtures. A Rb$_x$C$_{60}$ sample prepared and analyzed in an analogous way exhibited evidence for superconductivity with T$_c$ = 30 K and a diamagnetic shielding of 7 percent could be obtained.