Meissner-effect and microwave-absorption measurements on bulk samples show that ${\mathrm{Rb}}_{\mathit{x}}$${\mathrm{C}}_{60}$ is superconducting with a maximum transition temperature of 28 K. This is a 10-K (60%) increase over the K-doped material. Only ${\mathrm{Ba}}_{0.6}$${\mathrm{K}}_{0.4}$${\mathrm{BiO}}_3$ and the cuprate superconductors have higher transition temperatures.

• Received 26 April 1991

DOI:https://doi.org/10.1103/PhysRevLett.66.2830