VORTEX-LATTICE MELTING IN SUPERCONDUCTING FULLERENE RB3C60

The temperature and held dependences of the dc magnetization have been characterized on a sample of Rb3C60 fullerene with T-c=30.5 K. The sa mple exhibits a rather large critical current density of about 2x10(6) A/cm(2) at T=5 K and H=0 T. An irreversibility line H-irr(T) is defin ed by merging points of two sets of zero-field-cooling and field-cooli ng magnetization curves. The temperature dependence of the irreversibi lity field is in fairly good agreement with a power-law relation: H-ir r(T)=H-irr(0)[1 -T(H)/T-0](gamma). The best fit of experimental data t o this equation provides the following values: gamma=2.04/-0.16, H-irr (0)=470+/-52 kOe, and T-0=30.4+/-0.2 R. According to the nonlocal elas ticity theory based on the Lindemann criterion, gamma=2 strongly sugge sts the occurrence of thermally activated vortex-lattice melting in ou r Rb3C60 sample. The Lindemann number of c less than or equal to 0.074 is comparable to those of conventional type-II superconductors, but i s much smaller than those for high-T-c oxides. Our data provide eviden ce of vortex-lattice melting in the superconducting fullerenes.