IRREVERSIBILITY LINES OF RB3C60 FULLERENE

The irreversibility line H-irr(T) of isotropic superconducting Rb3C60 fullerene (T-c = 30.5 +/- 0.1 K) was obtained from M(T) curves for low fields and M(H) curves for high fields, respectively. It can be descr ibed by a simple power-law equation, H-irr(T) H-irr(0)[1-T/T-o](gamma) . Based on experimental data the best fitting results to this scaling relation are determined as: gamma = 2.04 +/- 0.16, H-irr(0) = 47.0 +/- 5.2 T, and T-o = 30.4 +/- 0.2 K for H-irr less than or equal to 1 T ( T less than or equal to 0.8T(c)); and gamma = 1.50 +/- 0.03, H-irr(0) = 89.8 +/- 1.6 T, and T-o = 30.9 +/- 0.4 K for 1 T less than or equal to H-irr less than or equal to 5 T (T < 0.2T(c)). The characteristic c rossoverfield of H similar to 1 T separates the two regions of differe nt gamma values. The existence of gamma value near 2 suggests that the irreversibility line close to T-c is possibly due to thermally activa ted vortex-lattice melting that may have occurred in our Rb3C60 sample . The other value of gamma = 1.5 may be attributed to a vortex-glass p hase transition in low temperatures.