MULTI-CRITICAL BEHAVIOR OF IRREVERSIBILITY LINES FOR RB3C60 FULLERENES

The irreversibility lines of polycrystalline and single-crystal Rb3C60 fullerenes determined by M(T) curves and M(H) loops are well characte rized by a simple 3-dimensional power law function: H-irr(T) = H-0(1 - T/T-0)(n), with various n values obtained in different field regions and for different specimens. For polycrystalline Rb3C60 powders, the i rreversibility field in the low field region (Hi,, = 1 T) obeys the po wer function with n = 2, which suggests that a vortex-lattice melting transition may have occurred. However, in the high field region (H-irr > 1 T), n = 3/2. This reduced exponent may be explained by the model of thermally activated vortex motion with collective pinning. For Rb3C 60 crystals, the irreversibility line can be perfectly fitted to a lin ear relation, i.e., n = 1 : H-irr(T) = a(1 - T/T-0), where a = 4.8 T a nd T-0 = 60.0 K for H < 3.2 T; and a = 7.92 T and T-0 = 30.3 K for H > 3.2 T. This is the first reported linear temperature-dependence of th e irreversibility field.