THE TREND OF MAGNETIC IRREVERSIBILITY FOR RB3C60 AND ITS IMPLICATIONS

The irreversibility magnetic field (H-irr) as a function of the irreve rsibility temperature (T-irr) for two-dimensional superconductors, e.g ., the ones with a perovskite structure, can be characterized by an ex ponential law. However, with increased thermal agitation toward T-c, t he coupling strength between adjacent superconducting (i.e., Cu-O-2) l ayers would impart a three-dimensional like character. As a result, th e exponential law may be approximated by a simpler power law. The full erene superconductors are intrinsically isotropic so their H-irr trend s appear to follow the power law throughout the entire temperature ran ge. For example, our powdered Rb3C60 sample showed such a trend with a highly three-dimensional character. Moreover, a well crystallized Rb3 C60 sample exhibits an ideal isotropic feature as manifested by the li near dependence of H-irr on T-irr. Furthermore, this linearity may be represented by two superimposed straight lines that intersect the T-ir r axis at two T-0 values (30.5 K and 60.0 K). For most superconductors , T-0 is indicative of T-c. The cause of two T-0 values for our Rb int ercalated fullerene crystals is not known. If they represent two T-c v alues of superconducting phases, then, the phase with T-c = 30.5 K cor responds to the well-known composition of Rb3C60. However, the phase w ith T-c = 60.0 K may possess a different composition designated as Rbx C60 with x possibly higher than 3.