Superconductivity of Rb 2CsC 60 : ac response and upper critical field

Abstract

Ternary alkali metal doped fullerene samples of the composition Rb 2CsC 60 were prepared with optimized synthesis conditions. The superconducting transition occurs at T c = 32.5 K with a shielding fraction of ≈ 72 %. Detailed investigations of the complex susceptibility χ( T) = χ′( T) − iχ″( T) in dependence of different ac and dc magnetic fields are presented. The extrapolation of the upper critical field values B c2(T) measured up to 7 T leads to the zero temperature value B c2 (0) = 50±3) T and the Ginzburg-Landau coherence length ξ GL = (25±1) A ̊ . Furthermore the application of Bean's critical state model allows a first estimation of the critical current densities j c.