Upper critical field study of alkali-metal and Ba-doped fullerenes

The temperature dependence of the upper critical field Bc2 for Rb3C60 and K2CsC60 is determined by means of a.c. susceptibility measurements in external magnetic d.c. fields. The sample synthesis was optimized especially by using doubly sublimed C60 and handling in an atmosphere with low oxygen content. The sample structure was determined by X-ray diffraction. Using the Werthamer-Helfand-Hohenberg (WHH) theory, the zero-temperature values of the upper critical field and the Ginzburg-Landau coherence lengths were estimated. The results are discussed in comparison with those of K3C60- and Rb2CsC60- and Ba-doped C60, which shows a much smaller slope (dB c2(T)/dT)Tc. Furthermore, for all samples an analysis concerning the density of states N(EF) based on the upper critical field results is performed.