UPPER CRITICAL-FIELD STUDY OF ALKALI-METAL AND BA-DOPED FULLERENES

The temperature dependence of the upper critical field B-c2 for Rb3C60 and K2CsC60 is determined by means of a.c, susceptibility measurement s in external magnetic d.c. fields. The sample synthesis was optimized especially by using doubly sublimed C-60 and handling in an atmospher e with low oxygen content. The sample structure was determined by X-ra y 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 C-60 which shows a much smaller slope (dB(c2)(T)/dT)(TC). Furthermore, for all sa mples an analysis concerning the density of states N(E(F)) based on th e upper critical field results is performed.