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

摘要

The temperature dependence of the upper critical field B_c2 for Rb_3C_60 and K_2CsC_60 is determiend by means of a.c. susceptibility measurements in external magnetic d.c. fields. The sample synthesis was optimized especially by using doubly sublimed C_60 and handling in an atmosphere with low oxygen content. The sample structure was determined y X-ray diffraction. Using the Werthamer-Helfand-Holhenbeg (WHH) theory, the zero-temperature values of the upper critical field and the Ginzburg-Landau coherenece lengths were estimated. The results are discussed in comparison with those of K_3C_60~- and Rb_2CsC_60~- and Ba-doped C_60, which shows a much smaller slope (dB_c(T)/dT)_Tc. Furthermore, for all samples an analysis concerning the density of states N(E_F) based on the upper critical field results is performed.