Resistive upper critical field of high-T-c single crystals of Bi2Sr2CaCu2O8

The upper critical field H-c2(T) of Bi2Sr2CaCu2O8 crystals with T-c > 92 K has been measured from the out-of-plane resistivity in magnetic fields (H perpendicular to ab) up to 15 T. By the use of the empirical procedure the H-c2(T) curve has been extrapolated up to H-c2 similar or equal to 230 T and T/T-c similar or equal to 0.35 which is independent of the choice of the R/R(N) ratio. We found that H-c2(T) does not follow the conventional theory with or without fluctuations but is consistent with the prediction based on the Bose-Einstein condensation of charged bosons formed above T-c. Our results together with the heat capacity measurements provide an evidence for the possibility of 2e Bose liquid ground state of high-T-c oxides.