Resistive upper critical field of high- Tc single crystals of Bi2Sr2CaCu2O8

The upper critical field Hc2(T) of Bi2Sr2CaCu2O8 crystals with Tc>92 K has been measured from the out-of-plane resistivity in magnetic fields (H ⊥ ab) up to 15 T. By the use of the empirical procedure the Hc2(T) curve has been extrapolated up to Hc2≃230 T and T/Tc≃0.35 which is independent of the choice of the R/RN ratio. We found that Hc2(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 Tc. Our results together with the heat capacity measurements provide an evidence for the possibility of 2e Bose liquid ground state of high-Tc oxides.