Kinetic Energy Driven Superconductivity in the Electron Doped Cobaltate NaxCoO2·yH2O

Within the charge-spin separation fermion-spin theory, we show that the mechanism of superconductivity in the electron doped cobaltate NaxCoO2·yH2O is ascribed to its kinetic energy. The dressed fermions interact occurring directly through the kinetic energy by exchanging magnetic excitations. This interaction leads to a net attractive force between dressed fermions, then the electron Cooper pairs originating from the dressed fermion pairing state are due to the charge-spin recombination, and their condensation reveals the superconducting ground state. The superconducting transition temperature is identical to the dressed fermion pair transition temperature, and is suppressed to a lower temperature due to the strong magnetic frustration. The optimal superconducting transition temperature occurs in the electron doping concentration δ≈0.29, and then decreases for both underdoped and overdoped regimes, in qualitative agreement with the experimental results.