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Kinetic energy driven superconductivity in the electron doped cobaltate Na$_{x}$CoO$_{2}\cdot y$H$_{2}$O

Within the charge-spin separation fermion-spin theory, we have shown that the mechanism of superconductivity in the electron doped cobaltate Na$_{x}$CoO$_{2}\cdot y$H$_{2}$O 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 $δ\approx 0.29$, and then decreases for both underdoped and overdoped regimes, in qualitative agreement with the experimental results.