Kinetic Energy Driven Superconductivity in the Electron Doped Cobaltate Na _x CoO ₂ · y H ₂ O

Abstract

Within the charge-spin separation fermion-spin theory, we have shown 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. The discovery of superconductivity in the doped cobaltate Na 0.35 CoO 2 · 1.3H 2 O has engendered great interest in transition metal oxides 1 . Although the superconducting (SC) transition temperature is much less than that in the doped cuprate superconductors, both systems share many common features. The doped cobaltate has a lamellar structure consisting of the two-dimensional (2D) CoO 2 layers separated by a thick insulating layer of Na + ions, where the one-half spin Co 4+ ions are arranged on a triangular lattice, and it becomes a superconductor after hydration that significantly enhances the distance between CoO 2 layers 1 . This structure is similar to the cuprate superconductors in the sense that they also have a layered structure of the square lattice of the CuO 2 plane separated by insulating layers 2 . The undoped cuprate is a Mott insulator with the antiferromagnetic (AF) longrange-order (AFLRO), then changing the carrier concentration by ionic substitution or increasing the oxygen content turns these compounds into the SC state leaving the AF short-range correlation still intact 2,3 . The optimal doping occurs at 0.15 holes (electrons) added to the half-filled band of the parent compound 2,3 . However, Na x CoO 2 · yH 2 O is viewed as an electron doped Mott insulator, where superconductivity appears with electron doping 1 . The optimal doping for superconductivity occurs at 0.3 electrons per Co above the ground-state Na 0.3 CoO 2 · 1.3H 2 O, which is a half-filled two-electron t 2g derived band 4,5 . Although the ferromagnetic correlation is present in Na x CoO 2 for the large electron doping concentration (x ≈ 0.7) 6 , the AF short-range spin correlation in Na x CoO 2 and Na x CoO 2 · yH 2 O in the low electron doping concentration (δ ≈ 0.35) has been observed from nuclear quadrupolar resonance and thermopower as well as other experimental measurements 1,4,5,7,8 . Moreover, the SC transition temperature displays the same kind of doping controlled behavior that is observed in the doped cuprates, i.e., the optimal SC transition temperature ...