Role of spin-orbit coupling on the spin triplet pairing in Na_{x}CoO_{2}yH_{2}O II: Multiple phase diagram under the magnetic field

Abstract

The possibility of multiple phase diagram in the novel superconductor Na_{x}CoO_{2}yH_{2}O is analyzed on the basis of the multi-orbital Hubbard model including the atomic spin-orbit coupling. We have shown that the spin triplet pairing state is stable in this model. The p-wave (f-wave) state is stabilized when the Hund's rule coupling is large (small). In the precedent paper, we have determined the direction of d-vector at T=Tc and H=0 within the linearized Dyson-Gorkov equation. In this paper, the pairing state below Tc and under the magnetic field is determined within the weak coupling approximation including the paramagnetic effect. We find that the p+f coexistent state is stabilized at low temperatures in a part of parameter range. We point out that the phase diagram in the H-T plane is quite different between the p-wave, f-wave and p+f-wave superconductivities. The characteristics of each phase are clarified by showing the magnetic susceptibility and specific heat. We discuss the comparison with experimental results and suggest some future experiments to detect the multiple phase transition.

arXiv comment14 pages, 15 figures

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• Valence and Na content dependences of superconductivity in NaxCoO2.yH2O
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• In-plane ferromagnetism in charge-ordering $N{a}_{0.55}Co{O}_2$
• Electron-phonon interaction dressed by electronic correlations near charge ordering as the origin for superconductivity in cobaltates
• 59Co, 23Na, and 1H NMR Studies of Double-Layer Hydrated Superconductors NaxCoO2-yH2O
• In-plane Anisotropy of the Magnetic Fluctuations in NaxCoO2-yH2O