Unconventional superconductivity in Na$_{0.35}$CoO$_{2}\cdot$1.3D$_{2}$O and proximity to a magnetically ordered phase

Source: arXiv


Muon spin relaxation ($\mu$SR) measurements on the new layered cobalt oxide superconductor Na$_{0.35}$CoO$_{2}\cdot$1.3H$_{2}$O and its parent, non-superconducting compounds, have revealed unconventional nature of superconductivity through: (1) a small superfluid energy which implies a surprisingly high effective mass of the charge carriers, approximately 100 times the bare electron mass; (2) the superconducting transition temperature $T_{c}$ scaling with the superfluid energy following the correlations found in high-$T_{c}$ cuprate and some other two-dimensional superconductors; (3) an anisotropic pairing without broken time-reversal symmetry; and (4) the proximity of a magnetically ordered insulating phase at Na$_{0.5}$CoO$_{2}$ below $T_{N}$ = 53 K.

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Available from: Graeme Luke, Apr 05, 2013
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    ABSTRACT: Motivated by the recent discovery of superconductivity in NaxCoO2.yH(2)O, we use series expansion methods and cluster mean-field theory to study spontaneous charge order, Neel order, ferromagnetic order, dimer order and phase-separation in the triangular-lattice t-J-V model at 2/3 electron density. We find that, for t<0, the charge ordered state, with electrons preferentially occupying a honeycomb lattice, is very robust. Quite surprisingly, hopping to the third sublattice can even enhance Neel order. At large negative t and small V, the Nagaoka ferromagnetic state is obtained. For large positive t, charge and Neel order vanish below a critical V, giving rise to an itinerant antiferromagnetically correlated state.
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    ABSTRACT: We propose that the spin-triplet pairing mechanism due to disconnected Fermi surfaces proposed in our previous study [Phys. Rev. BPRBMDO0163-1829 63, 174507 (2001)10.1103/PhysRevB.63.174507] may be at work in a recently discovered superconductor NaxCoO2·yH2O. We introduce a single band effective model that takes into account the pocketlike Fermi surfaces along with the van Hove singularity near the K point found in the band calculation results. Applying the fluctuation exchange method and solving the linearized Éliashberg equation, the most dominant pairing is found to have spin-triplet f-wave symmetry, where the nodes of the gap function do not intersect the pocket Fermi surfaces. The presence of finite Tc is suggested in sharp contrast to cases when the gap nodes intersect the Fermi surface.
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    ABSTRACT: The low-energy Fermi-surface states in layered cobaltate were analyzed from the Mott insulating unit. Investigations show that the fermi-surface topology at low doping were strongly influenced by the spin-orbit coupling of the correlated electrons at t2g level. It was found that the low-energy electronic states of the CoO2 layer were derived from the Kramers pseudospin doublets of the Co4+ ion. The results show that the projection of the J interaction on the pseudospin states leads to the novel pseudospin-triplet pairing.
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