Article

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

04/2004;

Source: arXiv

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**ABSTRACT:**Motivated by the recent discovery of superconductivity in NaxCoO2.yH2O , we use series expansion methods and cluster mean-field theory to study spontaneous charge order, Néel 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 tPhysical review. B, Condensed matter 07/2004; 70. · 3.66 Impact Factor - [Show abstract] [Hide abstract]

**ABSTRACT:**We show that finite angular momentum pairing chiral superconductors on the triangular lattice have point zeroes in the complex gap function. A topological quantum phase transition takes place through a nodal superconducting state at a specific carrier density $x_c$ where the normal state Fermi surface crosses the isolated zeros. For spin singlet pairing, we show that the second nearest neighbor $d+id$-wave pairing can be the dominant pairing channel. The gapless critical state at $x_c\simeq0.25$ has six Dirac points and is topologically nontrivial with a $T^3$ spin relaxation rate below $T_c$. This picture provides a possible explanation for the unconventional superconducting state of Na$_x$CoO$_2\cdot y$H$_2$O. Analyzing a pairing model with strong correlation using the Gutzwiller projection and symmetry arguments, we study these topological phases and phase transitions as a function of Na doping.12/2007; - [Show abstract] [Hide abstract]

**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.Physical Review Letters 08/2004; 93(7). · 7.73 Impact Factor

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