Extended s-wave pairing originating from the a_1g band in Na_xCoO₂·yH₂O: Single-band U-V model with fluctuation exchange method

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Kazuhiko Kuroki,¹ Seiichiro Onari,² Yukio Tanaka,² Ryotaro Arita,^3,4 and Takumi Nojima¹
¹Department of Applied Physics and Chemistry, The University of Electro-Communications, Chofu, Tokyo 182-8585, Japan
²Department of Applied Physics, Nagoya University, Nagoya, 464-8603, Japan
³Max Planck Institute for Solid State Research, Heisenbergstrasse 1, Stuttgart 70569, Germany
⁴Department of Physics, University of Tokyo, Hongo 7-3-1, Tokyo 113-0033, Japan

Received 24 February 2006; published 2 May 2006

Motivatedby recent experiments and band calculation results, we investigate thepossibility of superconductivity originating from the a_1g band of Na_xCoO₂·yH₂Oassuming the absence of e <sub>g</sub><sup>[prime]</sup> pockets. We adopt a singleband U-V model that takes into account the local minimumof the a_1g band dispersion at the Gamma point. Usingthe fluctuation exchange method, we find that an unconventional s-wavepairing with a gap that changes sign between the innerand the outer Fermi surfaces is enhanced due to thecoexistence of spin fluctuations at wave vectors bridging the twoFermi surfaces and the charge fluctuations near the K point.Possible relevance to the experimental results is discussed.

URL:	http://link.aps.org/doi/10.1103/PhysRevB.73.184503
DOI:	10.1103/PhysRevB.73.184503
PACS:	74.20.Mn; 71.10.Fd; 74.20.Rp; 74.25.Ha 74.20.Mn Nonconventional mechanisms of superconductivity including spin fluctuations, polarons and bipolarons, resonating valence bond model, anyon mechanism, marginal Fermi liquid, Luttinger liquid, etc 71.10.Fd Lattice fermion models (condensed matter) including Hubbard model, etc 74.20.Rp Pairing symmetries (other than s-wave) in superconductivity theory 74.25.Ha Magnetic properties of superconductors YEAR: 2006
KEYWORDS:	sodium compounds, type II superconductors, superconducting energy gap, spin fluctuations, Fermi surface

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