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Seminars & Lectures

* TITLE

[APCTP JRG Seminar] Magnetic fluctuation and superconducting-gap symmetry in iron-based superconductors

* SPEAKERS

Name	Affiliation	E-mail
Dr. Hiroaki Ikeda	Kyoto University	Prof. Tetsuya Takimoto (APCTP) takimoto(at)apctp.org

* HOST(Applicant)

Name	Affiliation	E-mail
-

* DATE / TIME

2010-03-09, 10:30am

* PLACE

512 Seminar Room, APCTP Headquarters, POSTECH, Pohang, Korea

* ABSTRACT

The recent discovery of iron-pnictide superconductors with high transition temperature (T_c), some over 50K, opened a highly intensive research field. The appearance of high-T_c superconductivity in close proximity to the anti-ferromagnetic (AF) phase, induced by chemical substitution and applied pressure, reminds us of the cuprates and the heavy-fermion superconductors.
The sign-reversing s_\\pm-wave state, mediated by AF fluctuation, has been suggested as a promising candidate. However, the NMR-1/T_1 study revealed
that the strong AF fluctuation in the undoped case is dramatically suppressed with electron doping, and a pseudogap behavior is observed.
This implies that the pairing mechanism cannot be attributed merely to the AF fluctuations. Motivated thereby, we investigate the overall features of the normal-state properties and the superconducting phase diagram for the effective five-band Hubbard model, obtained from the first-principle band structure calculation in LaFeAsO, on the basis of the fluctuation-exchange approximation.
We find that the NMR-1/T_1 on the hole-doped side is enhanced due to the strong AF fluctuation, and suppressed on the electron-doped side. In contrast, the eigenvalue of the s_\\pm-wave in the Eliashberg equation is relatively insensitive to a carrier doping. Roughly speaking, the gap function is fully-gapped on the hole-doped side, and remarkably anisotropic on the electron-doped side. The overall trends are consistent with the experimental observations.

References
[1] H. Ikeda, J. Phys. Soc. Jpn. 77, 123707 (2008).
[2] H. Ikeda, R. Arita, and J. Kunes, Phys. Rev. B 81, 054502 (2010).
[3] H. Ikeda, R. Arita, and J. Kunes, arXiv:1002.4471.

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