April 7th, 2009
Tohoku Univ. WPI-AMIR
Tohoku Univ. Faculty of ScienceJSTDirect determination of superconducting-pairing symmetry inelectron- and hole-doped iron-based superconductors-Important step toward complete understanding of superconducting mechanism-＜Content＞
The research group headed by Prof. T. Takahashi in WPI and Dr. T. Sato in Department ofPhysics has revealed the superconducting mechanism of new iron-based high-temperature
superconductors. They have succeeded in determining the superconducting-pairing symmetry by using
ultrahigh-resolution angle-resolved photoemission spectroscopy.
The research result is published online in Proceedings of National Academy of Sciences (PNAS)
in the week of April 6th in the U.S. eastern time.
＜Background＞
After the discovery of iron-based high-temperature superconductor LaFeAsO1-xFx by Prof. Hosono inTokyo Institute of Technology in February 2008, intensive researches have been performed to elucidate
the mechanism of superconductivity and also to search for new superconducting materials with higher
superconducting transition temperatures (Tc). The Tc value has already exceeded 50 K and is actually
the highest except for copper-oxide superconductors discovered 23 years ago. Characteristic propertiesof iron-based superconductors are (i) electrons of iron atoms, which are believed to be the foe of thesuperconductivity, play a central role in realizing the superconductivity, and (ii) the superconductivity
emerges by both hole- and electron-doping into the parent compound.

To elucidate thesuperconducting mechanism, intensive researches to determine the pairing symmetry of Cooper pairs
are now urgently in progress because the pairing symmetry is the key ingredient to establish thesuperconducting mechanism. One of essential questions in iron-based superconductors is whether theparing symmetry is the same or not between electron- and hole-doped materials. This problem iscrucial in understanding the superconducting mechanism of iron-based superconductors, but has not1

been resolved yet.

＜Research＞
The research group of Tohoku University has directly observed the superconducting gap of anelectron-doped Fe-based superconductor BaFe2-xCoxAs2 and has succeeded for the first time in directly
determining the pairing symmetry. The pairing symmetry exhibits a s-wave character as in thehole-doped counterpart. The present result unambiguously indicates the electron-hole symmetry andthe multi-band nature of superconductivity in iron-based superconductors.
＜Future aspect＞
The present research result will certainly contribute to establishing of the model to explain thehigh-temperature superconductivity in iron-based materials. In addition, the present result definitely
gives a clear strategy to synthesize a new iron-based superconductor with a higher Tc based on themulti-band s-wave mechanism.
This project is supported by the CREST-JST and TRiP-JST.

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＜Contact＞＜About research＞
Professor Takashi Takahashi
WPI-AMIR, Tohoku University
Tel：022-795-6417
E-mail：t.takahashi＠arpes.phys.tohoku.ac.jp
Assistant Professor Takafumi Sato
Department of Physics, Tohoku University
Tel：022-795-6477
E-mail：t-sato＠arpes.phys.tohoku.ac.jp＜About JST＞
About CREST
Hiroyuki KanekoJST
Tel: 03-3512-3524 Fax：03-3222-2064
E-mail: crest@jst.go.jp
About TRiP
Takayuki Saito
JST
Tel: 03-3512-3528
E-mail: pnictide@jst.go.jp

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＜Figures＞

Fig.1: Angle-resolved photoemission spectroscopy of superconductors

Fig.2: Electronic structure and superconducting pairing symmetry ofelectron-doped FeAs superconductor.
Top: Fermi-surface
dependence of the magnitude of superconducting gap.
Superconducting gap shows s-wave symmetry with the size of 6.5 and5.0 meV on the β Fermi surface at Γ point, and electronlike Fermi
surfaces at M point, respectively. Bottom: Band structure andFermi surface of electron-doped FeAs superconductor.

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