Kozo Okazaki, Yoshiaki Ito, Yuichi Ota, Yoshinori Kotani, Takahiro Shimojima, Takayuki Kiss, Shuntaro Watanabe, Chuangtian Chen, Seiji Niitaka, Tetsuo Hanaguri, Hidenori Takagi, Ashish Chainani, Shik Shin
We investigate the band dispersions in FeTe_{0.6}Se_{0.4} (T_c = 14.5 K \sim 1.2 meV) in an accessible range below and above the Fermi level (E_F) using high-resolution laser angle-resolved photoemission spectroscopy. We find an electron band lying just 0.7 meV (\sim 8 K) above E_F at the \Gamma-point. This electron band shows a sharp superconducting coherence peak with gap formation below T_c, quite like the band crossings of the \Gamma centered hole Fermi surface. Further, the estimated values of superconducting gap \Delta and Fermi energy \epsilon_F are significantly different, while the associated Bogoliubov quasiparticle dispersions get merged. The results indicate composite superconductivity in an iron-based superconductor, consisting of strong-coupling Bose-Einstein condensation (BEC) in the electron band while the hole band superconductivity lies closer to the weak-coupling Bardeen-Cooper-Schrieffer (BCS) limit. We discuss the band structure required for a possible route to composite BCS-BEC superconductivity.
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http://arxiv.org/abs/1307.7845
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