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Laser-ARPES study of band dispersions and superconductivity in optimally doped FeTe0.6Se0.4 : Possible route to BCS-BEC superconductivity

The Bardeen-Cooper-Schrieffer (BCS) theory of superconductivity for electrons pairing in momentum-space, and the superfluidity due to Bose-Einstein condensation(BEC) of atoms paired in real-space, are connected by the so-called BCS-BEC crossover and has been recently discussed for Fe-based superconductivity[1]. We investigate the band dispersions in optimally doped FeTe0.6Se0.4 (TC = 14.5 K 1.2 meV) in an accessible range below and above the Fermi level (EF) using ultra-high resolution laser angle-resolved photoemission spectroscopy (ARPES). We find an electron band lying just 0.7 meV( 8 K) above EF at the Γ-point and this band shows a sharp superconducting coherence peak with gap formation below TC. For the hole band, in contrast to earlier ARPES studies but consistent with thermodynamic results, the momentum dependence shows a cos(4φ) modulation of the SC-gap anisotropy [2a]. The estimated values of superconducting gap (Δ) and Fermi energy (ϵF) indicate composite superconductivity in FeTe0.6Se0.4, suggestive of strong-coupling BEC in the electron band while the hole band superconductivity lies closer to the weak-coupling BCS limit. The study identifies a possible route to BCS-BEC superconductivity [2b].


[1] Y. Lubashevsky, E. Lahoud, K. Chashka, D. Podolsky, and A. Kanigel, Nat. Phys. 8, 309 (2012).

[2] K. Okazaki, Y. Ito, Y. Ota, Y. Kotani, T. Shimojima, T. Kiss, S. Watanabe, C. -T. Chen, S. Niitaka, T. Hanaguri, H. Takagi, A. Chainani, and S. Shin, Phys. Rev. Lett. 109, 237011(2012) ; Scientific Reports 4, 4109 ; DOI:10.1038/srep04109 (2014).