Abstract
One of the ultimate goals of the study of iron-based superconductors is to identify the common feature that produces the high critical temperature (Tc). In the early days, based on a weak-coupling viewpoint, the nesting between hole- and electron-like Fermi surfaces (FSs) leading to the so-called $s\pm$ state was considered to be one such key feature. However, this theory has faced a serious challenge ever since the discovery of alkali-metal-doped FeSe (AFS) superconductors, in which only electron-like FSs with a nodeless superconducting gap are observed. Several theories have been proposed, but a consistent understanding is yet to be achieved. Here we show experimentally that a hole-like band exists in KxFe2-ySe2, which presumably forms a hole-like Fermi surface. The present study suggests that AFS can be categorized in the same group as iron arsenides with both hole- and electron-like FSs present. This result provides a foundation for a comprehensive understanding of the superconductivity in iron-based superconductors.
Highlights
Band-structure calculations predict that iron arsenide superconductors have holeand electron-like Fermi surfaces (FSs) at the Brillouin zone (BZ) center and corner, respectively1) [Fig. 1(a)]
Theoretical studies based on a weak-coupling approach have proposed that spin fluctuation arising from FS nesting induces the s± superconducting state, where the gap function has s-wave symmetry with its sign reversed between the hole- and electron-like FSs.3,4) This is consistent with the nearly isotropic s-wave superconducting gap in ARPES2) and the spin resonance mode at the nesting vector observed in inelastic neutron scattering (INS) measurements.5)
In KxFe2-ySe2, it is known that a minority superconducting phase (~10%) and a majority insulating phase (~90%) coexist due to intrinsic phase separation.27,28) Very recently, an increased area of the superconducting phase (~30%) has been found in a K0.62Fe1.7Se2 single crystal obtained by a one-step method with the quenching technique.29) there is a possibility that the ARPES measurement of K0.62Fe1.7Se2 will detect an intrinsic electronic structure responsible for the superconductivity, which has eluded previous ARPES experiments because of the smaller superconducting region in the sample surface
Summary
Band-structure calculations predict that iron arsenide superconductors have holeand electron-like Fermi surfaces (FSs) at the Brillouin zone (BZ) center and corner, respectively1) [Fig. 1(a)]. 2(d)-2(f)], we observed a small electron pocket (α) and a hole-like band (γ).
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