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One of the ultimate goals of the study of iron-based superconductors is toidentify the common feature that produces the high critical temperature (Tc).In the early days, based on a weak-coupling viewpoint, the nesting betweenhole- 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 faceda serious challenge ever since the discovery of alkali-metal-doped FeSe (AFS)superconductors, in which only electron-like FSs with a nodelesssuperconducting gap are observed. Several theories have been proposed, but aconsistent understanding is yet to be achieved. Here we show experimentallythat a hole-like band exists in KxFe2-ySe2, which presumably forms a hole-likeFermi surface. The present study suggests that AFS can be categorized in thesame group as iron arsenides with both hole- and electron-like FSs present.This result provides a foundation for a comprehensive understanding of thesuperconductivity in iron-based superconductors.

Observation of a Hidden Hole-Like Band Approaching the Fermi Level in K-Doped Iron Selenide Superconductor