Iron pnictides are the only known family of unconventional high-temperature superconductors besides cuprates. Until recently, it was widely accepted that superconductivity is driven by spin fluctuations and intimately related to the fermiology, specifically, hole and electron pockets separated by the same wavevector that characterizes the dominant spin fluctuations, and supporting order parameters (OP) of opposite signs^1,2. This picture was questioned after the discovery of intercalated or monolayer form of FeSe-based systems without hole pockets, which seemingly undermines the basis for spin-fluctuation theory and the idea of a sign-changing OP^{3,4,5,6,7,8,9,10,11}. Using the recently proposed phase-sensitive quasiparticle interference technique, here we show that in LiOH-intercalated FeSe compound the OP does change sign, albeit within the electronic pockets. This result unifies the pairing mechanism of iron-based superconductors with or without the hole Fermi pockets and supports the conclusion that spin fluctuations play the key role in electron pairing.

除铜酸盐外，铁素化物是唯一已知的非常规高温超导体系列。直到最近，人们普遍认为超导是由自旋涨落驱动的，并且与铁电学密切相关，特别是，由同一波矢分隔的空穴和电子口袋表征了主要的自旋涨落，并支持相反符号的有序参数（OP）^{1 ，2}。在发现无孔穴的基于FeSe的系统的插层或单层形式后，此图片受到质疑，这似乎破坏了自旋涨落理论的基础和改变符号的理论^{3、4、5、6、7， 8,9,10,11}。使用最近提出的相敏准粒子干涉技术，我们在这里表明，在LiOH嵌入的FeSe化合物中，OP确实会改变符号，即使在电子口袋中也是如此。该结果统一了有或没有孔费米袋的铁基超导体的配对机理，并支持了自旋涨落在电子配对中起关键作用的结论。