铁基超导体能隙结构和机理的统一认识

铁基超导体作为除铜氧化物超导体之外的第二类高温超导体，自从被发现以来其超导电性的成因就备受关注。早期有人针对同时具有空穴型费米口袋和电子型费米口袋的铁砷基超导体提出了S^±配对模式，而且得到中子散射和隧道谱实验的初步支持。这一配对模型的基础是需要在布里渊区同时具有空穴型和电子型费米面。但是铁基超导体的费米面因材料而异，新发现的很多铁硒基超导体因缺乏空穴型费米面，对S^±电子配对模型来说是一个挑战。文章综述了过去几年来作者在这方面取得的工作进展。运用高精度的扫描隧道显微镜，在不同铁基超导体系中，针对超导能隙结构，或超导序参量的性质进行了仔细研究。首先作者在铁砷基超导体NaFe_1-xCo_xAs和Ba_1-xK_xFe ₂As₂中利用无磁性杂质态测量和隧道谱测量，发现S^±的明确证据。进一步，在只有电子型费米面的(Li_1−xFe_x)OHFeSe超导材料中发现两个各向异性的超导能隙，利用准粒子相干散射实验第一次把这两个超导能隙对应到由电子型费米面套叠或杂化后形成的内外两套费米面上。通过非磁性杂质诱导产生的能隙内杂质态和新型电子驻波相位敏感实验的探测，证明该材料中超导能隙符号也发生反转。因此作者的系列工作统一了有和没有空穴型费米面的铁基超导体的能隙形式，支持排斥势是导致电子配对和超导电性产生的关键因素。

Unified picture for the order parameter and mechanism of iron based superconductors

Iron-based superconductors are the only family of high-temperature unconventional superconductors besides cuprates, and their pairing symmetry has attracted considerable attention since their discovery. The widely accepted pairing model is S^±, which requires both hole and electron pockets. However, this picture was questioned after the discovery of the intercalated or monolayer form of FeSe-based systems which have no hole pockets.We have conducted a series of experiments to study the characteristics of the gap structure or order parameter in various iron-based superconductors with different Fermi surface topologies. In two typical FeAs-based superconductors, namely NaFe_1-xCo_xAs and Ba_1-xK_xFe₂As₂, by measuring the scanning tunneling spectra or detecting non-magnetic impurity induced bound states, we found clear evidence supporting the S^± pairing. Concerning the systems without hole pockets, we took the (Li_1−xFe_x)OHFeSe system as a platform and conducted extensive investigations. Two anisotropic superconducting gaps were observed in (Li_1−xFe_x)OHFeSe by our scanning tunneling microscopy measurements. Detailed experimental data based on the Fourier transformed quasiparticle interference (FT-QPI) allowed us for the first time to assign two superconducting gaps to the outer and inner electron Fermi pockets after folding or hybridization. In addition, a sign reversal of the superconducting order parameter in (Li_{1 − x}Fe_x)OHFe_1-yZn_ySe is evidenced according to the further data and analysis based on the in-gap bound state from a non-magnetic impurity as well as the phase-sensitive analysis of FT-QPI data. These results provide a unified picture for the gap structure and pairing mechanism of iron-based superconductors with or without Fermi hole pockets and support the conclusion that repulsive interaction plays the key role in electron pairing.