(Li0.8Fe0.2)OHFeSe is a newly-discovered intercalated iron-selenide superconductor with a Tc above 40 K, which is much higher than the Tc of bulk FeSe (8 K). Here we report a systematic study of (Li0.8Fe0.2)OHFeSe by low temperature scanning tunneling microscopy (STM). We observed two kinds of surface terminations, namely FeSe and (Li0.8Fe0.2)OH surfaces. On the FeSe surface, the superconducting state is fully gapped with double coherence peaks, and a vortex core state with split peaks near EF is observed. Through quasi-particle interference (QPI) measurements, we clearly observed intra- and inter-pocket scatterings in between the electron pockets at the M point, as well as some evidence of scattering that connects gamma and M points. Upon applying magnetic field, the QPI intensity of all the scattering channels are found to behave similarly. Furthermore, we studied impurity effects on the superconductivity by investigating intentionally introduced impurities and intrinsic defects. We observed that magnetic impurities such as Cr adatoms can induce in-gap states and suppress superconductivity. However, nonmagnetic impurities such as Zn adatoms do not induce visible in-gap states. Meanwhile, we show that Zn adatoms can induce in-gap states in thick FeSe films, which is believed to have an (s+-)wave pairing symmetry. Our experimental results suggest it is likely that (Li0.8Fe0.2)OHFeSe is a plain s-wave superconductor, whose order parameter has the same sign on all Fermi surface sections.
Robust superconductivity and transport properties in (Li1-xFex)OHFeSe single crystals
最近发现的（Li ${_{1-x}}$ Fe ${_x}$）OHFeSe超导体（$T_c$约为40 K）为研究FeSe基超导体的磁化和电输运性质提供了一个很好的平台。采用水热离子交换法成功地生长了（Li ${_{1-x}}$ Fe ${_x}$）OHFeSe晶体。X射线衍射分析表明，样品为PbO型单晶结构，具有c轴择优取向。磁化率和电阻测量表明，超导转变的起始温度约为${T_c}$ =38.3K。利用磁滞回线和Bean临界态模型，在低温区观察到大的临界电流${J_s}$。临界电流密度随磁场的增大呈指数规律地减小。测量了不同电流和磁场下电阻率的温度依赖性，揭示了强大的超导电流密度和体超导性。
Superconductivity and Phase Diagram in (Li$_{0.8}$Fe$_{0.2}$)OHFeSe$_{1-x}$S$_x$
采用水热法合成了一系列（Li $_{0.8}$ Fe $_{0.2}$）OHFeSe $_{1-x}$ S $_x$（0 $\leq$ x $\leq$ 1）样品，并绘制了相图.磁化率表明，由（Li $_{0.8}$ Fe $_{0.2}$）OH层产生的反铁磁性与超导电性共存，反铁磁性转变温度在不同S掺杂水平下几乎保持不变.此外，a和c轴的晶格参数减小和超导转变温度T $_c$逐渐被抑制与S的Se的替代，并最终在$x$ = 0.90的超导电性消失。T $_c$的降低可归因于S的离子尺寸相对于Se的离子尺寸较小引起的化学压力的影响，这与流体静压对（Li $_{0.8}$ Fe $_{0.2}$）OHFeSe的影响一致。但对$T_{\rm c}$与晶体学事实之间关系的详细研究表明，$T_{\rm c}$对阴离子离Fe_2层高度或$Ch$-Fe_2- $Ch$角的依赖性与FeAs基超导体中的依赖性非常不同。
Ab-initio Studies of (Li$_{0.8}$Fe$_{0.2}$)OHFeSe Superconductors: Revealing the Dual Roles of Fe$_{0.2}$ in Structural Stability and Charge Transfer
最近发现的（Li $_{0.8}$ Fe $_{0.2}$）OHFeSe超导体为探索FeSe衍生体系中高$T_c$超导性的微观机制提供了一个新的平台。使用密度泛函理论计算，我们首先表明，由Fe取代Li不仅显着增强（Li $_{0.8}$ Fe $_{0.2}$）OH间隔层和FeSe超导层之间的吸引力沿\emph{c}轴，但也最大限度地减少了两者之间的晶格失配在\emph{ab}平面，都有利于稳定的整体结构。接下来，我们探讨了从间隔层到FeSe的电子注入，并明确地确定Fe $_{0.2}$组分是显著增强的层间电荷转移的主要原子来源。我们进一步揭示，该系统强烈有利于共线反铁磁有序的FeSe层，但间隔层可以是反铁磁或铁磁的Fe $_{0.2}$的空间分布而定。基于这些理解，我们还预测（Li $_{0.8}$ Co $_{0.2}$）OHFeSe具有更大的电子注入和潜在的更高的$T_c$的结构稳定性。
Simultaneous emergence of superconductivity, inter-pocket scattering and nematic fluctuation in potassium-coated FeSe superconductor
超导性起源于电子配对。因此，费米面上的配对沟道和配对胶是理解超导体的两个关键问题。最近，在电子掺杂的FeSe超导体中发现了超过40 K的高温超导性，包括K $_x$ Fe $_{2-y}$ Se $_2$、Li $_{0.8}$ Fe $_{0.2}$ OHFeSe和单层FeSe薄膜。然而，它们的配对机制仍然存在争议。本文研究了包钾FeSe单晶电子结构的系统演化。通过在样品表面上原位蒸发钾来精确控制掺杂水平。我们发现当费米面的Lifshitz跃迁使两个电子袋之间的袋间散射打开时，超导性就出现了。在相同的掺杂浓度下，电子有序度显著降低，在较宽的掺杂范围内，电子有序度仍有较大的涨落。我们的研究结果表明，在电子掺杂的FeSe超导体的超导电性，袋间散射，和涨落之间的潜在的相关性。
Evidence for d-wave superconductivity in single layer FeSe/SrTiO3 probed by quasiparticle scattering off step edges
de Gennes外推长度是超导体边界附近配对能隙空间演化的方向依赖性度量，因此提供了探测其对称性的可行手段。对于平面s波对，它可能是无限的和各向同性的，而对于d波对，它可能是有限的和各向异性的。在这里，我们合成单层FeSe薄膜SrTiO 3（001）（STO）衬底上的分子束外延，并测量德热内外推长度的扫描隧道显微镜/光谱。我们发现镜面[110]Fe边缘附近的超导能隙减少了40%，得到的外推长度为8.0 nm。然而，在镜面[010]Fe边缘附近，外推长度几乎是无限的。这些发现与具有2重对称性的相变配对一致，表明d波超导性。这进一步得到了镜面[110]Fe边缘附近的带隙状态的存在的支持，但不是[010]Fe边缘。这一工作为单层FeSe/STO中d波超导电性提供了直接的实验证据，并证明了准粒子边界散射是铁基超导体中对对称性的一种可行的相敏探针。
