高分辨率角分辨光电子能谱对单原子层 FeSe 薄膜高临界温度超导体的电子结构研究

The recent discovery of high-temperature superconductivity with a Tc higher than 77K in FeSe monolayer has attractted great attention. With the simplest crystal structure and chemical formula, it is a superior member among the Fe-based superconductors discovered so far. Therefore, it might become a model system in exploring the iron-based superconductivity mechanism. In this project, we propose to make a systematic and deep investigation on the electronic structure of the various few-layer FeSe films, by using the VUV laser-based angle resolved photoemission spectroscopy system and the deep UV laser-based spin- and angle-resolved photoemission spectroscopy system (Both of them are successfully developed by our team). We will mainly focus on a few points: (1).to study the electron doping origin and characteristics as well as the essential factors of controlling Tc in physics for the monolayer FeSe film; (2).to capture the doping evolution of electronic structures in the two-layer and multilayer FeSe films；(3). to investigate the substrate dependence of electronic structure and possible superconductivity in the FeSe, FeTe and Fe(Se,Te) monolayer film. The advantages of Our two advanced ARPES systems are nicely matching to this project, due to their ultra-high resolution and being equipped with two kinds of bright light sources (laser and Helium discharging lamp) and the in-situ MBE growth subsystems. Such studies are helpful to find those physical parameters to directly induce high Tc superconductivity in the monolayer FeSe film. It will also shed a light on raising the Tc further up. Meanwhile, the realization of this project might supply some key ingredients in solving the Fe-based superconductivity mechanism.

单原子层FeSe薄膜由于可能拥有超过液氮温区(77K)的高临界温度超导电性而备受瞩目。它也是迄今为止最简单最干净的铁基超导体，可能成为实验和理论上研究和解决铁基超导机理的一个模型材料。本项目拟采用我们自主研制成功的两台国际领先深紫外激光角分辨光电子能谱仪,对不同层数的单原子层FeSe和相关薄膜电子结构进行系统和深入的研究。内容主要包括: 1.研究单原子层FeSe薄膜电子掺杂的起源和特征及影响Tc的关键物理因素；2.获得两层及两层以上一系列不同层数 FeSe薄膜电子结构随掺杂的演化情况；3.研究FeSe,FeTe和Fe(Se,Te)单原子层薄膜在不同衬底上的电子结构和超导电性。我们的两台谱仪具有超高分辨率, 并配备专业MBE原位生长系统, 非常适合开展上述研究工作。我们相信结果将有助于澄清导致单层FeSe薄膜极高Tc的机制，为进一步提高Tc指明方向，为铁基超导机理的解决提供重要的实验素材。

铁基超导体作为第二类高温超导体，其高温超导机制是目前凝聚态物理研究的热点。FeSe类铁基超导体由于其简单的晶体结构、电子结构和高的超导转变温度以及其宽范围的可掺杂性，为铁基超导体机理研究提供了一个模型系统，对其研究必然会对铁基超导机理的理解提供重要信息。我们利用自主研制的高分辨激光角分辨光电子能谱系统对原子层厚 FeSe 薄膜和新型“11111”FeSe基超导体进行了系统研究。. 首次在单层FeSe/SrTiO3薄膜中发现了载流子浓度变化诱导的绝缘体-超导体转变，这与其二维特性及衬底对电子关联的加强相关，表明了电子掺杂和电子间关联对单原子层FeSe 薄膜的高Tc超导发生作用重大。结果也为理解铁基超导体中的电子关联效应及其作用提供了重要信息。我们研究了双层FeSe薄膜电子结构随掺杂的演化，获得了电子相图。结果表明，对于双层FeSe薄膜，同样的载流子则要在两个FeSe层之间分配， 从而导致对其中单个FeSe层掺杂效率显著降低，从而难以实现从绝缘相到超导相的转变和超导电性的产生。此外，我们最先对新型FeSe 基超导材料(Li0.84Fe0.16)OHFe0.98Se 电子结构和超导能隙展开了研究，结果发现其电子结构，能带结构跟超导单层FeSe/SrTiO3薄膜具有惊人的相似性，超导能隙的对称性与超导单层FeSe/SrTiO3薄膜也一样，表明这样的纯电子型费米面结构包含着高温超导的基因。最后，我们系统总结了FeSe基超导体的电子结构和超导电性,对其研究现状及前景进行了综述，有助于推动此类超导体在阐明超导机制和进一步提高Tc两方面的深入研究。. 项目共发表4篇高水平SCI 论文，包括2篇Nature Communications，1篇PNAS，1篇Journal of Physics: Condensed Matter （Topical Review）。