铁硒体系中的掺杂和界面效应及计算方法研究

FeSe-based Materials have become a hot topic in condensed matter physics in recent years due to that they have demonstrated a variety of novel properties such as hight Tc superconductivity and topological properties. So far, there has no consensus on the mechanism of the superconductivity in these materials. A deep understanding of their electronic structures is a prerequisite for exploring the underlying mechanism of the superconductivity. However, there is a significant discrepancy in the band structures of bulk FeSe as well as FeSe thin films and intercalated FeSe systems between first-principles calculations and ARPES results. We propose to investigate the electronic properties, i.e., magnetism, band structure and topological properties, of FeSe-based materials including doped FeSe and their interface structures and intercalated systems, in which electronic correlation and spin-orbit coupling play an important role and interplay with each other. A new method, which combines first-principles method and k-projection method, will be developed for the investigations of the band structure of materials with complex magnetism, e.g., co-existence of multiple magnetic orders, spin fluctuation and spin spiral. Based on this method a code will be developed for the simulation of angle-resolved photonemission spectroscopy. The method and the code will be used to study the effects of doping and interface on the electronic structures of the proposed materials. Then they will be used to explore the underlying physics for the significant discrepancy in the band structures of FeSe-based materials between first-principles calculations and ARPES results. Moreover, topological properties of the proposed materials will be studied and tuned by making use of spin-orbit proximity effect to assist the exploration of high temperature topological superconductors.

铁硒基材料展现出高温超导电性与拓扑性质等新奇的物理性质，因此成为近几年凝聚态物理领域的研究热点。关于其超导机理目前人们尚未取得共识。深入理解其电子结构是进一步研究超导机理的前提。然而，关于铁硒基材料的电子结构，当前的第一性原理计算与角分辨光电子谱实验存在巨大差异。本项目拟研究电子关联与自旋轨道耦合共同作用下，铁硒掺杂体系及其界面结构、铁硒插层体系的电子性质。我们将结合我们已有的k投影方法和第一性原理方法发展一种新的计算方法，研究复杂磁性下（多种磁序共存、自旋涨落、自旋螺旋态）材料的能带结构，开发一套模拟角分辨光电子谱的程序；研究掺杂效应和界面效应对铁硒基体系电子结构的影响和调控规律；探索关于铁硒及其薄膜、铁硒插层体系电子结构计算与角分辨光电子谱实验结果不相符的物理起源；探索可能存在的拓扑态以及掺杂和界面自旋轨道邻近效应对其拓扑性质的调控规律，为探索高温拓扑超导体提供理论指导。

第一性原理超胞方法被广泛用于模拟晶体中的掺杂和界面结构。然而，超胞方法会产生能带折叠，由此掩盖能带的某些重要特征，并给人们理解角分辨光电子谱实验结果带来困难。本项目中，我们主要针对晶体中的掺杂和界面效应开展研究工作。我们发展了用于消除能带折叠的能带反折叠方法和程序，并将其应用于FeSe、过渡金属硫族化合物以及类CuInP2S6化合物等层状材料。主要研究成果有：.1、进一步完善和拓展自主开发的能带反折叠方法和程序，提出结合傅里叶变化和其逆变换加快界面结构能带反折叠计算的新算法，大大加快了计算速度。进一步将其推广到真空部分波函数投影和权重的计算，使其能够用于模拟扫描隧道谱。.2、将自主发展的能带反折叠方法应用于表面单层的能谷极化的研究，揭示了衬底表面重构和晶面取向对表面单层能谷极化的影响；提出了利用过渡金属硫族化合物单层中的自旋-动量锁定特性并引入能谷与自旋极化态的杂化诱导能谷依赖的能隙。该特征可为人们实现电控能谷极化翻转、自旋和能谷阀及过滤器等新效应和新概念器件。.3、研究了界面效应对表面单层的Rashba自旋-轨道劈裂、拓扑及铁电性质的影响；与实验合作研究了转角对层状材料电子结构的影响。.4、预言了一系列新型单层结构，发现这类体系具有丰富的电子性质，包括非平庸的拓扑性质、二维铁磁性、Ising超导及鲁棒性的电子能谷。.在该项目的支持下，总共发表了SCI论文18篇，包括Nature Physics 1篇，Nature Communications 2篇，Nano Letters 1篇，Physical Review系列6篇。