多轨道复杂体系中的新奇量子态

This project will focus on the novel quantum states in multi-orbital complex systems, including iron-based superconductors, single layer and bi-layer graphene, Bi-S superconductors and Iridium compounds, discovered in recent years. In such systems, impressive physical properties emerge due to orbital and spin degrees of freedom and the coupling between them, as well as the electron correlations. For this reason, they have attracted much attention from the physics community. .In iron-based superconductors, the common agreement on the fact that electron pairing and hence superconductivity is produced by antiferromagnetic fluctuations has been reached. However, the question on the paring symmetry in AFe2Se2 and FeSe is still open, and the debates between local spin moment picture and itinerant spin theory has not ended. Moreover, how to solve such an intermediate coupling system itself is already a challenging question independent on superconductivity..The researches on graphene have made great achievements in recent years. The special band dispersion with Dirac-cone and consequences have shown us plenty of exciting results. However, the effect of electron correlations, spin-orbital coupling and doping are still unclear. .Spin-orbital couping is a raising star in condensed matter physics also. In the Bi-S superconductors and Iridium compounds, it is possible that spin-orbital coupling may produce topological superconductivity. And together with the electron correlations, topological magnetism may be found. .This project will study the roll of spin-orbital coupling and electron correlations in physical properties including magnetism and superconductivity in such systems. Especially, the electron correlation has always been a tough point in condensed matter theory. This project will adapt a series of numerical techniques which are known to be outstanding for strongly correlated electron systems, including Variational Monte Carlo, Quantum Monte Carlo and Exact Diagonalization, together with analytical calculations, and study the complex systems deeply and systematically from a theoretical point of view, in order to understand the anomalous physical properties in them and reveal new physical laws.

本项目集中研究近年来所发现的多轨道复杂系统中的奇异量子态。包括铁基超导体、单双层的石墨烯、铋硫超导体以及铱系化合物等体系。在这些体系中，由于轨道自由度、自旋自由度以及它们之间的耦合与电子关联的交互作用而呈现出一系列奇异的物理行为，从而引起物理学界极大的关注。本项目重点关注在这些多轨道复杂体系中的磁性、超导电性等物理性质以及自旋--轨道耦合、电子关联对体系物性的影响。在理论上，如何处理电子-电子关联一直是凝聚态理论中的难点。本项目采用适合于强关联电子系统的一整套的数值技术，包括变分蒙特卡罗(VMC)和量子蒙特卡罗(QMC)方法以及精确对角化，并结合解析计算，对这些体系进行系统深入的理论研究，以试图理解这些体系中所表现出来的反常物性，揭示新的物理规律。

在该基金的支持下，我们关于多轨道复杂体系新奇物理性质的研究进展顺利。按照研究计划，我们主要开展了1，铁基超导材料超导机理及其物性的研究； 2，石墨烯相关体系磁性和超导电性的研究；3，拓扑超导新体系的探索研究；4，新颖超导体系的探索。共发表Nature Physics 2篇，PNAS 1篇，Phys. Rev. Lett. 5篇，Nature Communications 2篇，Phys. Rev. X 3篇，Phys. Rev. B 27篇，Science Bulletin等20余篇。.研究计划执行情况总体较好。部分研究内容完成得很好或较好，部分内容由于实际情况的变化而被拓展或深化。对原研究计划中完成得很好的部分在于铁基超导的磁性与超导部分，石墨烯相关材料的磁性与超导的研究。在原定研究计划的基础上，由于实际科研情况而大大深化和拓展了研究内容，取得了一系列新的相关课题的研究成果。在Phys. Rev. Lett, Phys. Rev. B等期刊发表论文数篇。对原定计划中完成较好的部分为Bi-系化合物中可能拓扑超导的研究，根据实际情况略有调整，研究了BiH中可能的拓扑超导，研究成果发表在Phys. Rev. B。对于原定计划中关于铁基超导的磁性与超导部分，该部分获得重大研究成果。相关论文发表在Nature Physics，Phys. Rev. B等上。根据实际研究进展，拓展了新的、和本基金课题相关的研究，并取得重要进展，比如关于重费米子方面的研究获得突破，相关成果发表在Phys. Rev. Lett等期刊上。