具有强自旋轨道耦合的强关联电子体系中的新奇物态

Materials with strong spin-orbit coupling has been a recent research focus in condensed matter physics, topological insulator is the most outstanding example. However topological insulators have rather weak electron correlations, so they can be well described by non-interacting electrons. Theoretically it is known that many more exotic phases exist with closely competing spin-orbit coupling and correlation effects, for example the topological Mott insulator and the Kitaev spin liquid. These phases have exotic elementary excitations like spinons and Majorana fermions, and might be used for topological quantum computation. Currently the most promising systems for this proposals are various iridium oxides. However by so far most theoretical studies on these materials are phenominological, reliable studies with first-principle approaches are extremely lacking.We plan to employ large scale first-principle calculations to build reliable electronic models for several interesting iridium oxides, and then study these models by various theoretical tools for strongly correlated electron systems, including the random phase approximation(RPA), the fluctuation-exchange(FLEX) approximation, functional renormalization group, Monte Carlo calculation for projected variational wavefunctions, exact diagonalizations and so on. We hope to obtain reliable theoretical phase diagrams and predict observable physical properties for these systems, which could provide guidance for experimental realization of the exotic phases by manipulation of the structures and compositions of these materials.

有强自旋轨道耦合的材料是凝聚态物理近年来的一个研究热点,最突出的例子是拓扑绝缘体。但是拓扑绝缘体的电子关联效应很弱，可以用无相互作用电子很好地描述。理论上已知在自旋轨道耦合和电子关联强度相当的体系中有许多更新奇的物相，如拓扑Mott绝缘体和Kitaev自旋液体等。这些奇异物相具有更新颖的元激发如自旋子、Majorana费米子，并有可能用于实现拓扑量子计算。目前最有希望实现这些理论预言的体系是各种铱氧化物，但是目前关于这些材料的理论研究大多局限于现象学，非常缺少基于第一性原理方法的可靠理论工作。我们计划应用大规模的第一性原理电子结构计算为多种铱氧化物构建可靠的电子模型，然后利用多种强关联理论方法包括随机相位近似、涨落交换近似、泛函重整化群、投影变分波函数蒙特卡罗、严格对角化、微扰级数展开等研究这些模型，得到可靠的理论相图并预言可观测的物性行为，为实验上通过调控结构和组分以实现新物相提供指导。

本项目的初衷是研究有强自旋轨道耦合的强关联电子系统如铱氧化物，从理论上探索其中可能出现的新奇物理现象。在这方面，我们研究了Na2IrO3的磁序，发现其赝自旋各向异性相互作用造成的磁矩方向与过去的假设不同，这个结果已被同期一个独立实验验证；我们还研究了Tb掺杂的Sr2IrO4，发现Tb和Ir之间有强烈各向异性的自旋-赝自旋相互作用，这为设计新奇磁性体系如compass模型提供了思路。项目进行过程中我们还研究了铁基超导材料中的向列序、非常规量子相变、拓扑绝缘体中的量子振荡等问题。我们提出FeSe中的磁矩可能因为阻挫构成“向列量子顺磁态”，构成向列序而没有磁长程序，但是有低能的Neel序和stripe序自旋激发，这个预言被后续的中子散射实验部分证实。我们首次用量子Monte Carlo研究了一个对称保护的拓扑相发生的对称破缺相变，发现其体态的临界行为和非拓扑相没有区别，但是“边界临界行为”完全不同。我们提出能带反转构成的拓扑绝缘体，虽然没有体态Fermi面，但是在有限温度或极强磁场下可以表现出体态量子振荡。这些结果推进了对相关理论问题的理解，对将来的实验和理论研究有比较重要的指导作用，均已发表在主要学术期刊上并在相关研究领域产生了一定影响。