具有巡游和局域特性的多轨道超导体的超导机理与新颖量子态

Studies on the properties of the high-temperature superconductivities and elucidation of their underlying mechanism are currently challenging and hot issues on the frontier of the condensed matter physics. The recently discovered iron-based superconductors not only possess the high transition temperature, but also exhibit the dichotomy of itinerant and localized electrons as well as a series of other exotic phenomena. The project is aimed to carry out a systematic studies on the physical properties and the novel quantum states of one family of the multi-orbital superconductors with the dichotomy of itinerant and localized d-electrons, as represented by the newly discovered FeSe-based superconductors, which appear to show some unique features in their structure and Fermi surfaces as compared with those of other iron-based superconductors. Based on the comparative studies between the FeSe-based, FeAs-based, cuprate and other unconventional superconductors, the project will firstly elucidate the anomalous properties and their experimental characterization, as caused by the coupling between the itinerant electrons and localized magnetic moments in the multi-orbital superconductors. Secondly, we will explore theoretically the possible mechanism underlying the sharp increase of the superconducting transition temperature in the intercalated FeSe-based superconductors or FeSe films. And thirdly, the project will reveal the impacts of the orbital selective phase transitions on the properties of the superconductors. Through the researches on these issues, we expect to acquire thorough understanding of the origin of magnetism, the pairing symmetry, and the relationship between the magnetism and superconductivity in the multi-orbital and/or multi-component superconductors, and to provide the important basic research data for the ultimate elucidation of the mechanism underlying the high-temperature superconductivities.

对高温超导性质及超导微观机理的研究是当前凝聚态物理中极富挑战性的前沿热点课题。近年来人们发现一类包含铁元素的超导材料不仅具有较高的超导转变温度，还表现出从巡游到局域的过渡行为，并伴随一系列新的物理现象。本项目拟以新近发现的具有独特结构和费米面的铁硒基超导体为代表，系统地研究一类同时包含巡游和局域d-电子的铁基超导体的物理性质和新颖量子态。通过对比研究铁硒基、铁砷基、铜氧化物以及其他非常规超导体，明确多轨道超导体中巡游电子和局域磁矩耦合所导致的反常性质及可能的实验表征；从理论上探索插层或者薄膜铁硒基超导体超导转变温度大幅度提高的可能机制；揭示轨道选择性相变对超导体各种属性的影响，全面理解具有多轨道多分量超导体的磁性序起源、超导配对对称性、磁性序与超导的相互关系。并希望这些从不同角度对比不同材料的研究，为促成高温超导微观机理的突破提供重要的基础研究资料。

我们在与项目原计划的多轨道超导体相关的方向上进行了探索并取得了一定进展。首先，我们研究了铜氧化物高温超导体中赝能隙态的双向电荷密度波框架下的自旋动力学行为；其次，我们还研究了铜氧化物高温超导体中Yang-Rice-Zhang框架下赝能隙态的不同掺杂浓度时RVB和超导能隙的自洽确定；再次，我们对过渡金属二硫化物中的Ising超导和的轨道自旋耦合可能出现的拓扑量子态进行了研究。接着，我们对蜂巢晶格中可能存在的手征超导进行了探索，发现具有蜂巢晶格的二维超导体在磁涡旋附近有可能诱导手征超导。当诱导的手征超导为次近邻的d波时，涡旋中心没有磁矩出现。而当诱导的手征超导为次近邻的p波时，则在涡旋中心有磁矩出现。最后，我们在外加垂直于kagome晶格平面的磁场情况下发展了磁平移周期性条件，并在此边界条件下研究了kagome晶格的量子霍耳效应。对比这些与项目原计划相关的研究有助于全面理解具有多轨道多分量超导体的磁性序与超导序的相互关系，对深入了解铁基和其他的非常规超导体的超导机理提供有益的启示。