铁基高温超导材料SmFeAsO1-xFx中量子临界点的同步辐射谱学研究

The discovery of the new family of iron-based high-Tc superconductors (HTSC) opens new window to reveal the superconducting mechanism of HTSC. Recently, more and more scholars believe that the superconductivity of HTSC is closely related to the quantum critical point (QCP), and that studying the QCP can be expected to uncover the veil over HTSC. Take the samarium-oxide-iron-arsenic series (SmFeAsO1-xFx) as an example, researchers found that there exists QCP correlated with superconductivity in this series of materials. This quantum behavior incurs exotic properties in the atomic and electronic structures of materials. Given the fact that quantum behavior manifests on the atomic or molecular scale, it is essential to study the QCP at the same level. Currently, some studies were carried out on the quantum critical point; however, it is not evident to reveal the superconducting mechanism owing to lack of systematic investigation regarding the local and electronic structure near QCP. We propose to perform systematic investigations of the local structures of FeAs superconducting layer in the vicinity of QCP by using X-ray absorption spectroscopy, in attempt to reveal the superconducting mechanism. Meanwhile, the correlation between local structure at atomic level and the QCP can be extended to other strongly-correlated systems. Therefore, it is both innovative and academically valuable to perform such study.

新型铁基超导材料的出现为揭示高温超导机理打开了新的窗户。最近，越来越多的学者认为高温超导材料的超导特性与量子临界点（QCP）密切相关，研究量子临界点的行为，有望揭开高温超导体的神秘面纱。以掺杂钐氧铁砷系列(SmFeAsO1-xFx)为例，研究表明，这类超导材料中存在与超导特性关联的量子临界点，这种量子行为导致材料的原子、电子结构呈现出奇异特性。由于量子行为的呈现尺度主要在原子、分子量级，对量子临界点进行同等尺度上的研究成为关键。最近国际上有一些对量子临界点的研究，但对量子临界点附近的局域及电子结构，仍然缺乏系统的研究，揭示超导机理证据不足。本项目拟采用X射线吸收谱，系统地研究FeAs超导层的局域结构在量子临界点浓度附近的行为，以期揭示体系的超导机理。同时，这种原子尺度上的局域结构与量子临界点的关联研究，也可以推广至其他强关联体系，具有独特的创新性和重要的学术价值。

研究超导材料的关键在于揭示其局域结构与超导特性间的关系，基于同步辐射的X射线吸收谱是从微观角度探讨超导材料局域结构与超导特性关联的绝佳技术手段。本项目从最根本的微观结构出发，依托X射线吸收谱技术对铁基SmO1-xFxFeAs材料的量子临界点、铁基材料FeAs层面内和面外掺杂的差异性、BiS2基材料SrFBiS2和EuFBiS2的Ce掺杂效应进行了研究，从原子尺度上找寻超导电性存在的基本根源。此外，本项目还对理论预言可能存在非常规超导电性Sr2IrO4基材料的La掺杂和Sr缺位效应进行了初步的探索工作。取得的主要创新性成果如下：1）建立了SmO1-xFxFeAs材料量子临界点和Fe-As键长的内在联系，从微观原子尺度上找寻了其量子临界点存在的基本根源；2）揭示了铁基超导体FeAs层面外和面内掺杂的差异性，面外掺杂会导致超导层和载流子层的间距缩小，而面内掺杂仅仅使FeAs层变得更紧凑；3）以Sr1-xCexFBiS2和Eu0.5Ce0.5FBiS2为例，阐明了BiS2基材料中Bi-S1超导面平整度和面内结构对超导性能的重要性；4）发现了Sr2IrO4基材料中规则的IrO6八面体可有效调制晶体场和自旋轨道耦合的相对强度，进而改变其导电状态。在本项目的资助下，共发表研究论文10篇。