铁基超导体中的自旋涨落及其与超导的关联

Superconductivity is one of the most fantastic phenomena in condensed matter physics. Understanding high temperature superconductivity will not only significantly advance the natural science but also lead to important practical applications. The recently discovered iron-based superconductors have offered us an new playground to uncover the mechanism of high temperature superconductivity. Currently, the mechanism of superconductivity in iron-based superconductors is hotly debated, with proposals of spin fluctuation, orbital fluctuation or electron-phonon coupling being the pairing glue. To identify the most plausible mechanism, accurate theory and method are needed. However, the multi-orbital multi-band nature of the iron-based superconductors accompanied by strong electronic correlation poses a strong challenge to conventional band theory and model Hamilton approaches as these methods are not able to account for the complexity arising from non-perturbative strength of the interaction in these compounds. The combination of many-body theory, namely, dynamical mean field theory and density functional theory (DFT+DMFT) has been proven to be best suited for the iron-based superconductors by successfully predicting their strength of electronic correlation, magnitude of ordered moments, photoemission spectra and Fermi surfaces, charge response such as optical conductivity, non Fermi liquid behavior such as orbital-dependent coherence-incoherence crossover upon increasing temperature, and more importantly the spin dynamics such as the dynamical spin susceptibility or in other words, spin fluctuation. In this project, we will develop and apply the state-of-the-art DFT+DMFT tools to study the physical properties of three interesting families of the iron-based superconductors. While most iron-based superconductors have a single-dome superconducting phase diagram as a function of doping or pressure, the H-doped LaFeAsO and K-doped FeSe thin film, and KxFe2-ySe2 under pressure all exhibit an usual double-dome superconducting phase diagram with doping or applied pressure. Their special double-dome superconducting phase diagram put extra constrains on the mechanism of their high temperature superconductivity. By using many-body first-principles DFT+DMFT calculations to accurately trace the changes of their electronic structures, strength of electronic correlations, and the intensity and momentum distribution of spin fluctuations upon doping and applying pressure of these three families of iron-based superconductors, we aim at discovering the correlation of these physical properties and their superconductivity and identifying the most plausible pairing glue for the high temperature superconductivity in iron-based superconductors as a whole. Our findings are expected to shed new lights on designing and discovering novel high temperature superconductors.

理解高温超导机制不仅会显著地推动科学的发展还会产生重要的实际应用。近年发现的铁基超导体给我们提供了一个新的平台去揭开高温超导机制。自旋涨落，轨道涨落和电声子耦合都被提出来作为可能的铁基超导机制。区分这些机制需要准确的理论和方法。多体理论-动力学平均场理论-结合密度泛函理论(DFT+DMFT)被证明能很好的描述铁基超导体，包括关联强度，有序磁矩，光电子能谱，自旋涨落等物性。我们将发展和应用最新的DFT+DMFT工具去研究三类特殊的铁基超导体系（1）LaFeAsO的H掺杂，（2）FeSe薄膜的K掺杂，（3）KxFe2-ySe2加压。它们特殊的双拱形相图对可能的超导机制有更多的限制。我们将通过准确计算它们的电子结构，关联强度和自旋涨落随掺杂和加压的变化来建立这些物理量和超导电性的关联并找到最可能的适用于铁基超导体的超导机制。我们的成果将为设计和发现新的高温超导体系奠定重要的基础。

本项目中，我们利用密度泛函理论结合动力学平均场理论来研究多个铁基超导体系中的电子结构、自旋涨落、关联强度随掺杂的变化，并进一步总结自旋涨落和超导的关联。我们研究的体系包括电子掺杂的LaFeAsO，S掺杂的FeSe，FeSe加压，(Li,Fe)OHFeSe, 插层的FeSe体系，空穴掺杂的LiFeAs，电子掺杂的NaFeAs，P掺杂的BaFe2As2, BaFe2As2、SrFe2As2, KFe2As2, CaKFe4As4, SrCo2As2，BaCo2As2，CaCo2As2母体及掺杂。我们发现一些铁基超导体包括(Li,Fe)OHFeSe和CaKFe4As4中存在非平庸拓扑能带结构和拓扑超导表面态。通过总结这些体系中的自旋涨落特性，我们发现铁基超导体系的自旋涨落高度依赖电子结构的细节并具有很强的轨道依赖性。因此掺杂和加压可以非常有效的调节铁基超导体中的自旋涨落。我们的研究表明自旋涨落在铁基超导体中具有至关重要的作用，它与铁基超导体的能带结构和强电子关联效应密切相关，同时主导了铁基超导体中多个量子态的共存与竞争，包括超导相，相列相，和各种反铁磁相。我们的研究为理解铁基超导体中的超导机制奠定了重要基础。

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