Co取代$\textrm{Fe}\textrm{Se}$超导体电子结构的软X射线光谱和密度泛函理论研究
The electronic structure of Co-substituted $\textrm{Fe}\textrm{Se}$ superconductor probed by soft X-ray spectroscopy and density functional theory
摘要：
我们用X射线衍射、X射线光谱和密度泛函理论研究了$\textrm{Fe}_{1-x}\textrm{Co}_x\textrm{Se}$体系（$x=0$，0.25，0.5，0.75和1.0）的晶体和电子性质。我们发现，在FeSe中引入Co $3d$态使键强度松弛，并诱导从四角到六角的结构转变，其交叉发生在$x\approx0.38$。这种结构转变反过来又改变了与自旋状态相关的磁序。使用共振非弹性X射线光谱，我们估计的自旋状态的系统，FeSe被发现是在一个高自旋状态（S=2），但Fe被减少到一个低自旋状态后，Co取代的$x \le 0.25$，远低于结构转变。最后，我们证明了FeSe是一个适度相关的系统，但引入钴到主机晶格减弱的相关强度为$x\ge0.25$。这些新的发现对于揭示铁硫系超导体中超导态的机制具有重要意义。
英文摘要：
We study the crystalline and electronic properties of the $\textrm{Fe}_{1-x}\textrm{Co}_x\textrm{Se}$ system ($x=0$, 0.25, 0.5, 0.75, and 1.0) using X-ray diffraction, X-ray spectroscopy and density functional theory. We show that the introduction of Co $3d$ states in FeSe relaxes the bond strengths and induces a structural transition from tetragonal to hexagonal whose crossover takes place at $x\approx0.38$. This structural transition in turn modifies the magnetic order which can be related to the spin state. Using resonant inelastic X-ray spectroscopy we estimate the spin state of the system; FeSe is found to be in a high spin state (S=2), but Fe is reduced to a low spin state upon Co substitution of $x \le 0.25$, well below the structural transition. Finally, we show evidence that FeSe is a moderately correlated system but the introduction of Co into the host lattice weakens the correlation strength for $x\ge0.25$. These novel findings are important to unravel the mechanisms responsible for the superconducting state in iron-chalcogenide superconductors.
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