氢在CaFeAsH-based超导体电子性质中的作用
Role of Hydrogen in the Electronic Properties of CaFeAsH-based Superconductors
摘要：
采用密度泛函方法研究了新型氢化物超导体CaFeAsH（La掺杂时超导温度可达47 K）和等价合金体系CaFeAsH $_{1-x}$ F $_x$的电子和磁性.嵌套函数$ξ(\vec q)$的$\vec Q = (π,π,0)$峰被发现是非常强和尖锐的，和额外的结构在$ξ(\vec q)$与近圆形费米面（FS），可能会影响低能量激发进行量化。不寻常的频带引入的H，这表明强烈的色散垂直于FeAs层，被证明是连接到一个奇特的范霍夫奇点费米能级以下。该能带提供了在其他铁基超导材料中不存在的三维电子椭球费米面，也不存在于CaFeAsF中。25% La或Co的电子掺杂对这种椭球费米面的影响很小，但强烈抑制了FS嵌套，这与消除强嵌套和相关磁序允许出现高T $_c$超导性的观点一致。从初始能带的影响角度讨论了等价合金系统CaFeAsH $_{1-x}$ F $_x$的各个方面和电子掺杂方法。
英文摘要：
The electronic and magnetic properties of the new hydride superconductor CaFeAsH, which superconducts up to 47 K when electron-doped with La, and the isovalent alloy system CaFeAsH$_{1-x}$F$_x$ are investigated using density functional based methods. The $\vec Q = (π,π,0)$ peak of the nesting function $ξ(\vec q)$ is found to be extremely strong and sharp, and additional structure in $ξ(\vec q)$ associated with the near-circular Fermi surfaces (FSs) that may impact low energy excitations is quantified. The unusual band introduced by H, which shows strong dispersion perpendicular to the FeAs layers, is shown to be connected to a peculiar van Hove singularity just below the Fermi level. This band provides a three dimensional electron ellipsoid Fermi surface not present in other Fe-based superconducting materials nor in CaFeAsF. Electron doping by 25\% La or Co has minor affect on this ellipsoid Fermi surface, but suppresses FS nesting strongly, consistent with the viewpoint that eliminating strong nesting and the associated magnetic order allows high T$_c$ superconductivity to emerge. Various aspects of the isovalent alloy system CaFeAsH$_{1-x}$F$_x$ and means of electron doping are discussed in terms of influence of incipient bands.
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