采用第一性原理电子结构计算方法研究了K $_{0.8+x}$ Fe $_{1.6}$ Se $_2$的电子结构和磁性.铁空位有序的KPDMATHABMP Fe $_{1.6}$ Se $_2$的基态被发现是一个准二维块棋盘式反铁磁（AFM）半导体的能隙为594 meV和一个大的有序磁矩为3.37 $μ_B$为每个Fe原子，在良好的协议与中子散射测量。潜在的机制是化学键驱动的四聚体晶格畸变。K $_{0.8+x}$ Fe $_{1.6}$ Se $_2$ with finite $x$是一种掺杂的AFM半导体，具有低导电载流子浓度，其与过量的钾含量近似成比例，定性地与红外观察一致。我们的研究揭示了这些材料中反铁磁性和超导性之间相互作用的重要性。这表明$_{0.8}$ Fe $_{1.6}$ Se $_2$而不是KFe $_2$ Se $_2$应被视为母体化合物，在电子或空穴掺杂时从其出现超导性。
英文摘要：
We have studied electronic and magnetic structures of K$_{0.8+x}$Fe$_{1.6}$Se$_2$ by performing the first-principles electronic structure calculations. The ground state of the Fe-vacancies ordered K$_{0.8}$Fe$_{1.6}$Se$_2$ is found to be a quasi-two-dimensional blocked checkerboard antiferromagnetic (AFM) semiconductor with an energy gap of 594 meV and a large ordering magnetic moment of 3.37 $μ_B$ for each Fe atom, in excellent agreement with the neutron scattering measurement. The underlying mechanism is the chemical-bonding-driven tetramer lattice distortion. K$_{0.8+x}$Fe$_{1.6}$Se$_2$ with finite $x$ is a doped AFM semiconductor with low conducting carrier concentration which is approximately proportional to the excess potassium content, consistent qualitatively with the infrared observation. Our study reveals the importance of the interplay between antiferromagnetism and superconductivity in these materials. This suggests that K$_{0.8}$Fe$_{1.6}$Se$_2$, instead of KFe$_2$Se$_2$, should be regarded as a parent compound from which the superconductivity emerges upon electron or hole doping.
Electronic and magnetic structures of ternary iron selenides AFe$_2$Se$_2$ (A=K, Cs, or Tl)
通过第一性原理电子结构计算，我们发现三元铁硒化物AFe $_2$ Se $_2$（A=K，Cs，或Tl）的基态为双共线反铁磁有序，其中Fe局域磁矩（$\sim2.8μ_B$）沿Fe-Fe正方晶格的一个对角线方向铁磁排列，而沿另一个对角线方向反铁磁排列.这种双共线反铁磁序是由Se $4p$轨道介导的最近邻、次最近邻和次最近邻超交换相互作用之间的相互作用引起的。
First-principles study of pressure-induced magnetic phase transitions in ternary iron selenide K$_{0.8}$Fe$_{1.6}$Se$_2$
采用第一性原理电子结构计算方法研究了三元铁硒化物K $_{0.8}$ Fe $_{1.6}$ Se $_2$的电子结构和磁序的压力效应.在低压下，该化合物是在阻断棋盘式反铁磁（AFM）半导体相，所观察到的中子散射测量。施加压力诱导两个相变，首先从阻断的棋盘形AFM半导体相到约12 GPa的共线AFM金属相，然后分别到约25 GPa的非磁性金属相。我们的结果有助于澄清最近的实验测量压力下。
Since the discovery of high-temperature superconductivity in the thin-film FeSe/SrTiO$_3$ system, iron selenide and its derivates have been intensively scrutinized. Using ab initio density functional theory calculations we review the electronic structures that could be realized in iron-selenide if the structural parameters could be tuned at liberty. We calculate the momentum-dependence of the susceptibility and investigate the symmetry of electron pairing within the random phase approximation. Both the susceptibility and the symmetry of electron pairing depend on the structural parameters in a…
Electronic structures and magnetic orders of Fe-vacancies ordered ternary iron selenides TlFe$_{1.5}$Se$_2$ and AFe$_{1.5}$Se$_2$ (A=K, Rb, or Cs)
通过第一性原理电子结构计算，我们发现Fe空位有序TlFe $_{1.5}$ Se $_2$的基态是一个准二维共线反铁磁半导体，其能隙为94 meV，与实验测量相一致.这种反铁磁有序是由Fe磁矩之间的Se桥反铁磁超交换相互作用驱动的。同样地，我们发现晶体AFe $_{1.5}$ Se $_2$（A=K，Rb，或Cs）也是反铁磁半导体，但具有零能隙半导体态或与基态近似简并的半金属态。因此，丰富的物理性质和相图的预期。
We report polarized neutron scattering measurements of the low energy spin fluctuations of the iron-selenide superconductor Li$_{0.8}$Fe$_{0.2}$ODFeSe below and above its superconducting transition temperature $T_c=41$ K. Our experiments confirmed that the resonance mode near 21 meV is magnetic. Moreover, the spin excitations are essentially isotropic in spin space at 5$\leq E\leq$ 29 meV in the superconducting and normal states. Our results suggest that the resonance mode in iron-based superconductors becomes isotropic when the influence of spin-orbit coupling and magnetic/nematic order is mi…
In this editorial, we first give a brief survey of the field of iron-selenide superconductivity, both in the case of bulk FeSe characterized by the co-existence of superconductivity and nematic order, and in the case of electron-doped FeSe characterized by high-temperature superconductivity. We next review the 8 contributions to the Frontiers in Physics research topic "High-Tc Superconductivity in Electron-Doped Iron Selenide and Related Compounds".
The electronic structure of the vacancy-ordered K$_{0.5}$Fe$_{1.75}$Se$_2$ iron-selenide compound (278 phase) is studied using the first-principles density functional method. The ground state of the 278 phase is stripe-like antiferromagnetic, and its bare electron susceptibility shows a large peak around $(π, π)$ in the folded Brillouin zone. Near Fermi level, the density of states are dominated by the Fe-3d orbitals, and both electron-like and hole-like Fermi surfaces appear in the Brillouin zone. Unfolded band structure shows limited similarities to a hole doped 122 phase. With 0.1e electron…
Intrinsic high-temperature superconductivity in ternary iron selenides
本文研究了三元铁硒化物KPDMATHABMM Fe $_{2-x}$ Se $_{2}$在介观混合反铁磁（AF）和超导（SC）相中的超导电性。结果表明，层间跳跃和AF阶数是决定SC相$T_{c}$的关键因素。一般来说，跳跃会产生变形的费米面（FS），这往往会抑制超导性。然而，与通常的预期相反，我们发现较大的AF阶数实际上导致较大的SC阶数，这解释了在这些相位中观察到的相对较高的$T_{c}$。此外，我们的研究结果表明，通过适当地减少层间跳跃，相分离的K $_{y}$ Fe $_{2-x}$ Se $_{2}$可以表现出其固有的SC相在二维极限与更高的$T_{c}$（$\sim 65K$）比已经观察到的。
Berg, Metlitski and Sachdev, Science 338, 1606 (2012), have shown that the exchange of hidden spin fluctuations by conduction electrons with two orbitals can result in high-temperature superconductivity in copper-oxide materials. We introduce a similar model for high-temperature iron-selenide superconductors that are electron doped. Conduction electrons carry the minimal 3d xz and 3d yz iron-atom orbitals. Low-energy hidden spin fluctuations at the checkerboard wavevector Q_AF result from nested Fermi surfaces at the center and at the corner of the unfolded (one-iron) Brillouin zone. Magnetic …
An essential step toward elucidating the mechanism of superconductivity is to determine the sign/phase of superconducting order parameter, as it is closely related to the pairing interaction. In conventional superconductors, the electron-phonon interaction induces attraction between electrons near the Fermi energy and results in a sign-preserved s-wave pairing. For high-temperature superconductors, including cuprates and iron-based superconductors, prevalent weak coupling theories suggest that the electron pairing is mediated by spin fluctuations which lead to repulsive interactions, and there…
Superconducting properties of sulfur-doped iron selenide
最近在单层铁硒化物中发现的高温超导性引起了人们对通过晶格改性优化铁基超导体超导性能的重大实验兴趣。为了模拟类似的效果，通过改变化学成分，由于S掺杂，我们研究了高品质的FeSe $_{1-x}$ S $_{x}$（$x$ =0，0.04，0.09，和0.11）单晶的超导性能，使用磁化强度，电阻率，伦敦穿透深度，和低温比热测量。我们发现，S的FeSe的引入提高了超导转变温度$T_{c}$，各向异性，上临界场$H_{c2}$，和临界电流密度$J_{c}$。上临界场$H_{c2}(T)$及其各向异性与温度有很强的相关性，表明该体系具有多带超导性。通过对伦敦穿透深度$λ_{ab}(T)$和比热的测量和分析，我们得到了强耦合双能隙$s$波超导的明确证据。由低临界场和电子比热计算得到的$λ_{ab}(T)$的温度依赖性可以用具有$s$类波隙的双带模型来描述。我们发现在弱耦合近似下的$d$波和单能隙BCS理论不能描述我们的实验。由磁场引起的比热变化只能用多带超导来理解。
Nanometer-sized particular structures are generated on the surfaces of FeSe epitaxial films directly after exposure to air; this phenomenon was studied in the current work because these structures are an obstacle to field-induced superconductivity in electric double-layer transistors using FeSe channel layers. Chemical analyses using field-effect scanning Auger electron spectroscopy revealed no clear difference in the chemical composition between the particular structures and the other flat surface region. This observation limits the possible origins of the particulate formation to light eleme…
Infrared spectrum and its implications for the electronic structure of the semiconducting iron selenide K$_{0.83}$Fe$_{1.53}$Se$_2$
本文报道了新铁硒化物体系的半导体母体化合物K $_{0.83}$ Fe $_{1.53}$ Se $_2$的红外光谱研究。主要的光谱特征被发现是明显不同于所有其他铁基超导系统。我们的测量揭示了两种特殊的光谱结构：在4000-6000 cm $^{-1}$之间的双峰结构和比122结构更多的丰富的声子模。我们阐述了这些功能可以自然地解释从阻塞的反铁磁性，由于Fe空位的存在下，最近的中子衍射实验确定的顺序。双峰反映了相邻Fe位之间铁磁耦合和反铁磁耦合的共存。
Structure and composition of the superconducting phase in alkali iron selenide K$_y$Fe$_{1.6+x}$Se$_2$
本文用中子衍射方法研究了绝缘超导钾铁硒化物KPDMATHABMM Fe $_{1.6+x}$ Se $_2$的平均结构和局域晶格畸变的温度演化。在高温顺磁状态下，两种材料都具有单相，其晶体结构类似于铁磷属元素化物的BaFe $_2$ As $_2$家族。而绝缘K $_y$ Fe $_{1.6+x}$ Se $_2$在低温下形成$\sqrt{5}\times\sqrt{5}$铁空位有序块反铁磁（AF）结构，超导化合物自发相分离为具有$\sqrt{5}\times\sqrt{5}$铁空位有序绝缘部分和化学组成为KPDMATHABMP的超导相MQ Fe $_{2}$ Se $_2$和BaFe $_2$为$_2$结构。因此，碱性铁硒化物的超导电性来自于绝缘块AF相的基质中的缺碱K $_z$ Fe $_{2}$ Se $_2$。
Coexistence of localized and itinerant magnetism in newly discovered iron-selenide superconductor LiFeO2Fe2Se2
采用第一性原理方法研究了LiFeO $_{2}$ Fe $_{2}$ Se $_{2}$的电子结构和磁性.在LiFeO $_{2}$层中，Fe 1的基态为N $\acute{e}$ el反铁磁（AFM）Mott绝缘态，具有局域磁性;在Fe $_{2}$ Se $_{2}$层中，Fe 2的基态为条纹状AFM金属态，具有巡游磁性;此外，层状LiFeO $_{2}$被发现是超过一个绝缘块层，但负责增强AFM相关的Fe $_{2}$ Se $_{2}$层通过层间磁耦合。Fe 1和Fe 2的磁性之间的相互作用引入了与铁基超导体中的超导性相关的自旋涨落的控制机制。
Recent inelastic neutron scattering studies by Pan et al., Nature Communications 8, 123 (2017), find evidence for spin excitations at energies above the quasi-particle gap in an iron-selenide high-Tc superconductor. The momenta of the spin excitations form a diamond around the checkerboard wavevector, Q_AF, that is associated with the square lattice of iron atoms that makes up the system. It has been suggested that such a "hollowed-out" spin-excitation spectrum is due to hidden Neel order. We study such a hidden spin-density wave (hSDW) state that results from nested Fermi surfaces at the cent…
The Unexpected Properties of Alkali Metal Iron Selenide Superconductors
铁基超导体含有作为晶体结构中的基本构建块的FeAs层，在过去已经使用基于当存在弱Hubbard $U$相互作用时空穴和电子袋的费米表面嵌套的想法来合理化。考虑到母体化合物中的磁矩值很小，以及基于所需电子和空穴袋的光电发射实验的明确证据，这种方法似乎是合适的。然而，最近的结果在碱金属铁硒化物的背景下，具有通用化学组成$A_x$ Fe $_{2-y}$ Se $_2$（$A$ =碱元素），已经彻底挑战了这些先前的想法，因为在特定的组成$y$的低温基态是绝缘的，并显示具有大的铁磁矩的反铁磁性磁序。此外，角分辨光电发射研究已经揭示了在这些材料中的费米能级处没有空穴袋。这一令人兴奋的研究领域的现状，有可能改变概念上我们的铁基超导体的理解，在这里进行审查，包括实验和理论研究。本文还简要地介绍了近年来有关的其他进展，如硒化物双梯结构的研究和单层FeSe超导性的发现。文中讨论了为碱金属硒化铁建立的概念问题，以及仍需进一步工作的几个问题。
Pairing Symmetry and Spin Excitations of Iron Selenide Superconductors
本文对新发现的铁硒化物超导体APDMATHABMM Fe $_{2-y}$ Se $_2$（A=Tl，K，Rb，Cs）提出了一个有效的单带模型.基于该最小模型和无规相位近似，从理论上揭示了$d_{x^2-y^2}$对对称性，并在费米面拓扑的框架下加以理解。一个共同的起源的超导性阐明这种化合物和其他高T $_c$材料。在超导态的$(π,π/2)$上观察到自旋激发，与中子散射实验符合得很好。自旋共振只出现在d波配对中，这为这类超导体的d波配对对称性提供了额外的指示。
Nematic phase intertwines closely with high-Tc superconductivity in iron-based superconductors. Its mechanism, which is closely related to the pairing mechanism of superconductivity, still remains controversial. Comprehensive characterization of how the electronic state reconstructs in the nematic phase is thus crucial. However, most experiments focus only on the reconstruction of band dispersions. Another important characteristic of electronic state, the spectral weight, has not been studied in details so far. Here, we studied the spectral weight transfer in the nematic phase of FeSe$_{0.9}$S…
The electronic structure and magnetism of a new magnetic intercalation compound (Li0.8Fe0.2)OHFeSe are investigated theoretically. The electronic structure calculations predict that the Fe in the (Li,Fe)OH intercalated layer is in +2 valence state, i.e. there is electron doping to the FeSe layer, resulting in the shrinking of the Fermi surface (FS) pocket around Gamma and a strong suppression of dynamical spin susceptibility at M in comparison with the bulk FeSe compound. The ground state of the FeSe layer is a striped antiferromagnetic (SAFM) metal, while the (Li,Fe)OH layer displays a very w…
It is known that iron selenide superconductors exhibit unique characteristics distinct from iron pnicitides, especially in the electron-doped region. However, acomprehensive study of continuous carrier doping ang the corresponding crystal structures of FeSe is still lacking, mainly due to the difficulties in controlling the carrier density in bulk materials. Here, we report the successful synthesis of a new family of bulk Lix(C3H10N2)0.37FeSe, which features a continue superconducting dome harboring Lifshitz transition within the wide range of 0.06~0.68. We demonstrate that with electron-doped…
Magnetic phase separation and superfluid density measurements in alkali metal-organic solvent intercalated iron selenide superconductor Lix(C5H5N)yFe2-zSe2
Modulated superconductivity due to vacancy and magnetic order in $A_x$Fe$_{2-x/2}$Se$_2$ [$A=$Cs, K, (Tl,Rb), (Tl,K)] iron-selenide superconductors
本文计算了铁硒化物超导体的“调制”超导态。零动量$d-$波配对打破了$I4/mmm$空间群的常规BaFe $_2$ Se $_2$类晶体的平移对称性。当Fe空位形成有序状态时，这种配对状态变得相容，并且晶体对称性变为低温$I4/m$对称性。对于在$Q_v=(1/5, 3/5)$在K $_{0.82(2)}$ Fe $_{1.626(3)}$ Se $_2$的一个无公度空位顺序的具体情况下，我们发现，它诱导块棋盘形反铁磁相波矢量${\bf Q}_m=4{\bf Q}_v$。空位有序和磁有序的共存导致了一个重建的基态，它自然地耦合到$d-$波超导在一个统一的相位，我们建议将是一个一般的耦合所有的铁硒化物超导体。我们的结果与迄今为止的大量实验数据一致。因此，我们建议，不可分割性导致多个相的均匀共存，作为一个可行的替代方案，在高- $T_c$超导体的纳米级相分离，并发挥重要作用，在增强超导性。
Conformity of spin fluctuations in alkali-metal iron selenide superconductors inferred from the observation of a magnetic resonant mode in K(x)Fe(2-y)Se(2)
An important challenge in condensed matter physics is understanding iron-based superconductors. Among these systems, the iron selenides hold the record for highest superconducting transition temperature and pose especially striking puzzles regarding the nature of superconductivity. The pairing state of the alkaline iron selenides appears to be of $d$-wave type based on the observation of a resonance mode in neutron scattering, while it seems to be of $s$-wave type from the nodeless gaps observed everywhere on the Fermi surface (FS). Here we propose an orbital-selective pairing state, dubbed $s…
Hidden spin-density waves (hSDW) with Neel ordering vector (pi,pi) have been proposed recently as parent groundstates to electron-doped iron-selenide superconductors. Doping such groundstates can result in visible electron-type Fermi surface pockets and faint hole-type Fermi surface pockets at the corner of the folded Brillouin zone. A Cooper pair instability that alternates in sign between the electron-type and the hole-type Fermi surfaces has recently been predicted. The previous is due to the interaction of electrons and holes with hidden spin fluctuations connected with hSDW order that is …
