Synthesis and physical properties of the new potassium iron selenide superconductor K0.80Fe1.76Se2
Rongwei Hu, E. D. Mun, D. H. Ryan, K. Cho, H. Kim, H. Hodovanets, W. E. Straszheim, M. A. Tanatar, R. Prozorov, W. N. Rowan-Weetaluktuk, J. M. Cadogan, M. M. Al… 展开作者
Rongwei Hu, E. D. Mun, D. H. Ryan, K. Cho, H. Kim, H. Hodovanets, W. E. Straszheim, M. A. Tanatar, R. Prozorov, W. N. Rowan-Weetaluktuk, J. M. Cadogan, M. M. Altarawneh, C. H. Mielke, V. S. Zapf, S. L. Bud'ko, P. C. Canfield
备注：
30 pages, 15 figures, to be a chapter in the book titled "Iron Pnictide Superconductors: materials, properties and mechanism" by Pan Stanford Publishing
In this article we review our studies of the K0.80Fe1.76Se2 superconductor, with an attempt to elucidate the crystal growth details and basic physical properties over a wide range of temperatures and applied magnetic field, including anisotropic magnetic and electrical transport properties, thermodynamic, London penetration depth, magneto-optical imaging and Mossbauer measurements. We find that: (i) Single crystals of similar stoichiometry can be grown both by furnace-cooled and decanted methods; (ii) Single crystalline K0.80Fe1.76Se2 shows moderate anisotropy in both magnetic susceptibility and electrical resistivity and a small modulation of stoichiometry of the crystal, which gives rise to broadened transitions; (iii) The upper critical field, Hc2(T) is ~ 55 T at 2 K for H||c, manifesting a temperature dependent anisotropy that peaks near 3.6 at 27 K and drops to 2.5 by 18 K; (iv) Mossbauer measurements reveal that the iron sublattice in K0.80Fe1.76Se2 clearly exhibits magnetic order, probably of the first order, from well below Tc to its Neel temperature of Tn = 532 +/- 2 K. It is very important to note that, although, at first glance there is an apparent dilemma posed by these data: high Tc superconductivity in a near insulating, large ordered moment material, analysis indicates that the sample may well consist of two phases with the minority superconducting phase (that does not exhibit magnetic order) being finely distributed, but connected with in an antiferromagnetic, poorly conducting, matrix, essentially making a superconducting aerogel.
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 $_2$。
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。
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。
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".
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)
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 …
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。
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…
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。
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 …
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。
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…
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 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…
Effect of iron content and potassium substitution in A$_{0.8}$Fe$_{1.6}$Se$_2$ (A = K, Rb, Tl) superconductors: a Raman-scattering investigation
我们对超导体K $_{0.8}$ Fe $_{1.6}$ Se的高质量单晶进行了拉曼散射测量MO（$T_c$ = 32 K），T1 $_{0.5}$ K $_{0.3}$ Fe $_{1.6}$ Se $_2$（$T_c$ = 29 K）和T1 $_{0.5}$ Rb $_{0.3}$ Fe $_{1.6}$ Se $_2$（$T_c$ = 31 K），以及绝缘化合物KFe $_{1.5}$ Se $_2$。为了解释我们的结果，我们对K $_{0.8}$ Fe $_{1.6}$ Se $_2$有序铁空位结构中的声子模进行了第一性原理计算.我们观察到的模式可以很好地分配从我们的对称性分析和计算，使我们能够比较拉曼活性声子的AFeSe化合物。我们发现超导和非超导钾晶体之间的大多数声子模式的频率有明显的差异，表明铁含量的根本影响。相比之下，在A $_{0.8}$ Fe $_{1.6}$ Se $_2$中用Tl或Rb取代K不会导致高于60 cm $^{-1}$的任何模式的实质频移，表明碱金属型金属对FeSe层的微结构几乎没有影响。在Tl和Rb取代的样品中，在60 cm $^{-1}$以下出现了几个额外的模式，这些模式是较重的Tl和Rb离子的振动。最后，我们的计算揭示了“手征”声子模式的存在下，其起源在于手征性质的K $_{0.8}$ Fe $_{1.6}$ Se $_2$结构。
