Angle resolved photoemission spectroscopy (ARPES) reveals the features of the electronic structure of quasi-two-dimensional crystals, which are crucial for the formation of spin and charge ordering and determine the mechanisms of electron-electron interaction, including the superconducting pairing. The newly discovered iron based superconductors (FeSC) promise interesting physics that stems, on one hand, from a coexistence of superconductivity and magnetism and, on the other hand, from complex multi-band electronic structure. In this review I want to give a simple introduction to the FeSC physics, and to advocate an opinion that all the complexity of FeSC properties is encapsulated in their electronic structure. For many compounds, this structure was determined in numerous ARPES experiments and agrees reasonably well with the results of band structure calculations. Nevertheless, the existing small differences may help to understand the mechanisms of the magnetic ordering and superconducting pairing in FeSC.
Iron-based superconductors are well known for their complex interplay between structure, magnetism and superconductivity. FeSe offers a particularly fascinating example. This material has been intensely discussed because of its extended nematic phase, whose relationship with magnetism is not obvious. Superconductivity in FeSe is highly tunable, with the superconducting transition temperature, $T_\mathrm{c}$, ranging from 8 K in bulk single crystals at ambient pressure to almost 40 K under pressure or in intercalated systems, and to even higher temperatures in thin films. In this topical review…
本文报道了YFe $_2$ Ge $_2$的电子结构和磁性的计算结果，并根据在磁性附近观察到的超导电性讨论了结果。基于预测磁性的标准密度泛函结果与实验的比较，我们发现YFe $_2$ Ge $_2$是一种接近磁量子临界点的材料。的能带结构和费米面是非常三维的和更高的电导率被预测在$c$轴方向。磁性是斯通纳型，主要是从一个平面内的铁磁倾向。层间耦合是弱的，给出可能的2D字符的磁性，这是在相反的导电性，并可能是重要的抑制有序化的趋势。这与由自旋涨落介导的三重态超导态是相容的。
Unique to certain unconventional superconductors is the coexistence of magnetism and superconductivity. We have previously found ferromagnetism and superconductivity in hydrogenated graphitic materials. Herein we present similar as well as completely new findings this time applicable to hydrogenated graphite foils. As the strength of the magnetic field is increased, the temperature-dependent magnetization shows several important transitions. From a Neel paramagnetic-antiferromagnetic transition, to a ferromagnetic superconductor state, to an orbital paramagnetic glass high-temperature supercon…
It is widely believed that, in contrast to its electron doped counterparts, the hole doped compound Ba1-xKxFe2As2 exhibits a mesoscopic phase separation of magnetism and superconductivity in the underdoped region of the phase diagram. Here, we report a combined high-resolution x-ray powder diffraction and volume sensitive muon spin rotation study of underdoped Ba1-xKxFe2As2 (0 \leq x \leq 0.25) showing that this paradigm is wrong. Instead we find a microscopic coexistence of the two forms of order. A competition of magnetism and superconductivity is evident from a significant reduction of the …
Magnetism and Disorder Effects on muSR Measurements of the Magnetic Penetration Depth in Iron-Based Superconductors
结果表明，利用横场μ子自旋旋转（TF-muSR）方法精确推导过掺杂BaFe_{1.82} Co_{0.18} As 2单晶的磁穿透深度的尝试，由于磁场有序性和强的涡旋晶格无序性而受阻.我们解释了如何从磁场分布的一个或两个这些因素几乎完美的涡旋晶格的实质性偏差也是显着的其他铁基超导体，这引入了相当大的不确定性TF-muSR获得的磁穿透深度的值。
The iron-based superconductors had a significant impact on condensed matter physics. They have a common structural motif of a two-dimensional square iron lattice and exhibit fruitful physical properties as a strongly correlated electron system. During the extensive investigations, quasi-one-dimensional iron-based spin-ladder compounds attracted much attention as a platform for studying the interplay between magnetic and orbital ordering. In these compounds, BaFe2S3 and BaFe2Se3 were found to exhibit superconductivity under high pressure, having a different crystal and magnetic structure at low…
Intense debate has recently arisen regarding the photoinduced changes to the iron-chalcogenide superconductors, including the enhancement of superconductivity and a metastable state. Here, by employing high energy resolution, we directly observe the melting of superconductivity on ultrafast timescales. We demonstrate a distinctly nonequilibrium response on short timescales, where the gap fills in prior to the destruction of the superconducting peak, followed by a metastable response. We propose that the former is due to pair phase decoherence and speculate that the latter is due to the increas…
Pseudo-Ising superconductivity induced by $p$-wave magnetism
The Hong Kong University of Science and Technology（香港科技大学）；RIKEN Center for Emergent Matter Science (CEMS)（理化学研究所涌现物质科学中心）；The Hong Kong University of Science and Technology (Guangzhou)（香港科技大学（广州））
The iron-based superconductor FeTe$_{1-x}$Se$_{x}$ has attracted considerable attention as a candidate topological superconductor owing to a unique combination of topological surface states and bulk high-temperature superconductivity. The superconducting properties of as-grown single crystals, however, are highly variable and synthesis dependent due to excess interstitial iron impurities incorporated during growth. Here we report a novel physicochemical process for pumping this interstitial iron out of the FeTe$_{1-x}$Se$_{x}$ matrix and achieving bulk superconductivity. Our method should have…
Phenomenological Theory of Superconductivity and Magnetism in Ho$_{1-x}$Dy$_x$Ni$_2$B$_2$C
用Ginzburg-Landau理论研究了Ho $_{1-x}$ Dy $_x$ Ni $_2$ B $_2$ C的超导性和磁性共存。该合金表现出超导有序和磁有序的共存和复杂的相互作用。我们提出了一个唯象模型，该模型包括两个磁序参数和两个超导序参量，解释了该材料的多能带结构。我们唯象地描述了磁涨落和磁序，并证明了它们导致了上临界场的反常行为。分析了具有再进入行为的Ho $_{1-x}$ Dy $_x$ Ni $_2$ B $_2$ C中$T_c$的掺杂依赖性，得到了与实验数据非常一致的结果.
Neutron scattering has played a significant role in characterizing magnetic and structural correlations in Fe$_{1+y}$Te$_{1-x}$Se$_x$ and their connections with superconductivity. Here we review several key aspects of the physics of iron chalcogenide superconductors where neutron studies played a key role. These topics include the phase diagram of Fe$_{1+y}$Te$_{1-x}$Se$_{x}$, where the doping-dependence of structural transitions can be understood from a mapping to the anisotropic random field Ising model. We then discuss orbital-selective Mott physics in the Fe chalcogenide series, where temp…
Van der Waals heterostructures have risen as a tunable platform to combine different electronic orders, due to the flexibility in stacking different materials with competing symmetry broken states. Among them, van der Waals ferromagnets such as CrI3 and superconductors as NbSe2 provide a natural platform to engineer novel phenomena at ferromagnet-superconductor interfaces. In particular, NbSe2 is well known for hosting strong spin-orbit coupling effects that influence the properties of the superconducting state. Here we put forward a ferromagnet/NbSe2/ferromagnet heterostructure where the inte…
Superconductivity and magnetism in Iron Sulfides Intercalated by Metal Hydroxides
受天然硫化物矿物的启发，我们提出了一个新的铁基超导体家族。亚稳态形式的FeS被称为矿物mackinawite形成二维片，可以很容易地插入各种阳离子客体物种。在水热条件下，使用碱金属氢氧化物，我们制备了三种不同的阳离子和金属氢氧化物插层的FeS相，包括（Li $_{1-x}$ Fe $_x$ OH）FeS，[（Na $_{1-x}$ Fe $_x$）（OH）$_2$ ]FeS和K $_x$ Fe $_{2-y}$ S $_2$。在成功嵌入FeS层后，（Li $_{1-x}$ Fe $_x$ OH）$^{δ+}$嵌入物的超导临界温度$T_c$从5 K提高到8 K. [（Na $_{1-x}$ Fe $_x$）（OH）$_2$ ]FeS的层状异质结构类似于天然矿物托奇林石，其包含与六方氢氧化物晶格交错的铁正方形晶格。而异质结构的[（Na $_{1-x}$ Fe $_x$）（OH）$_2$ ]FeS在15 K附近显示长程磁有序，K $_x$ Fe $_{2-y}$ S $_2$显示短程反铁磁性。
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。
The effective attractive interaction between electrons, mediated by electron-phonon coupling, is a well-established mechanism of conventional superconductivity. In metals exhibiting a Fermi surface, the critical temperature of superconductivity is exponentially smaller than the characteristic phonon energy. Therefore such superconductors are found only at temperatures below a few Kelvin. Systems with flat energy bands have been suggested to cure the problem and provide a route to room-temperature superconductivity, but previous studies are limited to only BCS models with an effective attractiv…
