High pressure electrical resistivity and x-ray diffraction experiments have been performed on Fe single crystals. The crystallographic investigation provides direct evidence that in the martensitic $bcc \rightarrow hcp$ transition at 14 GPa the $\lbrace 110\rbrace_{bcc}$ become the $\lbrace 002\rbrace_{hcp}$ directions. During a pressure cycle, resistivity shows a broad hysteresis of 6.5 GPa, whereas superconductivity, observed between 13 and 31 GPa, remains unaffected. Upon increasing pressure an electronic instability, probably a quantum critical point, is observed at around 19 GPa and, close to this pressure, the superconducting $T_{c}$ and the isothermal resistivity ($0<T<300\,$K) attain maximum values. In the superconducting pressure domain, the exponent $n = 5/3$ of the temperature power law of resistivity and its prefactor, which mimics $T_{c}$, indicate that ferromagnetic fluctuations may provide the glue for the Cooper pairs, yielding unconventional superconductivity.
Unconventional pairing in the iron arsenide superconductors
我们用磁长程序作为工具来探测砷化铁超导体中的库珀对波函数。我们从理论上证明，只有当库珀对形成一个非常规的，符号变化的状态，反铁磁性和超导性可以共存于这些材料。在Ba（Fe $_{1-x}$ Co $_{x}$）$_{2}$ As $_{2}$中共存的观察则证明了该材料中的非常规配对。理论和中子衍射实验之间的详细协议，特别是不寻常的行为的磁序低于$T_{c}$，证明了我们的结论的鲁棒性。我们的研究结果强烈地表明，在砷化铁家族的所有成员中，超导性都是非常规的。
Unconventional superconductivity in the layered iron germanide YFe$_2$Ge$_2$
自从2006年在LaFePO中发现超导性以来，已经在不同的结构家族中鉴定出许多铁基超导体，所有这些都将铁与V族（磷属元素）或VI族（硫属元素）元素结合。非常规的超导性在这些层状铁系统和铜酸盐之外的过渡金属化合物中是极其罕见的，并且它几乎普遍与高度各向异性的电子性质和接近2D的费米表面几何形状相关联。铁基金属间化合物YFe $_2$ Ge $_2$具有3D费米面和强烈增强的低温热容，这表明强的电子相关性。我们目前的数据从新一代的高品质样品的YFe $_2$锗$_2$，这表明超导转变异常低于1.8 K的热力学以及运输测量，建立超导性是内在的，在这个层状铁化合物以外的已知的超导铁磷属元素化物或硫属元素化物家庭。YFe $_2$ Ge $_2$的费米面几何形状类似于KFe $_2$ As $_2$在高压下坍缩的四方相的费米面几何形状，其中最近报道了在高达10 K的温度下的超导性，这表明两个系统之间的潜在联系。
Evidence for Unconventional Superconductivity in Arsenic-Free Iron-Based Superconductor FeSe : A ^77Se-NMR Study
本文报道了具有与LaFeAsO $_{1-x}$ F $_x$超导体相似的晶体结构的$α$的核磁共振（NMR）结果.核自旋晶格弛豫速率$1/T_1$在超导转变温度$T_c$以下表现出$T^3$行为，没有相干峰。在高于$T_c$的宽温度范围内，可以看到指示费米液态的$T_1T=$恒定行为。$α$ -FeSe的超导电性与LaFeAsO $_{1-x}$ F $_x$及相关材料一样，也是一种非常规的超导电性. FeAs层对非常规超导性的发生不是必需的。
Superconductivity is a remarkably widespread phenomenon observed in most metals cooled down to very low temperatures. The ubiquity of such conventional superconductors, and the wide range of associated critical temperatures, is readily understood in terms of the celebrated Bardeen-Cooper-Schrieffer (BCS) theory. Occasionally, however, unconventional superconductors are found, such as the iron-based materials, which extend and defy this understanding in new and unexpected ways. In the case of the iron-based superconductors, this includes a new appreciation of the ways in which the presence of m…
Evidence of unconventional low-frequency dynamics in the normal phase of Ba(Fe1-xRhx)2As2 iron-based supercondutors
本文报道了Ba（Fe_（1-x）Rh_x）_2As_2超导体的~（75）As核磁共振自旋回波衰减率（1/T_2）的测量结果，其中0.041 < x < 0.094。结果表明，1/T2增加冷却后，在正常阶段，这表明一个非常规的非常低的频率激活动态的发病。导出了涨落的关联时间及其能垒。该运动有利于在大的Rh含量，而它是由垂直于FeAs层的磁场的应用程序阻碍。在自旋-晶格弛豫速率中以定量一致的方式观察到相同的动态。这些结果进行了讨论，在光的磁波动涉及畴壁运动。的类比与观察到的铜酸盐的行为也概述了。
Unconventional superconducting gap via spin fluctuations in iron-vacancy ordered A$_{y}$Fe$_{2-x}$Se$_{2}$
基于有效的12轨道紧束缚模型，研究了铁空位有序A $_{y}$ Fe $_{2-x}$ Se $_{2}$的反铁磁涨落诱导的超导态.结果表明，由于铁空位引起的反射对称性破缺，出现了具有$C_{4h}$对称性的新超导态。特别是，我们表明，在$C_{4h}$对称性，对称轴的配对动量不需要与轴的晶胞。结果，除了配对能隙的大小之外，配对波函数与晶格的相对取向形成了用于表征超导能隙的另一个自由度，并且可以进一步帮助获得凝聚能。尽管如此，与其他铁基超导体类似，单重基态仍然由s波或d波主导，它们几乎是简并的，具有各向异性的间隙。此外，s-波和d-波超导态被Hund规则耦合$J_{H}$控制的量子临界点分开。
Unconventional pairing originating from disconnected Fermi surfaces in the iron-based superconductor
对于铁基高$T_c$超导体LaFeAsO $_{1-x}$ F $_x$，我们构造了一个极小模型，其中所有的五个Fe $d$带都参与其中.然后，我们研究了超导性的起源与五带随机相位近似通过求解Eliashberg方程。我们得出结论，所产生的自旋波动模式之间的嵌套断开费米口袋实现，基本上，一个扩展的s波配对，其中的间隙改变整个嵌套矢量的符号。
Quantum critical point lying beneath the superconducting dome in iron-pnictides
量子临界点（QCP）是否位于超导穹顶之下一直是一个长期存在的问题，在许多非常规超导体中仍然没有解决，特别是铜酸盐，重费米子化合物和最近的铁磷属元素化物。QCP的存在可能提供一种理解的途径：它们异常的非费米液体性质的起源，非常规超导性和磁性或某种奇异秩序之间的微观共存，以及最终超导性本身的机制。等价取代的铁磷属元素化物BaFe $_2$（As $_{1-x}$ P $_x$）$_2$为量子临界性的研究提供了一个新的平台，为研究相图中电子性质的演化提供了一个独特的机会。最近在BaFe $_2$（As $_{1-x}$ P $_x$）$_2$中的实验首次提供了超导圆顶下存在二级量子相变的明确证据。
Antiferromagnetic Spin Fluctuations and Unconventional Nodeless Superconductivity in an Iron-based New Superconductor (Ca_4Al_2O_{6-y})(Fe_2As_2):75As-NQR Study
Three-dimensional quantum Griffiths singularity in bulk iron-pnictide superconductors
Peking University（北京大学）；Beijing Academy of Quantum Information Sciences（北京量子信息科学研究院）；Dalian University of Technology（大连理工大学）；Southern University of Science and Technology（南方科技大学）；Hefei National Laboratory（合肥国家实验室）
Quasiparticle excitations provide viable information on the physics of unconventional superconductors. Higgs and Leggett modes are some of the classic examples. Another important bosonic excitation is the spin exciton originating from the sign-changing superconducting gap structure. Here we report a direct observation of the temperature-dependent spin exciton in the Andreev spectra of iron-based superconductors. Combined with the other experimental evidence, our observation confirms the extended $s$-wave ($s_\pm$) order parameter symmetry and indirectly proves the spin-fluctuation mechanism of…
The strange metal Hall effect connects quantum criticality and superconductivity in an iron-based superconductor
许多非常规超导体表现出一组共同的异常电荷输运性质，将它们描述为“奇怪的金属”，这为存在描述它们的单一理论提供了希望。然而，奇怪的金属和超导性之间的模型无关的联系仍然难以捉摸。在这封信中，我们表明，非常规超导体BaFe $_2$（As $_{1-x}$ P $_x$）$_2$的霍尔效应包含一个奇怪的金属内的关联引起的异常贡献。这个项对磁场有着独特的依赖性，这使得我们能够跟踪它在掺杂温度相图上的行为，即使在超导圆顶下。这些测量表明，奇怪的金属霍尔组件从量子临界点发出，在零温度极限，超导临界温度成比例地衰减。这在量子临界性和超导性之间建立了一种清晰而新颖的联系，并表明类似的联系存在于其他奇怪的金属超导体中。
Unconventional pairing originating from disconnected Fermi surfaces in superconducting LaFeAsO$_{1-x}$F$_x$}
对于新发现的铁基高$T_c$超导体LaFeAsO $_{1-x}$ F $_x$，我们构造了一个极小模型，其中包含所有五个Fe $d$带是必要的.随机相位近似被应用到该模型中，以研究超导电性的起源。我们的结论是，多自旋涨落模式所产生的嵌套在断开的费米面实现了扩展的s波配对，而d波配对也可以是另一个候选者。
There are several FeSe based superconductors, including the bulk FeSe, monolayer FeSe thin film, intercalated KxFe2-ySe2 and Li1-xFexOHFeSe, etc. Their normal states all show metallic behavior. The key player here is the FeSe layer which exhibits the highest superconducting transition temperature in the form of monolayer thin film. Recently a new FeSe based compound, CsFe4-xSe4 with the space group of Bmmm was found. Interestingly the system shows a strong insulator-like behavior although it shares the same FeSe planes as other relatives. Density functional theory calculations indicate that it…
Drastic field-induced resistivity upturns as signatures of unconventional magnetism in superconducting iron chalcogenides
University of Oxford（牛津大学）；Max Planck Institute for Solid State Research（马克斯·普朗克固体研究所）；Institute of High-Pressure Physics, Polish Academy of Sciences（波兰科学院高压物理研究所）；Institute for Quantum Materials and Technologies, Karlsruhe Institute of Technology（卡尔斯鲁厄理工学院量子材料与技术研究所）
Non-trivial topology and unconventional pairing are two central guiding principles in the contemporary search for and analysis of superconducting materials and heterostructure compounds. Previously, a topological superconductor has been predominantly conceived to result from a topologically non-trivial band subject to intrinsic or external superconducting proximity effect. Here, we propose a new class of topological superconductors which are uniquely induced by unconventional pairing. They exhibit a boundary-obstructed higher-order topological character and, depending on their dimensionality, …
The effect of disorder on electronic Raman scattering in the superconducting state of iron pnictides
电子拉曼散射测量偏振相关的散射强度，其可以提供关于非常规超导体的能隙中的节点位置以及其整体对称性的信息。本文计算了几种铁磷属元素化物超导态模型在无序情况下的喇曼强度。我们包括，为了完整性，$d$波和各向同性的$s_\pm$响应除了更现实的扩展$s_\pm$超导间隙。使用自洽的$T$矩阵近似模型的影响的障碍，并在各向同性和带内仅散射的限制进行了研究。我们表明，最近的实验Ba（Fe $_{1-x}$ Co $_x$）$_2$作为$_2$可能是一致的“节点提升”的带内无序。
Disorder-induced topological change of the superconducting gap structure in iron pnictides
在具有非常规配对机制的超导体中，激发谱中的能隙通常具有节点，这允许在低能量下的准粒子激发。在许多情况下，例如$d$波铜氧化物超导体，节点的位置和拓扑结构由对称性强加，因此无间隙激发的存在被保护免于无序。在这里，我们报告的观察铁磷族化合物超导体的间隙结构的明显变化，增加杂质散射。通过电子辐照在BaFe $_2$（As $_{1-x}$ P $_x$）$_2$晶体中连续引入点缺陷，从低温穿透深度测量中发现，结态转变为完全带隙激发的无结态。此外，在进一步的辐射下，带隙状态演变成另一个无隙状态，提供了大量的证据，非传统的符号变化$s$波超导性。这表明，超导能隙的拓扑结构可以由无序控制，这是铁磷属化物的一个惊人的独特特征。
Critical Role of Hydrogen in Unconventional Superconductors: The Case of Hydrogenated FeSe Layers
Beijing National Laboratory for Condensed Matter Physics and Institute of Physics, Chinese Academy of Sciences（中国科学院物理研究所 和 北京凝聚态物理国家研究中心）；School of Physical Sciences, University of Chinese Academy of Sciences（中国科学院大学 物理科学学院）；Songshan Lake Materials Laboratory（松山湖材料实验室）
The orbital characters of bands in iron-based superconductor BaFe1.85Co0.15As2
新发现的铁基超导体的非常规超导电性与其多带/多轨道性质密切相关。本文通过对角分辨光电子能谱数据的偏振态和光子能量依赖性的研究，报道了BaFe $_{1.85}$ Co $_{0.15}$ As $_2$中低能三维电子结构的综合轨道特征。虽然$d_{xz}$、$d_{yz}$和$d_{3z^2-r^2}$轨道的分布与密度泛函理论的预测一致，但$d_{xy}$和$d_{x^2-y^2}$轨道的分布与理论明显不一致。我们的结果指出了现有能带结构计算的不足，更重要的是，为构建正确的铁磷属元素化物微观模型提供了基础。
Electronic nematicity is often found in unconventional superconductors, suggesting its relevance for electronic pairing. In the strongly hole-doped iron-based superconductors, the symmetry channel and strength of the nematic fluctuations, as well as the possible presence of long-range nematic order, remain controversial. Here, we address these questions using transport measurements under elastic strain. By decomposing the strain response into the appropriate symmetry channels, we demonstrate the emergence of a giant in-plane symmetric contribution, associated with the growth of both strong ele…
Emergent loop-nodal s$_\pm$-wave superconductivity in CeCu$_2$Si$_2$: similarities to the iron-based superconductors
重费米子超导体由于具有自旋轨道耦合自由度和电子关联，是新电子配对态的主要候选者。1979年发现的CeCu $_2$ Si $_2$中的超导电性是强关联电子系统中非常规（非BCS）超导体的原型，至今仍没有解决。在这里，我们提供了基于先进的第一性原理理论方法的超导性的第一份报告。我们发现，有希望的候选者是一个$s_\pm$波态的费米面上的环形节点，不同于广泛预期的线节点$d$波态。主要的配对胶是磁性的，但高秩八极波动。该系统与铁基超导体共享多轨道自由度的重要性。我们的发现不仅揭示了这种材料中长期存在的谜团，而且促使我们重新考虑所有重费米子超导体中的配对状态和机制。
The role of nematic fluctuations for unconventional superconductivity has been subject of intense discussions for many years. In iron-based superconductors, the most established probe for electronic-nematic fluctuations, i.e. the elastoresistivity seems to imply that superconductivity is reinforced by electronic-nematic fluctuations, since the elastoresistivity amplitude peaks at or close to optimal $T_C$. However, on the over-doped side of the superconducting dome, the diminishing elastoresistivity suggests a negligible importance in the mechanism of superconductivity. Here we introduce the N…
Unconventional Pairing in Doped Band Insulators on a Honeycomb Lattice : the Role of the Disconnected Fermi Surface and a Possible Application to Superconducting $β$-MNCl (M=Hf,Zr)
