We use transport measurements to determine the in-plane anisotropy of the upper critical field Hc2 in detwinned superconducting Ba(Fe1-xMx)2As2 (M = Co, Ni) single crystals. In previous measurements on twinned single crystals, the charge carrier doping dependence (x) of the upper critical field anisotropy for fields along the inter-planar (c-axis) and in-plane field directions was found to increase in the overdoped regime. For underdoped samples, which exhibit a spin nematic phase below the tetragonal to orthorhombic structural transition temperature Ts , we find that Hc2 along the a-axis is considerably lower than that along the b-axis. The upper critical field anisotropy disappears in the over-doped regime when the system becomes tetragonal. By combining these results with inelastic neutron scattering studies of spin excitations, and angle-resolved photoemission spectroscopy, we conclude that superconductivity in under-doped iron pnictides is orbital selective - with a dominant contribution from electrons with the dyz orbital character and being intimately associated with spin excitations.
Energy-Resolved Real-Space Imaging of Orbital Nematicity in an Fe-Based Superconductor
University of Tokyo（东京大学）；Hitachi High-Tech Corporation（日立高新技术公司）；Material Innovation Research Center (MIRC), University of Tokyo（东京大学材料创新研究中心）；National Institute of Advanced Industrial Science and Technology (AIST)（国立产业技术综合研究所）
Nematic fluctuations mediated superconductivity revealed by anisotropic strain in Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$
各向异性应变是一种能够选择性地解决超导涨落在促进超导性中的作用的外部场。我们证明了这一点，使用偏振分辨的弹性拉曼散射探测下的正常和超导状态的典型的铁基超导体Ba（Fe $_{1-x}$ Co $_x$）$_2$ As $_2$的应变下的电子涨落的演变。在非超导的母体化合物BaFe $_2$ As $_2$中，我们观察到了应变引起的超导磁化率的抑制，它遵循了对称破缺场下伊辛序参量的预期行为。对于超导化合物，超导磁化率的抑制与超导临界温度$T_c$的降低有关。我们的研究结果表明，一个显着的贡献，电子配对和增强$T_c$附近的一个量子临界点的理论方案验证的涨落。
Compatible Instability: Gauge Constraints of Elasticity Inherited by Electronic Nematic Criticality
University of Illinois Urbana-Champaign（伊利诺伊大学厄巴纳-香槟分校）；The Grainger College of Engineering, University of Illinois Urbana-Champaign（伊利诺伊大学厄巴纳-香槟分校格兰杰工程学院）；Anthony J. Leggett Institute for Condensed Matter Theory, The Grainger College of Engineering, University of Illinois Urbana-Champaign（伊利诺伊大学厄巴纳-香槟分校格兰杰工程学院安东尼·J·利格特凝聚态理论研究所）
Electric Transport of a Single Crystal Iron Chalcogenide FeSe Superconductor: Evidence of Symmetry Breakdown Nematicity and Additional Ultrafast Dirac cone-Like Carriers
在FeAs超导体中普遍观察到SDW反铁磁（SDW-AF）低温相变，AF自旋涨落对超导配对机制起着重要作用.然而，在FeSe超导体中没有观察到类似的磁相变，这引起了相当多的讨论。我们报告的内在电子状态的铁硒阐明运输测量下磁场使用高品质的单晶。一个迁移率谱分析，从头算方法，不作假设的多载流子系统中的输运参数，提供了非常重要和明确的证据，另一个隐藏的顺序，最有可能的对称性破缺从四碳C4对称性的C2对称向列性与选择性d-轨道分裂，存在于超导FeSe的情况下以外的AF磁序自旋涨落。FeSe中的本征低温相处于几乎补偿的半金属态，但另外伴随有狄拉克锥状超快电子$\sim$ 10 $^4$ cm $^2$（VS）$^{-1}$作为少数载流子。
Conditions for orbital selective altermagnetism in Sr$_2$RuO$_4$: tight binding model, similarities with cuprates and implications on superconductivity
Orbital-dependent modulation of the superconducting gap in uniaxially strained Ba$_{0.6}$K$_{0.4}$Fe$_2$As$_2$
表征超导配对机制的配对对称性通常通过测量超导能隙结构（$|Δ_k|$）来确定。在这里，我们报告的测量应变引起的间隙调制（$\partial|Δ_k|$）在单轴应变Ba $_{0.6}$ K $_{0.4}$ Fe $_2$作为$_2$利用角分辨光电子能谱和$in$ - $situ$应变调谐。我们发现，单轴应变驱动Ba $_{0.6}$ K $_{0.4}$ Fe $_2$ As $_2$进入超导态，打破了超导对的四重旋转对称性。超导能隙在$d_{yz}$电子和空穴袋上增大，而在$d_{xz}$对应物上减小。这种轨道选择性表明，轨道选择性配对存在于非铁基超导体的内在。$d_{xz}$和$d_{yz}$配对通道在原始超导状态下是平衡的，但在单轴应变下变得不平衡。我们的研究结果突出了铁基超导体中轨道内散射在介导超导配对中的重要作用。它还强调了测量的$\partial|Δ_k|$作为一种有效的方法来表征超导配对从微扰的角度来看。
