Fluctuations and pairing in Fe-based superconductors: light scattering experiments
Fluctuations and pairing in Fe-based superconductors: light scattering experiments
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DOI：
10.1088/1361-648x/ab8849
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发表时间：
2019-08
期刊：
影响因子：
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通讯作者：
Nenad Lazarević;Rudi Hackl
中科院分区：
文献类型：
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作者：
Nenad Lazarević;Rudi Hackl
可见光的非弹性散射(拉曼效应)为了解相关金属的性质，如自旋、电子和晶格动力学及其相互作用提供了一个窗口。在这篇综述中，我们集中讨论了铁基锡矿和硫系化合物中的电子和自旋激发，特别是但不只是超导体。在简要介绍了基本理论，包括各种散射过程的选择规则之后，我们对主要的实验结果进行了概述。在低于转变温度T_c的超导态，可以观察到对破裂效应，并可求出能隙，并与电子和空穴能带上的能隙相关联。尽管整体带结构相似，但结果强烈依赖于家族，甚至可能在一个家族内发生质的变化。在某些化合物中，强集体模出现在T_c以下。在所有铁基超导体中具有最各向同性带隙的Ba1−xKxFe2As2中，有迹象表明这些模式是在存在一系列配对倾向的情况下出现的类激子状态。在Co掺杂NaFeAs中观察到的强带隙模式可以用四极轨道激发来解释，这些激发在超导态下变得无阻尼态。在Ba(Fe1−xCox)2As2中，散射强度的掺杂依赖于量子临界点以上的向列相共振，并用最大T_c处的临界增强来解释。在正常情况下，粒子-空穴激励的响应反映了电阻率。此外，KBT的能量范围内可能存在与弹性性质相当的临界涨落，这与温度有很强的相关性。目前，光散射观察到的涨落是与自旋有关还是与电荷有关，目前还没有定论。另一个争议与假定的双磁子激发有关，通常在0.5 eV以下的能量范围内。然而，在大多数铁基化合物中，这种响应可能源于电荷激发，理论和实验表明，FeSe在60 meV范围内的激发来源于近乎受挫的体系中的局域自旋。
Inelastic scattering of visible light (Raman effect) offers a window into properties of correlated metals such as spin, electron and lattice dynamics as well as their mutual interactions. In this review we focus on electronic and spin excitations in Fe-based pnictides and chalcogenides, in particular but not exclusively superconductors. After a general introduction to the basic theory including the selection rules for the various scattering processes we provide an overview over the major experimental results. In the superconducting state below the transition temperature Tc the pair-breaking effect can be observed, and the gap energies may be derived and associated with the gaps on the electron and hole bands. In spite of the similarities of the overall band structures the results are strongly dependent on the family and may even change qualitatively within one family. In some of the compounds strong collective modes appear below Tc. In Ba1−xKxFe2As2, which has the most isotropic gap of all Fe-based superconductors, there are indications that these modes are exciton-like states appearing in the presence of a hierarchy of pairing tendencies. The strong in-gap modes observed in Co-doped NaFeAs are interpreted in terms of quadrupolar orbital excitations which become undamped in the superconducting state. The doping dependence of the scattering intensity in Ba(Fe1−xCox)2As2 is associated with a nematic resonance above a quantum critical point and interpreted in terms of a critical enhancement at the maximal Tc. In the normal state the response from particle–hole excitations reflects the resistivity. In addition, there are strongly temperature-dependent contributions from presumably critical fluctuations in the energy range of kBT which can be compared to the elastic properties. Currently it is not settled whether the fluctuations observed by light scattering are related to spin or charge. Another controversy relates to putative two-magnon excitations, typically in the energy range below 0.5 eV. Whereas this response presumably originates from charge excitations in most of the Fe-based compounds theory and experiment suggest that the excitations in the 60 meV range in FeSe stem from localized spins in a nearly frustrated system.