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ICQM member Yuan Li's group publishes an article in Physical Review X reporting direct evidence for an itinerant origin of antiferromagnetism in iron pnictides superconductors
铁基超导体中巡游电子对反铁磁性的贡献

Iron pnictides, a class of high-temperature superconductors discovered only in 2008, have presented many fundamental puzzles for physicists working on superconductors. One of the puzzles is the proximity of an antiferromagnetic phase to the superconducting phase in these materials, raising the tantalizing possibility of a fundamental connection between magnetism and superconductivity in these materials. What is the microscopic origin of their antiferromagnetism, then? Existing experiments point to two possibilities: Either the antiferromagnetism arises from spin moments of electrons localized on (iron) nuclei or it comes from the collective ordering of the spins of itinerant (mobile) electrons. However, no “smoking-gun” evidence has been found for either of these possibilities.

In fact, the two scenarios leave fundamentally different signatures in how the microscopic magnetic moments (or spins) ordered into the antiferromagnetic state fluctuate when excited. In the local-moment picture, the moments have a fixed size, so their fluctuations are dominated primarily by their precessions around the ordering direction. In the itinerant-electron picture, the sizes of the moments themselves can additionally fluctuate, giving rise to “longitudinal spin excitations.” 

In the newly published paper' Longitudinal Spin Excitations and Magnetic Anisotropy in Antiferromagnetically Ordered BaFe2As2' , Prof. Yuan Li and his Ph.D. student Chong Wang in collaboration with Prof. Pengcheng Dai's group at the Institute of Physics, Chinese Academy of Sciences have investigated the spin excitations on BaFe2As2, a “parent” compound of iron pnictides, using spin-polarized inelastic neutron scattering (INS). The measurement was performed on spectrometer IN22 at Institute Laue Langevin in Grenoble, France, with the support of Dr. Regnault. Indeed, spin-polarized INS can selectively determine the direction of the excited spin fluctuations. Using a state-of-the-art implementation of this technique and a very large single-crystal sample, Prof. Yuan Li and his collaborators have achieved an unprecedented precision in determining the spin fluctuations and are thus able to uncover an unequivocal signature of longitudinal spin excitations in the experimental spectrum, in other words, smoking-gun evidence for a sizable contribution of itinerant electrons to the antiferromagnetism.

Since itinerant electrons are in no doubt important for superconductivity in doped iron pnictides, this finding puts on a firmer footing the notion of an intimate connection between the magnetism and the superconductivity.

The work at Peking University is supported by the NBRP of China and the NSF of China.





Figure 1, Intrinsic spin-excitation signals extracted from the neutron scattering data. Ma represents longitudinal spin excitations; Mb and Mc represent two transverse spin excitations.

量子中心李源老师组在《物理评论X》 杂志发表文章,报道了铁基超导体中巡游电子对反铁磁性具有贡献的直接实验证据。

铁基超导体中的超导相,与在铜基超导体中一样,总是发生在反铁磁转变的边缘。超导与磁性一直被认为有着很大的关系。但是不同于铜基超导体,铁基超导体中磁性的微观起源一直是一个存在争议的问题。已有的实验结果分别支持两种可能:一种认为反铁磁性来源于局域电子的自旋磁矩;而另一种认为是巡游电子的集体行为带来了这种磁性。虽然有很多实验试图去澄清这两种观点,但却没有一个确凿的直接证据能肯定其中任何一个说法。

这两种图像给出的磁有序态下的自旋激发谱具有不同的特点。对于局域电子图像来说,自旋有着确定的大小,因此自旋的激发只可能是围绕着自旋排布方向的幅值大小不变的进动,也就是“横模自旋激发”;而对于巡游电子图像,因为磁序是巡游电子给出的,其幅值依赖于参与磁序的巡游电子数量,所以自旋激发谱会另外多出一个沿着自旋排布方向幅值振动的模式,也就是“纵模自旋激发”。

在最新发表的文章“Longitudinal Spin Excitations and Magnetic Anisotropy in Antiferromagnetically Ordered BaFe2As2”,李源教授和他的博士生王冲与中科院物理所戴鹏程老师组合作,利用非弹性中子散射的方法对铁基超导的母体材料BaFe2As2的磁激发谱进行了研究。该实验在法国劳厄-郎之万实验室的IN22中子谱仪上与谱仪科学家勒尼奥博士合作完成。自旋极化的中子散射可以区分不同自旋激发模式的振动方向。使用目前最先进的实验手段和非常大的单晶样品,李源教授和他的合作者获得了足够好的数据精度,证明了纵模自旋激发的存在。这个发现为铁基超导体中反铁磁性的巡游电子起源找到了确凿的证据。

掺杂样品中的超导现象毫无疑问来自于巡游电子,而此项研究表明在母体材料中巡游电子对反铁磁性也有重要的贡献。因此,这更加确定了巡游电子在磁性和超导之间的重要纽带关系。

本项工作得到了中国国家基础研究项目和中国国家自然科学基金的支持。





1,从自旋极化中子散射数据中提取出的自旋激发信号。其中Ma代表了纵模自旋激发; MbMc代表另外两个横模自旋激发。