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1、铁基超导体 BaFe2-xNixAs2 中磁性相互作用的中子散射 罗会仟 中国科学院物理研究所北京凝聚态物理国家实验室 摘 要： 铁基超导电性起源于对母体中长程反铁磁序的压制, 理解其超导微观机理的关键在于清楚认识磁性相互作用的演化规律, 及其与超导电性的具体关系.本文以典型的电子型掺杂铁基超导体系 BaFe2-xNixAs2为对象, 介绍了中子散射实验针对其电子态相图、磁有序态、低能磁激发和高能自旋涨落谱等磁性物理的相关研究, 着重总结它们随掺杂的系统而演变的规律, 同时介绍了在自旋向列相中的最新研究进展.最后, 基于这些结果, 提出了磁性相互作用驱动的非常规铁基超导电性的可能物理图像.关键
2、词： 铁基超导; 中子散射; 磁有序; 磁激发; 自旋涨落; 作者简介：罗会仟 E-mail: 收稿日期：2017-07-25基金：国家重点基础研究发展计划 (2011CBA00110, 2012CB821400, 2015CB921302) Neutron scattering studies on the magnetic interaction in iron-based superconductor BaFe2-xNixAs2LUO HuiQian Beijing National Laboratory of Condensed Matter Physics, Institute of
3、 Physics, Chinese Academy of Sciences; Abstract： Iron-based superconductivity emerges from the suppression of the long-ranged antiferromagnetic order in the parent compounds. To solve the microscopic mechanism of the superconductivity, the key issue is fully understanding about the evolution of magn
4、etic interaction and its relationship between superconductivity. Here, we take the electron-doped iron-based superconductor BaFe2-xNixAs2 as atypical example, and summarize the neutron scattering results on the electronic phase diagram, magnetic ordered state, low-energy spin excitations and high-en
5、ergy spin fluctuations, particularly for their systematic evolution versus electron dopings and the recent progress on the spin nematicity. Firstly, we will introduce about the crystal structure and magnetic structure, detailed procedure of the single crystal growth is also presented. By focusing on
6、 the structural transition temperature (Ts) and magnetic transition temperature (TN) near the optimal doping level, we have found both Ts and TN vanish above the superconducting transition temperature (Tc) as a first order manner, due to the lattice distortion and magnetically ordered moment decreas
7、e beyond the lower limit of the instrument resolution, giving an avoid quantum critical point at the optimal doping. In addition, the magnetic order becomes incommensurate and short-ranged magnetic cluster and competes with superconducting order, suggesting strong interplay between magnetism and sup
8、erconductivity. Secondly, we will discuss about the itinerant magnetism and spin resonance mode at low energy for spin dynamics. The spin resonance mode in iron pnictides is explained as the collective quasiparticle excitations from the Fermi surface nesting between the hole pocket at the point and
9、electron pocket at the M point. This requires itinerant magnetism from the electrons near Fermi surfaces, which is further confirmed by neutron scattering by discovering the longitudinal mode and line shape change of low-energy spin excitations upon doping. Thirdly, we will present the doping evolut
10、ion of spin fluctuations throughout the whole phase diagram. Although the spin waves in the parent compound can be described by an effective Heisenberg mode with anisotropic exchange coupling in Fe As plane, the damping features and unexpected low total fluctuation moments reveal both local moments
11、and itinerant electrons contribute to the magnetism. Upon Ni doping, the high energy spin fluctuations is very robust, but the low energy spin excitations are suppressed quickly to zero when cross the boundary of the overdoped superconducting regime. The analysis on the change of the magnetic coupli
12、ng energy associated with the superconducting transition gives evidence that the magnetic fluctuations are strong enough to drive the condensation of the Cooper pairs. Fourthly, we will further introduce the recent progress on the spin anisotropy by polarized neutron scattering experiments, and spin
13、 nematicity measured on detwinned samples. Anisotropic spin excitations at low energy are discovered by polarized neutron scattering, which is caused by the in-plane orbital ordering and persists away up to the tetragonal paramagnetic state. Further experiments on the detwinned sample under uniaxial
14、 pressure give direct evidence of the spin nematicity shown as breaking C4 rotation symmetry spin excitations in the tetragonal phase, similar to the anisotropic resistivity and electronic nematic phase discovered in other probes. Such novel electron state in these materials suggests the iron pnicti
15、des are unconventional. Finally, we summarize the results about the magnetic phase transition and spin dynamics and discuss its physical origin. Then a possible picture about the spin driven unconventional superconductivity in these materials is proposed. Further perspectives on the research of magn
16、etism in iron-based superconductivity are also given.Keyword： iron-based superconductors; neutron scattering; magnetic order; magnetic excitations; spin fluctuations; Received： 2017-07-25引用格式:罗会仟.铁基超导体 Ba Fe2-xNixAs2中磁性相互作用的中子散射.科学通报, 2017, 62:39553966Luo H Q.Neutron scattering studies on the magnetic interaction in iron-based superconductor Ba Fe2-xNixAs2 (in Chinese) .Chin Sci Bull, 2017, 62:39553966, 以铁砷化物和铁硒化物为主的铁基超导体自 2008 年初发现以来, 受到了凝聚态物理领域的广泛关注1.由于 Sm Fe
