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ICQM faculty member Fa Wang publishes an article in Nature Physics reporting ‘Nematicity and Quantum Paramagnetism in FeSe’
《自然•物理》发表量子材料科学中心王垡及合作者的工作“FeSe中的向列量子顺磁性”

Most parent compounds of iron-based high temperature superconductors display a finite temperature “nematic” phase transition with spontaneous breaking of the 4-fold crystal rotation symmetry. This transition is usually accompanied by a stripe antiferromagnetic ordering transition at the same or slightly lower temperature. For this reason the driving force of nematicity in these iron-based materials is usually regarded as the antiferromagnetic correlation.

FeSe however shows very different phenomenology. It has a nematic phase transition at 90Kelvin but no magnetic order down to the lowest temperature. And recent nuclear magnetic resonance studies do not see enhancement of low energy spin fluctuation across the nematic transition in FeSe. People thus speculate that the nematic transition in FeSe is not driven by magnetic correlation but possibly orbital order.

Prof. Fa Wang of ICQM at Peking University, in collaboration with Prof. Kivelson of Stanford University and Prof. Dung-Hai Lee of UC Berkeley, challenged this perception of non-magnetic mechanism for nematicity in FeSe. They established by theoretical arguments and numerical results that a “nematic quantum paramagnet” phase can exist for frustrated spin-1 models on square lattice. This phase spontaneously breaks 4-fold crystal rotation symmetry but has no magnetic order and no low-energy spin excitations. They argued that the nematic quantum paramagnet phase can explain the unusual phase diagram of FeSe, and predicted the existence of finite energy spin fluctuations at both stripe and Neel ordering wavevectors. This work has been published in Nature Physics [Nature Physics 11, 959 (2015)].

The work done in PKU was supported by the National Science Foundation of China(Grant No. 11374018) and National Key Basic Research Program of China (Grant No. 2014CB920902), and the “Thousand Talents Program for Distinguished Young Scholars”.

Left: Schematic depiction of two prototypical wavefunctions of nematic quantum paramagnet states. The spin-1 moments on solid blue lines form AKLT chains. Right: Spin excitation gap(S=1) and spin-singlet excitation gap(S=0) for 4x4 square lattice J1-J2 Heisenberg model. Around J2/J1=0.6 the spin gap is large but the spin-singlet gap is very small, suggesting nematic quantum paramagnet ground states in the thermodynamic limit.

        大部分铁基超导的母体在有限温度会有“向列”相变,在此温度之下晶格的四重旋转对称性会自发破缺,而在此相变温度下略低或相同的温度会发生条纹反铁磁相变。所以通常认为这些铁基材料中的向列相是由反铁磁关联驱动。

    但是FeSe体材料的行为与典型的铁基超导材料非常不同。FeSe90K温度有向列相变,但直到可以测量的最低温度都没有磁性长程序。最近的核磁共振研究在向列相变温度附近也没有看到低能自旋涨落的增强。基于这些结果,一些人推测FeSe中的向列相和磁性无关而可能是由轨道序驱动。

    北京大学物理学院的王垡研究员和斯坦福大学的Kivelson教授以及加州大学伯克利分校的李东海教授最近合作提出了对FeSe中向列相起源的不同理解。他们通过理论论证和数值计算提出正方晶格上自旋为1的阻挫自旋模型可以有一种“向列量子顺磁相”。这个相自发地破缺晶格的四重旋转对称性,但是有自旋能隙,所以没有接近零能的自旋涨落。王垡及合作者指出这种物相可以解释FeSe体材料的不寻常物性,并且预言FeSe会在条纹反铁磁序和交错反铁磁序的两种波矢都具有有限能量的低能自旋涨落。该工作已在《自然•物理》上发表 [Nature Physics 11, 959 (2015)]

    这项研究受到国家重大科学研究计划、国家自然科学基金和中组部“青年千人”计划的支持。


左图:两个向列量子顺磁态的原型波函数示意图。蓝色直线上的自旋1磁矩构成AKLT自旋链。

右图:自旋1J1-J2海森堡模型在4x4晶格上的自旋(S=1)能隙和零自旋(S=0)能隙。在J2/J1接近0.6的区域自旋能隙较大而零自旋能隙非常小,预示了热力学极限下的向列量子顺磁基态。