Nematic fluctuations occur in a wide range physical systems from biological molecules to cuprates and iron pnictide high-T_c superconductors. It is unclear whether nematicity in pnictides arises from electronic spin or orbital degrees of freedom. We studied the iron-based Mott insulators La₂O₂Fe₂OM₂M = (S, Se), which are structurally similar to pnictides. Nuclear magnetic resonance revealed a critical slowing down of nematic fluctuations and complementary Mössbauerr spectroscopy data showed a change of electrical field gradient. The neutron pair distribution function technique detected local C₂ fluctuations while neutron diffraction indicates that global C₄ symmetry is preserved. A geometrically frustrated Heisenberg model with biquadratic and single-ion anisotropic terms provides the interpretation of the low temperature magnetic fluctuations. The nematicity is not due to spontaneous orbital order, instead it is linked to geometrically frustrated magnetism based on orbital selectivity. This study highlights the interplay between orbital order and spin fluctuations in nematicity.

向列波动发生在从生物分子到铜酸盐和铁锡高T _c超导体的广泛物理系统中。尚不清楚肽的向列性是由电子自旋还是轨道自由度引起的。我们研究了铁基莫特绝缘子La ₂ O ₂ Fe ₂ OM ₂ M =（S，Se），其结构与磷化物相似。核磁共振表明，向列波起伏明显减缓，补充的Mössbauerr光谱数据表明电场梯度发生了变化。中子对分布函数技术检测到局部C ₂波动，而中子衍射表明整体C₄对称性得以保留。具有双二次和单离子各向异性项的几何受挫的海森堡模型提供了低温磁场波动的解释。向列性不是由于自发的轨道顺序引起的，而是与基于轨道选择性的几何受阻的磁性有关。这项研究突出了向列性和自旋波动之间的相互作用。