发表机构
P.L. Kapitza Institute for Physical Problems, RAS; National Research University “HSE”; M.V. Lomonosov Moscow State University; National University of Science and Technology “MISiS”（俄罗斯科学院P.L.卡皮察物理问题研究所; 国立研究型大学高等经济大学; 莫斯科罗蒙诺索夫国立大学; 国立科技大学米西斯大学）
机构由 AI 辅助整理，请以论文原文为准。AI 中文总结
本研究通过实验与建模，揭示nabokoite家族中轻、重碱金属化合物的磁有序存在质的差异，重碱金属呈共线反铁磁有序，轻碱金属经铁电相变后形成两步相变的非共线磁有序。
AI 中文摘要
nabokoite家族化合物ACu$_7$(TeO$_4$)(SO$_4$)$_5$X（A=Na、K、Rb、Cs；X=Cl、Br）包含由额外层间磁性离子修饰的阻挫二维正方形kagomé晶格层。本研究通过多频电子自旋共振光谱及热力学测量（比热、磁化强度、介电常数）探究nabokoite的磁有序。研究表明，轻碱金属离子（K、Na）化合物与重碱金属离子（Rb、Cs）化合物的低温基态选择存在质的差异：重碱金属nabokoite呈现具有易轴各向异性的常规共线反铁磁模式，其有序反铁磁态参数在整个重碱金属亚族中非常接近；轻碱金属nabokoite则展现出更为复杂的有序态形成路径：首先在25-90K发生铁电相变以解除阻挫，从而预准备低温有序；随后通过两步相变形成异常非共线磁有序，第一步相变温度T$_{c1}$≃5-6K，第二步相变温度T$_{c2}$≃3-4K，非共线有序通过特征性非拉莫尔反铁磁共振模式的观测得到证实。轻碱金属nabokoite的自旋动力学以三个零场磁振子能隙和两个自旋重取向场为特征，不同化合物的磁振子能隙与临界场值差异显著。结合nabokoite结构的金字塔结构块的有限尺寸团簇建模表明，nabokoite交换耦合参数与金字塔结构块不同量子基态边界的临界接近度，可能是轻、重碱金属nabokoite选择质的不同有序态的关键。
英文摘要
Nabokoite family compounds ACu$_7$(TeO$_4$)(SO4)$_5$X (A=Na, K, Rb, Cs; X=Cl, Br) host frustrated 2D square kagom'{e} lattice layers decorated by additional inter-layer magnetic ions. We study magnetic order in nabokoites with multi-frequency electron spin resonance spectroscopy and thermodynamic measurement (specific heat, magnetization and dielectric permittivity). Our study reveals that the choice of the low-temperature ground state is qualitatively different in light-alkali-ion (K, Na) and heavy-alkali-ion (Rb, Cs) compounds. Heavy-alkali-ion nabokoites order in conventional collinear antiferromagnetic pattern with easy-axis anisotropy. The parameters of the ordered antiferromagnetic state are very close for all heavy-alkali-ion subfamily. Light-alkali-ion members of nabokoite family demonstrate much more complicated route to the ordered state: firstly, a ferroelectric transition at 25-90K lifts the frustration and thus pre-cooks the low-temperature ordering; secondly, an unusual noncollinear magnetic order develops via two-step phase transition with first transition temperature $T_{c1}\simeq 5-6$K and the second transition at $T_{c2}\simeq 3-4$K. Noncollinear order is evidenced by observation of characteristic non-Larmor antiferromagnetic resonance mode. Spin dynamics of light-alkali-ion nabokoites is characterized by three zero-field magnon gaps and two spin-reorientation fields, the values of magnon gaps and critical fields are quite different for different compounds. The finite-size cluster modeling of pyramidal structural block of nabokoite structure combined suggests that the critical closeness of the nabokoite exchange coupling parameters to the border-line between the different quantum ground state of pyramidal building block of nabokoite structure could be the clue to the choice of qualitatively different ordered state in light- and heavy-alkali-ion nabokoites.
Comments22 pages, 16 figures
