四方晶格YbMnBi$_2$中的自旋向列液晶与标量手性自旋
Spin nematic liquid crystal and scalar spin chirality in tetragonal lattice YbMnBi$_2$
AI总结：
本文通过极化中子散射研究YbMnBi₂的自旋序，发现其存在动态自旋向列相，该相与标量手性自旋耦合诱导反常霍尔效应等，为复合自旋序物理和室温自旋电子学提供新方向。
AI中文摘要：
自旋向列序类似于向列液晶，其特征是自发打破自旋空间旋转对称性，同时保留时间反演（$T$）对称性。相比之下，标量手性自旋（SSC）是一种复合三自旋序，会打破$T$对称性，已知其会诱导反常霍尔效应（AHE）。尽管已有研究在阻挫磁体和平方晶格铱酸盐中提出了自旋向列相，但它如何影响磁输运性质仍不清楚。本文中，我们利用极化中子散射表明，四方晶系$A$MnBi$_2$（$A$=Ca、Yb）是严格沿$c$轴排列的共线反铁磁体（C型），其奈尔温度$T_N$分别约为270 K和290 K。在从450 K冷却至$T_N$的过程中，YbMnBi$_2$的低能自旋激发在四方平面内从自旋空间各向同性自发变为各向异性，在400 K附近因重Yb诱导的自旋轨道耦合形成动态自旋向列相，之后在$T_N$以下出现能隙。对CaMnBi$_2$的类似测量显示，其在$T_N$以上为各向同性顺磁散射，无自旋向列相。在平面内磁场作用下，Yb$^{3+}$磁矩可能与动态自旋向列相相互作用，诱导非零SSC，从而在$T_N$以上的YbMnBi$_2$中产生CaMnBi$_2$所没有的AHE和反常能斯特效应（ANE）。基于对称性的金兹堡-朗道分析表明，向列序与SSC之间的耦合项在外加磁场下是允许的，这可以解释YbMnBi$_2$中AHE随磁场的快速增加。我们的结果为补偿型共线反铁磁体顺磁相中动态SSC诱导的AHE和ANE提供了有力证据，为无磁有序的复合自旋序物理和室温自旋电子学开辟了新途径。
英文摘要：
A spin nematic order, analogous to the nematic liquid crystal, characterizes the spontaneous breaking of spin-space rotational symmetry while preserving time-reversal ($T$) symmetry. In contrast, scalar spin chirality (SSC), a composite three-spin order, breaks $T$ symmetry and is known to induce an anomalous Hall effect (AHE). Although a spin nematic phase has been suggested in frustrated magnets and the square-lattice iridate, how it might affect magnetotransport properties is unknown. Here we use polarized neutron scattering to show that tetragonal $A$MnBi$_2$ ($A$ = Ca, Yb) is a strictly $c$-axis-aligned collinear antiferromagnet (C-type), with $T_N \approx 270$ K and 290 K, respectively. On cooling from 450 K to $T_N$, low-energy spin excitations in YbMnBi$_2$ spontaneously change from isotropic to anisotropic in spin space within the tetragonal plane, forming a dynamic spin nematic phase around 400 K due to heavy Yb-induced spin-orbit coupling, before gapping out below $T_N$. Similar measurements on CaMnBi$_2$ reveal isotropic paramagnetic scattering without a spin nematic phase above $T_N$. Under an in-plane magnetic field, the Yb$^{3+}$ moments may interact with the dynamic spin nematic phase to induce nonzero SSC, giving rise to AHE and an anomalous Nernst effect (ANE) in YbMnBi$_2$ that are absent in CaMnBi$_2$ above $T_N$. A symmetry-based Ginzburg-Landau analysis shows that coupling terms between the nematic order and SSC are allowed under an external magnetic field, which could explain the rapid increase of AHE with field in YbMnBi$_2$. Our results provide compelling evidence for dynamic SSC-induced AHE and ANE in the paramagnetic phase of a compensated collinear antiferromagnet, opening a new avenue for the physics of composite spin orders and room-temperature spintronics without magnetic order.
