标题： 外尔交替磁体中隐藏对称破缺的非线性输运特征 显示英文标题

标题： Nonlinear Transport Signatures of Hidden Symmetry Breaking in a Weyl Altermagnet

摘要： 固相中的相变通常伴随着结构变化，但微妙的晶格畸变可能在常规晶体学探测中隐藏起来，阻碍正确序参量的识别。一个典型的例子是钙钛矿型极性钌酸盐Ca$_3$Ru$_2$O$_7$，其具有明确表征的相变，而最近其基态结构已成为争议的焦点。这种不确定性源于竞争相之间极小的原子位移（$\sim$0.001 \AA ），超出了X射线衍射、中子散射或光学二次谐波产生的分辨率。在这项工作中，我们提出了一种通过量子几何诱导的非线性输运来检测隐藏的对称破缺的方法。 我们证明了Ca$_3$Ru$_2$O$_7$在两种相中都是一个外尔链半金属。 低对称性相被分类为一种由对称性决定的交替磁体，其拓扑表面态沿极化（$b$）轴呈不对称分布。 然而，由于交替磁性的存在，非相对论自旋劈裂太弱（$\sim$0.1 meV），无法直接分辨。 相比之下，在费米面处，外尔链会产生较大的量子度规，导致低对称性相中的非线性电导率比高对称性相强几个数量级。 沿着极轴方向出现了一个纵向非线性电导率，这仅在此相中出现，提供了一种敏感探针，可以定性地区分它与高对称结构，并展示交替磁性的出现，这一点已被最近的一项实验所证实。 我们的工作确立了一条通过晶体对称性、拓扑学和非线性量子输运之间的相互作用来识别复杂量子材料中隐藏对称性破缺的途径。 显示英文摘要

摘要： Phase transitions in solids are often accompanied by structural changes, but subtle lattice distortions can remain hidden from conventional crystallographic probes, hindering the identification of the correct order parameters. A case in point is Ca$_3$Ru$_2$O$_7$, a correlated polar ruthenate with well-characterized phase transitions, whose ground state structure has recently become a subject of debate. This uncertainty stems from extremely small atomic displacements ($\sim$0.001 \AA) between competing phases, beyond the resolution of X-ray diffraction, neutron scattering, or optical second-harmonic generation. In this work, we propose a method to detect hidden symmetry breaking by leveraging nonlinear transport induced by quantum geometry. We show that Ca$_3$Ru$_2$O$_7$ is a Weyl chain semimetal in both phases. The low-symmetry phase, classified as an altermagnet by symmetry, features distorted topological surface states that are asymmetric along the polar ($b$) axis. However, the nonrelativistic spin splitting is too weak ($\sim$0.1 meV) to be resolved directly, regarding the altermagnetism. In contrast, Weyl chains generate a large quantum metric at the Fermi surface, leading to nonlinear conductivities that are orders of magnitude stronger in the low-symmetry phase. A longitudinal nonlinear conductivity along the polar axis emerges exclusively in this phase, providing a sensitive probe to qualitatively distinguish it from the high-symmetry structure and demonstrate the emergence of altermangetism, which is confirmed by a recent experiment. Our work establishes a route for identifying hidden symmetry breaking in complex quantum materials through the interplay of crystal symmetry, topology and nonlinear quantum transport.