发表机构
Hong Kong University of Science and Technology（香港科技大学）
机构由 AI 辅助整理，请以论文原文为准。AI 中文总结
本文发现狄拉克交变磁体中对称性允许的线性响应实际缺失，而应变驱动的Lifshitz重构产生非解析的压磁效应（M∝sgn(ε)ε²），并以单层Fe₂S为候选材料，提出量子奇异性作为材料响应工程的新自由度。
AI 中文摘要
长期以来，对称性一直是理解材料如何响应外部扰动的组织原则。在此，我们表明，对称性所允许的并不一定是底层系统实际涌现的，而这种区别对于对称性相关的无间隙狄拉克费米子（例如交变磁体中的狄拉克费米子）变得尤为重要。一种使狄拉克点发生位移的扰动会驱动Lifshitz重构，并产生一个本质上非解析的有效势，从而引发未被发现的奇异响应。在狄拉克交变磁体中，这种非解析性通过$C$-配对的自旋极化狄拉克锥获得磁性表现：打破配对对称性$C$会解除其简并，并将相关的Lifshitz重构转化为巡游磁化。因此，压磁效应（即机械应变诱导净磁化的现象）变得非线性和非解析，满足$M\propto\mathrm{sgn}(\varepsilon)\varepsilon^2$，而线性响应尽管被对称性允许，却是不存在的。相应地，主导响应无法从基于诺伊曼原理和解析朗道展开为基石的传统对称性范式中推断出来。第一性原理计算确定单层Fe$_2$S为这种奇异磁性响应的候选材料实现。这些结果表明，量子奇异性可作为工程材料响应的一个新自由度。
英文摘要
Symmetry has long provided the organizing principle for understanding how materials respond to external perturbations. Here we show that what is permitted by symmetry need not be what actually emerges from the underlying system, and this distinction becomes particularly consequential for symmetry-related gapless Dirac fermions such as in altermagnets. A perturbation that shifts the Dirac points drives a Lifshitz reconstruction and generates an intrinsically nonanalytic effective potential, giving rise to undiscovered singular responses. In Dirac altermagnets, this nonanalyticity acquires a magnetic manifestation through $C$-paired spin-polarized Dirac cones: breaking the pairing symmetry $C$ lifts their degeneracy and converts the associated Lifshitz reconstruction into an itinerant magnetization. As a consequence, piezomagnetism, the induction of a net magnetization by a mechanical strain, becomes nonlinear and nonanalytic, with $M\propto\mathrm{sgn}(\varepsilon)\varepsilon^2$, whereas the linear response is absent despite being allowed by symmetry. Accordingly, the leading response cannot be inferred from the conventional symmetry-based paradigm of material response built upon the cornerstones of Neumann's principle and analytic Landau expansion. First-principles calculations identify the monolayer Fe$_2$S as a candidate material realization for such singular magnetic responses. These results suggest quantum singularities as a new degree of freedom for engineering material responses.
Comments21 pages, 4 figures
