标题： 镍磷化合物 Ba$_{1-x}$Sr$_x$Ni$_2$As$_2$ 中非公度电荷密度波的巨大向列响应 显示英文标题

标题： Giant nematic response of the incommensurate charge density wave in the nickel-pnictide Ba$_{1-x}$Sr$_x$Ni$_2$As$_2$

摘要： 电子向列性——在保持平移对称性的同时打破旋转对称性——是经典向列液晶的量子类似物。 电子向列性最早于1998年被预测，在多种材料中得到了证实，包括磁场中的二维电子气（2DEG）、铜氧化物超导体和铁基超导体。 一个长期存在的开放问题是是什么物理机制驱动了电子向列序。 在BaFe$_2$As$_2$和高度欠掺杂的YBa$_2$Cu$_3$O$_{6+y}$中，有强有力的证据表明，向列性起源于残余的自旋密度波（SDW）序。 然而，与电荷密度波（CDW）序相关的向列性的证据不够确凿，特别是在接近超导态的系统中。 在这里，我们提供了直接证据，证明在pnictide超导体Ba$_{1-x}$Sr$_x$Ni$_2$As$_2$ (BSNA)中存在由电荷密度波(CDW)驱动的向列涨落。BSNA是一种基于镍的铁基超导体的同系物，它表现出CDW而非自旋密度波(SDW)序。先前的弹电阻研究显示，BSNA在相图中占据的一个非公度CDW区域显示出一个显著增强的超导性——与六重增强相关——这与大的向列性易感性有关。利用单轴应变下的X射线散射，我们证明即使是最小的应变水平($\epsilon \sim 10^{-4}$)也显著破坏了CDW的四重对称性。在一个Ginzburg-Landau框架内，我们定义了一个基于对称相关的CDW超晶格反射的不对称响应的向列性易感性，这与弹电阻测量结果有很好的一致性。我们的研究首次明确地展示了电荷序和向列态之间的直接联系，为这些材料交织的超导相提供了关键见解。 显示英文摘要

摘要： Electron nematicity-the breaking of rotational symmetry while preserving translational symmetry-is the quantum analogue of classical nematic liquid crystals. First predicted in 1998, electronic nematicity has been established in a variety of materials, including two-dimensional electron gases (2DEGs) in magnetic fields, copper-oxide superconductors, and Fe-based superconductors. A long-standing open question is what physical mechanisms drive electronic nematic order. In BaFe$_2$As$_2$ and highly underdoped YBa$_2$Cu$_3$O$_{6+y}$, strong evidence suggests that nematicity arises from vestigial spin-density-wave (SDW) order. However, evidence for nematicity associated with charge-density-wave (CDW) order has been less conclusive, particularly in systems near a superconducting state. Here, we present direct evidence for CDW-driven nematic fluctuations in the pnictide superconductor Ba$_{1-x}$Sr$_x$Ni$_2$As$_2$ (BSNA), a Ni-based homologue of Fe-based superconductors that exhibits CDW rather than SDW order. Previous elastoresistance studies have shown that BSNA displays a large nematic susceptibility-linked to a six-fold enhancement of superconductivity-within a region of the phase diagram occupied by an incommensurate CDW. Using x-ray scattering under uniaxial strain, we demonstrate that even minimal strain levels ($\epsilon \sim 10^{-4}$) significantly break the fourfold symmetry of the CDW. Within a Ginzburg-Landau framework, we define a nematic susceptibility based on the asymmetric response of symmetry-related CDW superlattice reflections, showing strong agreement with elastoresistivity measurements. Our study provides the first clear demonstration of a direct link between charge order and a nematic state, offering key insights into the intertwined superconducting phases of these materials.