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

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.