Charge order-driven nematicity in the nickel-pnictide superconductor Ba$_{1-x}$Sr$_x$Ni$_2$As$_2$
Nematic order refers to the spontaneous breaking of rotational symmetry while preserving translational symmetry. First identified in classical liquid crystals, nematic order arises from the collective alignment of anisotropic molecules. Its quantum counterpart, electronic nematicity, has been observed in a variety of quantum materials, ranging from unconventional superconductors to kagome metals. Despite its prevalence, there is no universal understanding of the conditions under which nematic order occurs. Electronic nematicity is most firmly established in iron-based superconductors, where it is understood to be a consequence of vestigial spin density wave (SDW) order. However, direct evidence for nematicity arising from other types of order are lacking. Here, we report direct evidence for charge-order-driven electronic nematicity in Ba$_{1-x}$Sr$_x$Ni$_2$As$_2$, a nickel-based analog of the iron pnictides known to exhibit charge density wave (CDW) order. Using x-ray diffraction under applied uniaxial strain, we observe a pronounced symmetry-breaking response-up to $\sim 50 \%$-in the intensity of incommensurate CDW Bragg peaks, even at small strain levels ($\epsilon_{xy} \sim 10^{-3}$). This effect occurs within the same region of the phase diagram where a giant nematic susceptibility is observed in transport measurements. These results provide direct evidence that long-range CDW order can drive nematic behavior in quantum materials.