Soft-Phonon and Charge-Density-Wave Formation in NematicBaNi2As2

We use diffuse and inelastic x-ray scattering to study the formation of an incommensurate charge-density-wave (I-CDW) in BaNi2As2, a candidate system for charge-driven electronic nematicity. Intensediffuse scattering is observed around the modulation vector of the I-CDW,QI-CDW. It is already visible atroom temperature and collapses into superstructure reflections in the long-range ordered state where a smallorthorhombic distortion occurs. A clear dip in the dispersion of a low-energy transverse optical phononmode is observed aroundQI-CDW. The phonon continuously softens upon cooling, ultimately driving thetransition to the I-CDW state. The transverse character of the soft-phonon branch elucidates the complexpattern of the I-CDW satellites observed in the current and earlier studies and settles the debatedunidirectional nature of the I-CDW. The phonon instability and its reciprocal space position are wellcaptured by ourab initiocalculations. These, however, indicate that neither Fermi surface nesting, norenhanced momentum-dependent electron-phonon coupling can account for the I-CDW formation,demonstrating its unconventional nature