Soft-Phonon and Charge-Density-Wave Formation in Nematic BaNi₂As₂

Signatures of nematic fluctuations have now been observed in various families of superconductors and have triggered intense investigations on the role of nematicity in the pairing mechanism. The superconductor BaNi2As2 (Tc = 0.6 K), a candidate system for charge-driven electronic nematicity, is a nonmagnetic, isostructural analogy of BaFe2As2. Instead of a spin density wave an incommensurate charge density wave (I-CDW) that orders in vicinity to a tetragonal to orthorhombic phase transition is observed [1-3]. We use diffuse and inelastic x-ray scattering to study the formation of the I-CDW in BaNi2As2. We present the evolution of the diffuse scattering at the I-CDW wave vector which is already present at room temperature and the continuous softening of the unstable phonon branch [4]. For the same phonon branch at the Γ-point recently a unusual large splitting emerging from a coupling with nematic fluctuations is reported [5]. The phonon instability and its reciprocal space position is well captured by our ab initio calculations. These however indicate that neither Fermi surface nesting, nor enhanced momentum-dependent electron-phonon coupling can account for the I-CDW formation, demonstrating its unconventional nature.
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[1] A. S. Sefat et al. Phys. Rev. B 79, 094508 (2009)
[2] S. Lee et al. Phys. Rev. Lett. 122, 147601 (2019)
[3] C. Meingast Phys. Rev. B 106, 144507 (2022)
[4] S. M. Souliou et al. arXiv:2207.07191 (2022)
[5] Y. Yao et al. Nat. Comm. 13, 4535 (2022)