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 recentlly a unsual 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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[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)