Charge density waves and soft phonon evolution in the superconductor BaNi2(As1-xPx)2

The superconductor BaNi2As2 (Tc = 0.6 K), an isostructural analogue to the parent compound of Fe-based superconductors BaFe2As2, exhibits an incommensurate charge density wave (I-CDW) and a commensurate CDW (C-CDW) accompanied with a transition to an orthorhombic and triclinic phase, respectively [1-2]. The CDWs and structural phases can be tuned by chemical substitution of e.g. arsenide with phosphorus. For phosphorus concentrations larger than ~7% the transition from the I-CDW to the C-CDW is suppressed while Tc jumps to ~3.2 K [3]. Recent investigations revealed that the I-CDW is associated with the softening of a low-lying optical phonon, which also displays an anomalously large splitting at the Γ-point [4-6]. However, DFT calculations indicate that the mechanism underlying the formation of the I-CDW is unconventional in nature [5].
We use diffuse and inelastic x-ray scattering to study the formation of the I-CDW and C-CDW in BaNi2(As1-xPx)2 for different substitution levels up to x≈0.1. We present the evolution of the diffuse scattering related to the I-CDW through the complete substitution series up to x≈0.1, evidencing a continuous lowering of the I-CDW transition temperature. ... mehrOur inelastic x-ray scattering measurements at the transition from the I-CDW to the C-CDW show a sudden softening of the low-lying optical phonon branch at the C-CDW position without any indications of a softening at this wavevector above the transition temperature. Finally, we find a similar soft phonon driving the I-CDW in the absence of the transition to the C-CDW/triclinic phase (x≈0.1) down to 2.2 K.

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[2] S. Lee et al., Phys. Rev. Lett. 122, 147601 (2019)
[3] C. Meingast et al., Phys. Rev. B 106, 144507 (2022)
[4] Y. Yao et al., Nat. Comm. 13, 4535 (2022)
[5] S. M. Souliou et al., Phys. Rev. Lett 129 (24), 247602 (2022)
[6] Y. Song et al., Phys. Rev. B 107, L041113