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arXiv · 2207.07191

Soft-phonon and charge-density-wave formation in nematic BaNi$_2$As$_2$

S. M. Souliou·T. Lacmann·R. Heid·C. Meingast·M. Frachet·L. Paolasini·A. -A. Haghighirad·M. Merz·A. Bosak·M. Le Tacon
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Abstract

We use diffuse and inelastic x-ray scattering to study the formation of an incommensurate charge-density-wave (I-CDW) in BaNi$_2$As$_2$, a candidate system for charge-driven electronic nematicity. Intense diffuse scattering is observed around the modulation vector of the I-CDW, $Q_{I-CDW}$. It is already visible at room temperature and collapses into superstructure reflections in the long-range ordered state where a small orthorhombic distortion occurs. A clear dip in the dispersion of a low-energy transverse optical phonon mode is observed around $Q_{I-CDW}$. The phonon continuously softens upon cooling, ultimately driving the transition to the I-CDW state. The transverse character of the soft-phonon branch elucidates the complex pattern of the I-CDW satellites observed in the current and earlier studies and settles the debated unidirectional nature of the I-CDW. 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.% and suggest alternative origins of the I-CDW.

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S. M. Souliou, T. Lacmann, R. Heid, C. Meingast, M. Frachet, L. Paolasini, A. -A. Haghighirad, M. Merz, A. Bosak, M. Le Tacon. 2022-07-14. Soft-phonon and charge-density-wave formation in nematic BaNi$_2$As$_2$. https://doi.org/10.1103/physrevlett.129.247602

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