Soft-Phonon and Charge-Density-Wave Formation in Nematic BaNi_{2}As_{2}.

Area of Science:

• Condensed Matter Physics
• Materials Science
• Solid State Chemistry

Background:

• BaNi2As2 is a candidate material for charge-driven electronic nematicity.
• Understanding the formation of incommensurate charge-density-waves (I-CDW) is crucial for novel electronic properties.

Purpose of the Study:

• Investigate the mechanism behind I-CDW formation in BaNi2As2.
• Clarify the role of lattice dynamics and electronic interactions in this process.
• Determine the nature of the I-CDW state.

Main Methods:

• Utilized diffuse and inelastic x-ray scattering techniques.
• Performed ab initio calculations to model the system's behavior.
• Analyzed phonon modes and scattering patterns.

Main Results:

• Observed intense diffuse scattering around the I-CDW modulation vector, Q(I-CDW), present even at room temperature.
• Identified a low-energy transverse optical phonon mode that softens upon cooling, driving the I-CDW transition.
• Diffuse scattering collapses into superstructure reflections in the ordered state with orthorhombic distortion.
• Ab initio calculations reproduced the phonon instability but ruled out conventional mechanisms like Fermi surface nesting.

Conclusions:

• The I-CDW formation in BaNi2As2 is driven by a soft transverse optical phonon mode.
• The study elucidates the complex I-CDW satellite patterns and confirms its unidirectional nature.
• The findings highlight the unconventional mechanism underlying I-CDW formation in this material, distinct from Fermi surface nesting or standard electron-phonon coupling.