Phonon softening and slowing-down of charge density wave fluctuations in <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msub><mml:mi mathvariant="normal">BaNi</mml:mi><mml:mn>2</mml:mn></mml:msub><mml:msub><mml:mi mathvariant="normal">As</mml:mi><mml:mn>2</mml:mn></mml:msub></mml:math>
Yu Song, Shan Wu, Xiang Chen, Yu He, Hiroshi Uchiyama, Bai-Zhuo Li, Saizheng Cao, Jiayu Guo, Guang‐Han Cao, R. J. Birgeneau
Abstract
${\mathrm{BaNi}}_{2}{\mathrm{As}}_{2}$ is a nonmagnetic analog of the iron pnictide superconductors, and exhibits an incommensurate charge density wave (IC-CDW) and a sizable elastoresistance. In this Letter, phonons in ${\mathrm{BaNi}}_{2}{\mathrm{As}}_{2}$ associated with the IC-CDW and uniform in-plane lattice distortions are investigated using high-resolution inelastic x-ray scattering. The in-plane transverse acoustic phonons reveal no softening at temperatures where the elastoresistance increases strongly, indicating the latter to be electronically driven. Systematic phonon measurements suggest the IC-CDW occurs in two stages upon cooling: Underdamped phonons first soften to zero energy well above the IC-CDW ordering temperature, then the resulting quasielastic IC-CDW fluctuations gradually slow down and coalesce into the static IC-CDW order. A possible origin for our observations is the IC-CDW in ${\mathrm{BaNi}}_{2}{\mathrm{As}}_{2}$ being uniaxial, which provides an additional Ising degree of freedom favorable for disordered IC-CDW modulations, and accounts for the elastoresistance through a weak coupling to the lattice.