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arXiv · 2207.03289

Phonon softening and slowing down of charge-density-wave fluctuations in BaNi$_2$As$_2$

Yu Song·Shan Wu·Xiang Chen·Yu He·Hiroshi Uchiyama·Baizhuo Li·Saizheng Cao·Jiayu Guo·Guanghan Cao·Robert Birgeneau
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Abstract

BaNi$_2$As$_2$ is a nonmagnetic analogue of the iron pnictide superconductors, and in its tetragonal state exhibits an incommensurate charge-density-wave (IC-CDW) and a sizable elastoresistance. In this work, phonons in BaNi$_2$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 reveal 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 tetragonal BaNi$_2$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.

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Yu Song, Shan Wu, Xiang Chen, Yu He, Hiroshi Uchiyama, Baizhuo Li, Saizheng Cao, Jiayu Guo, Guanghan Cao, Robert Birgeneau. 2022-07-07. Phonon softening and slowing down of charge-density-wave fluctuations in BaNi$_2$As$_2$. https://doi.org/10.1103/physrevb.107.l041113

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