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

TL;DR

This study investigates phonon behavior in BaNi₂As₂, revealing a two-stage charge-density-wave formation process involving phonon softening and slowing down, and suggests a uniaxial IC-CDW origin linked to elastoresistance.

Contribution

It provides detailed phonon measurements showing the two-stage IC-CDW development and proposes a uniaxial IC-CDW as the origin, connecting lattice and electronic properties.

Findings

Phonons associated with IC-CDW soften and then slow down upon cooling.

Elastoresistance increases without phonon softening, indicating electronic origin.

IC-CDW occurs in two stages: softening and slow down of fluctuations.

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.