Origin of the charge density wave state in BaFe$_2$Al$_9$

TL;DR

This study investigates the origin of the charge density wave in BaFe$_2$Al$_9$ through ab initio comparisons with BaCo$_2$Al$_9$, revealing electronic instabilities and Fermi surface features that drive the CDW transition.

Contribution

It provides a detailed ab initio analysis showing that electronic structure differences, especially Fermi surface nesting, primarily cause the CDW in BaFe$_2$Al$_9$, unlike BaCo$_2$Al$_9$.

Findings

BaFe$_2$Al$_9$ exhibits a higher density of states at the Fermi level.

Fermi surface nesting at $ extbf{q}_{ m CDW}$ drives the CDW transition.

Electron-phonon interactions are weaker at the CDW wave vector, indicating electronic factors dominate.

Abstract

Recently, a first-order phase transition associated with charge density wave (CDW) has been observed at low temperatures in intermetallic compound BaFe$_2$Al$_9$. However, this transition is absent in its isostructural sister compound BaCo$_2$Al$_9$. Consequently, an intriguing question arises as to the underlying factors that differentiate BaFe$_2$Al$_9$ from BaCo$_2$Al$_9$ and drive the CDW transition in BaFe$_2$Al$_9$. Here, we set out to address this question by conducting a comparative \emph{ab initio} study of the electronic structures, lattice dynamics, \textcolor{black}{and electron-phonon interactions} of their high-temperature phases. We find that both compounds are dynamically stable with similar phonon dispersions. The electronic structure calculations reveal that both compounds are nonmagnetic metals; however, they exhibit distinct band structures around the Fermi level. In particular, BaFe$_2$Al$_9$ exhibits a higher density of states at the Fermi level with dominant partially filled Fe-$3d$ states and a more intricate Fermi surface. This leads to an electronic instability of BaFe$_2$Al$_9$ toward the CDW transition, which is manifested by the diverged electronic susceptibility at the CDW wave vector $\mathbf{q}_{\rm CDW}$=(0.5, 0, 0.3), observable in both the real and imaginary parts. Conversely, BaCo$_2$Al$_9$ does not display such behavior, aligning well with experimental observations. Although the electron-phonon interactions in BaFe$_2$Al$_9$ surpass those in BaCo$_2$Al$_9$ by two orders of magnitude, the strength is relatively weak at the CDW wave vector, suggesting that the CDW in BaFe$_2$Al$_9$ is primarily driven by electronic factors.