Recently, a first-order phase transition associated with charge density wave (CDW) has been observed at low temperatures in intermetallic compound BaFe2⁢Al9. However, this transition is absent in its isostructural sister compound BaCo2⁢Al9. Consequently, an intriguing question arises as to the underlying factors that differentiate BaFe2⁢Al9 from BaCo2⁢Al9 and drive the CDW transition in BaFe2⁢Al9. Here, we set out to address this question by conducting a comparative ab initio study of the electronic structures, lattice dynamics, 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, BaFe2⁢Al9 exhibits a higher density of states at the Fermi level with dominant partially filled Fe-3⁢𝑑 states and a more intricate Fermi surface. This leads to an electronic instability of BaFe2⁢Al9 toward the CDW transition, which is manifested by the diverged electronic susceptibility at the CDW wave vector 𝐪CDW=(0.5,0,0.3), observable in both the real and imaginary parts. Conversely, BaCo2⁢Al9 does not display such behavior, aligning well with experimental observations. Although the electron-phonon interactions in BaFe2⁢Al9 surpass those in BaCo2⁢Al9 by two orders of magnitude, the strength is relatively weak at the CDW wave vector, suggesting that the CDW in BaFe2⁢Al9 is primarily driven by electronic factors.

中文翻译：

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BaFe2Al9 中电荷密度波态的由来


最近，在低温下在金属间化合物 BaFe2Al9 中观察到与电荷密度波 （CDW） 相关的一阶相变。然而，这种转变在其同构姐妹化合物 BaCo2Al9 中不存在。因此，出现了一个有趣的问题，即区分 BaFe2Al9 和 BaCo2Al9 并驱动 BaFe2Al9 中 CDW 转变的潜在因素。在这里，我们通过对高温相的电子结构、晶格动力学和电子-声子相互作用进行从头开始的比较研究来解决这个问题。我们发现这两种化合物在相似的声子分散体中都是动态稳定的。电子结构计算表明，这两种化合物都是非磁性金属;然而，它们在费米能级周围表现出不同的能带结构。特别是，BaFe2Al9 在费米能级上表现出更高的态密度，具有占主导地位的部分填充 Fe-3d 态和更复杂的费米表面。这导致 BaFe2Al9 向 CDW 跃迁产生电子不稳定性，这表现为 CDW 波矢量 qCDW=（0.5,0,0.3） 处的发散电子磁化率，在实部和虚部都可以观察到。相反，BaCo2Al9 没有表现出这种行为，与实验观察结果非常一致。 尽管 BaFe2Al9 中的电子-声子相互作用比 BaCo2Al9 中的电子-声子相互作用高出两个数量级，但在 CDW 波矢量处的强度相对较弱，这表明 BaFe2Al9 中的 CDW 主要由电子因素驱动。