CuS $_2$中电荷密度波不稳定性的理论研究
A Theoretical Investigation of Charge Density Wave Instability in CuS$_2$
摘要：
过渡金属二硫属化物CuS $_2$的电荷密度波（CDW）的存在自50年前首次实验合成以来一直未被确定。尽管关于其低温结构的实验文献相互矛盾，但还没有对这种材料的声子性质和晶格稳定性的理论研究。通过研究CuS $_2$在不同电子温度下的第一性原理电子结构和声子性质，我们发现了温度敏感的软声子模，揭示了一个以前未报道的Kohn异常在大约100 K.电子温度的变化表明存在两个不同的相，其特征在于在低温下由与S $_2$二聚体的运动相关的$2\times2\times2$周期性电荷调制。费米面的调查提出了一个潜在的费米面嵌套矢量相关的位置的Kohn异常和观察到的带分裂展开的能带结构。这些结果的组合表明，在CuS $_2$的CDW秩序的可能性很大。进一步研究CuS $_2$在单层形式没有发现CDW相的证据，因为所确定的批量周期性扭曲不能在2D中实现。这种行为使这种材料与其他过渡金属二硫属化物材料不同，其他过渡金属二硫属化物材料表现出低至2D极限的电荷密度波相位。由于TMDC材料中的CDW被认为与超导性竞争，单层CuS $_2$中CDW的缺乏表明相对于其他过渡金属二硫属化物增强超导性的可能性。总体而言，我们的工作确定CuS $_2$作为一个以前未实现的候选人，研究超导性，CDW秩序和维度的相互作用。
英文摘要：
The existence of a charge density wave (CDW) in transition metal dichalcogenide CuS$_2$ has remained undetermined since its first experimental synthesis nearly 50 years ago. Despite conflicting experimental literature regarding its low temperature structure, there exists no theoretical study of the phonon properties and lattice stability of this material. By studying the first-principles electronic structure and phonon properties of CuS$_2$ at various electronic temperatures, we identify temperature-sensitive soft phonon modes which unveil a previously unreported Kohn anomaly at approximately 100K. Variation of the electronic temperature shows the presence of two distinct phases, characterized at low temperature by a $2\times2\times2$ periodic charge modulation associated with the motion of the S$_2$ dimers. Investigation of the Fermi surface presents a potential Fermi surface nesting vector related to the location of the Kohn anomaly and observed band splittings in the unfolded bandstructure. The combination of these results suggests a strong possibility of CDW order in CuS$_2$. Further study of CuS$_2$ in monolayer form finds no evidence of a CDW phase, as the identified bulk periodic distortions cannot be realized in 2D. This behavior sets this material apart from other transition metal dichalcogenide materials, which exhibit a charge density wave phase down to the 2D limit. As CDW in TMDC materials is considered to compete with superconductivity, the lack of CDW in monolayer CuS$_2$ suggests the possibility of enhanced superconductivity relative to other transition metal dichalcogenides. Overall, our work identifies CuS$_2$ as a previously unrealized candidate to study interplay of superconductivity, CDW order, and dimensionality.
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