Recently, charge-density wave (CDW) and superconductivity are observed to coexist in atomically thin metallic NbSe₂. Lacking of knowledge on the structural details of CDW, however, prevents us to explore its interplay with superconductivity. Using first-principles calculations, we identify the ground state 3 × 3 CDW atomic structure of monolayer NbSe₂, which is characterized by the formation of triangular Nb clusters and shows a scanning tunnelling microscopy (STM) image and Raman CDW modes in good agreement with experiments. We further demonstrate that from bulk to monolayer NbSe₂, as the layer thickness decreases, the CDW order is gradually enhanced with rising energy gain and strengthened Fermi surface gapping, while superconductivity is weakened due to the increasingly reduced Fermi level density of states in the CDW state. These results well explain the observed opposite thickness dependencies of CDW and superconducting transition temperatures and uncover the nature of competitive interaction between the two collective orders in two-dimensional NbSe₂.

中文翻译：

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在二维NbSe _2中揭示电荷密度波，超导性及其竞争性质

最近，观察到电荷密度波（CDW）和超导性共存于原子薄的金属NbSe _2中。然而，由于缺乏关于CDW的结构细节的知识，使我们无法探索其与超导性的相互作用。使用第一性原理计算，我们确定了单层NbSe ₂的基态3×3 CDW原子结构，其特征在于形成了三角形Nb团簇，并显示了扫描隧道显微镜（STM）图像和拉曼CDW模式，与实验。我们进一步证明了从块状到单层NbSe ₂随着层厚度的减小，CDW阶数随着能量增益的增加和费米表面间隙的增加而逐渐增强，而超导性由于CDW态中费米能级密度的不断降低而减弱。这些结果很好地解释了观察到的CDW和超导转变温度的相反厚度依赖性，并揭示了二维NbSe _2中两个集体级之间竞争性相互作用的性质。