Department of Physics and State Key Laboratory of Low-Dimensional Quantum Physics , Tsinghua University , Beijing 100084 , People's Republic of China.
School of Materials Science and Engineering , Beihang University , Beijing 100191 , People's Republic of China.
Nano Lett. 2018 May 9;18(5):2924-2929. doi: 10.1021/acs.nanolett.8b00237. Epub 2018 Apr 16.
Recently, charge-density wave (CDW) and superconductivity are observed to coexist in atomically thin metallic NbSe. 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 NbSe, 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 NbSe, 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 NbSe.
最近，在原子层薄的金属 NbSe 中观察到电荷密度波 (CDW) 和超导性共存。然而，由于对 CDW 的结构细节缺乏了解，我们无法探索它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 的基态 3×3 CDW 原子结构，其特征在于形成三角形 Nb 簇，并与实验中的扫描隧道显微镜 (STM) 图像和 Raman CDW 模式很好地吻合。我们进一步证明，从体相到单层 NbSe，随着层厚的减小，CDW 有序度逐渐增强，能量增益增加，费米面隙增大，而超导性由于 CDW 态下费米能级态密度的减小而减弱。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 中两种集体序之间竞争相互作用的本质。
