二维金属性二硫化物多重电子有序态的理论研究

Two-dimensional metallic transition-metal disulfides have attracted much research interest because of the rich collective electronic orders and the physical effects that differ from the bulk. Recent experiments found that superconductivity in 2H-TaS2 is gradually enhanced with decreasing layer thickness, but the underlying mechanism and the behavior of charge-density wave (CDW) in monolayer limit are still unclear; monolayer 1H-NbS2 shows a CDW order that is not present in the bulk, but whether it coexists with magnetic order or superconductivity is not known. In this project, based on first-principles electronic structure and lattice dynamics calculations, we perform a systematical theoretical study of collective electronic orders in the two-dimensional limit of the two disulfides. First, we will uncover the physical mechanism of the enhanced superconductivity in monolayer 1H-TaS2 by investigating the evolution of CDW order strength, electron-phonon coupling and Coulomb repulsion under reduced dimensionality. Second, we will determine the energetically most stable electronic order of monolayer 1H-NbS2 via studying its CDW and magnetic ground states and explore its superconducting properties in an environment of preexisting CDW order. Third, we will utilize biaxial strain and substitutional doping to modulate different electronic orders in the monolayer and investigate the interplay between them. This project will lay theoretical foundations for the electronic state modulation and device applications of metallic disulfides based nanoscale systems.

二维金属性过渡金属二硫化物因蕴含丰富的集体电子态、呈现与体相迥异的物理效应，引起近期国内外研究的广泛关注。最近的实验发现，2H-TaS2的超导电性随层厚降低不断增强，但相关机制和单层极限下电荷序的演化尚不明确；1H-NbS2单层存在体相没有的电荷密度波，但该电荷序是否与磁序或超导序共存还不清楚。本项目拟通过第一性原理电子结构与晶格动力学计算，对这两种二硫化物在二维极限下的多重电子有序态展开系统的理论研究，力图揭示超导序在1H-TaS2单层增强的微观物理机制，重点关注电荷序强度、电声耦合和库仑相互作用随维度降低的演变规律；确定1H-NbS2单层的电荷序和磁序基态结构，揭示其能量最稳定的电子有序态和在电荷序环境下的超导电性；进一步以应变、替换掺杂为调控手段探究单层不同电子态间的相互作用。本项目的实施可为金属性二硫化物相关纳米体系的电子态调控与器件应用提供理论基础。

利用第一性原理计算系统研究了V族过渡金属二硫属化物2H-TX2（T = Nb，Ta，X = S，Se）在二维极限下电荷序的起源和不同超导序维度效应的物理机制，声子线宽和裸电子磁化率的计算揭示单层1H-TX2的电荷序源于波矢依赖的电声耦合，而非费米面嵌套；通过分析基态电荷序结构的原子扭曲振幅与重整电子态密度，阐明不同1H-TX2单层电荷序的结构共性和相对强度；单层超导性质的计算揭示自旋轨道耦合效应可以显著修正电-声子相互作用的强度和各向异性程度以及每种体系的超导转变温度，同时给出部分体系的电子/空穴掺杂-温度相图；块体与单层的对比研究表明V族2H-TX2呈现普遍增强的二维电荷序，并揭示2H-NbX2和2H-TaX2的超导序在单层极限增强或减弱取决于电荷序调制的费米面电子态密度和电声耦合矩阵元随维度降低的具体演化。此外，基于Migdal-Eliashberg理论阐明了2M相VI族过渡金属二硫属化物普适的电-声子s波配对机制；通过模拟超导准粒子态密度并与实验隧穿谱比较，揭示2M-WS2实际表面存在较强的本征近邻效应；预言双层2M-WS2是二维Z2拓扑金属，且有比多数二维二硫属化物更高的超导转变温度。以上研究工作可为层状金属性二硫属化物多重电子有序态的认识、调控和它们在二维超导、拓扑超导等领域的应用研究提供重要参考。