二维晶体电子材料的输运性质与量子相变

The discovery of graphene has led to revolutionary advances in two-dimensional (2D) crystalline electronic materials. Even though graphene has some excellent properties, it also has some disadvantages such as a zero energy band gap. Recently discovered atomically thin crystals of transition metal chalcogenides prepared by mechanical exfoliation do have a band gap. The related two dimensional electron gas has a energy band structure, superconductivity, charge and spin density waves, and spin-valley coupling and corresponding novel physics. Building on our previous work and the nanofabrication and low-temperature measurement capabilities at SJTU, we propose to carry out a focused experimental study of atomically thin crystals of transition metal chalcogenides, exploring the effects of band structure, two-dimensionality, interface, and the carrier density on the low-temperature transport processes. We seek to identify factors limiting the mobility of the carriers so as to open pathways for high carrier mobility. We will also study the interplay between superconductivity and density waves, and the quantum phase transitions between semiconductor and metal, as well as that between superconductor and charge density waves. We will also explore possible device applications of these novel electronic materials.

石墨稀的发现引发了二维晶体电子材料的一个革命性进展。虽然石墨稀具有很好的性能，但存在零带隙等明显的不足。最近发现可以用机械解理方法获得的过渡金属硫族化合物二维晶体材料有较大的带隙，相关的二维电子气体有能带结构，以及超导，磁性和电荷、自旋密度波等有序物态和由于自旋和能谷的耦合导致的新物理现象。本项目将以过渡金属硫族化合物为主要研究对象，在已有的工作基础上全面系统地探讨过渡金属硫化物二维晶体的物理性质，利用上海交大的纳米制备和低温测量条件，研究二维晶体中的能带结构，维度, 界面，及载流子浓度对过渡金属硫化物二维晶体中载流子散射过程的影响，理解制约迁移率的因素，寻找过渡金属硫化物二维晶体中的提高迁移率的途径，探索电荷密度波、超导等不同有序态在二维超导体中的共存与竞争, 半导体－金属，超导体－电荷密度波等量子相变，以及这些新材料的器件应用。

通过对机械解理获得的过渡金属硫族化合物二维晶体及其微纳加工的器件的物理性质的研究，人们有望获得超越石墨烯的新型电子二维材料和器件。本项目以2H-NbS2 和2H-TaSe2为代表，研究了2H-NbS2的晶体生长特性及其微纳加工器件的制备和表征；基于2H-TaSe2制备了二维晶体的场效应晶体管器件，揭示其锂离子插层的微观过程及其对超导和电荷密度波的调制；并通过量子输运的研究，在2H-TaSe2二维晶体中发现了弱反局域化到弱局域化的过度的现象，证明了离子插层具有调节自旋弛豫性质的功能，并与该二维体系中超导温度的提高有潜在的联系。项目综合了对二维材料的晶体和薄膜的生长和器件制备、理解二维材料的长程有序态等方面，取得了一系列预期研究成果，为新型电子二维材料和器件物理的进一步研究提供了坚实的基础。