金属原子吸附对石墨烯电子态影响的量子输运研究

Owing to graphene’s unique band structure, its electrons can be described by a massless Dirac equation. Therefore, the electronic transport property is one of the most intriguing properties of graphene. After near 10 years of extensive research, much has been learned about Dirac electrons. Although a variety of interesting electronic properties have been discovered in graphene, a few important condensed matter phenomena, e.g., topological insulators and superconductivity have not been observed. It has been theoretically proposed that these phenomena can be realized in graphene. In this project, metal adatoms will be used to engineer the properties of graphene. Employing an ultra-low temperature transport measurement system with an in situ metal deposition capability to avoid clustering and oxidation, quantum transport of modified graphene will be studied. Heavy metal adatoms will be used to introduce strong spin-orbit coupling, which will be confirmed by weak localization, quantum Hall effect, spin Hall effect, etc. The goal is to realize 2D topological insulators in graphene. Transition metal adatoms will be used to introduce local magnetic moment. Attempts will be made to realize magnetic order by tuning the interaction between magnetic moments. Alkali and Alkaline earth metal adatoms will be used to introduce strong charge transfer, hence the Fermi level. Evolution of electron-phonon coupling will be studied, as well electron scattering by charge impurities. We will search for 2D superconductors based on graphene and investigate their properties.

由于石墨烯独特的能带结构，其中的电子满足无质量狄拉克粒子特征，因而电学性质是石墨烯最有特色的性质之一。经过近10年的研究，人们对其中电子的性质已经有了比较清楚的了解。尽管石墨烯表现出很多优异的电学性质，对于凝聚态物理中的几个重要现象，如拓扑绝缘体和超导，却没有在石墨烯中实现。基于已有理论预言，本项目将利用具备原位蒸镀能力的极低温输运测量平台，通过表面金属原子修饰的办法，调控石墨烯的性质。利用重金属元素吸附，在石墨烯中引入自旋-轨道耦合，通过测量弱局域化、量子霍尔效应、自旋霍尔效应等现象，研究自旋-轨道耦合强度，寻找基于石墨烯的二维拓扑绝缘体，并研究其边界态性质；利用过渡族元素吸附，在石墨烯中引入局域磁矩，并探索调控磁矩间的相互关联作用，实现宏观磁性；利用碱金属或碱土金属吸附产生的电荷转移，大范围改变费米能，从而调控电声相互作用，研究相关的电子输运过程，寻找基于石墨烯的二维超导体。

石墨烯具有诸多优异性质，但是其中的自旋轨道耦合非常弱，也不具备局域磁矩和磁有序，载流子浓度太低不利产生超导电性。在这个项目中，我们主要研究目标是如何通过原位表面原子修饰，在石墨烯中引入强自旋轨道耦合、载流子和局域磁矩相互作用、或者超高载流子浓度，最终实现基于石墨烯的二维拓扑绝缘体、二维磁有序、或者超导电性，并研究这些物态的性质。我们研究了铟、金、铋、钛、碲、硒、铁、钴、锰、铬、钙、镁等元素原子对石墨烯的原位修饰，结果表明由于石墨烯面外方向的化学惰性，这些原子和石墨烯的相互作用很弱，难以有效地引入自旋轨道耦合和磁性。同时原子更容易形成团簇，对石墨烯的载流子掺杂作用出现饱和。而且，由于电荷转移产生长程库伦势，导致短程势被屏蔽，进一步削弱了可能的自旋轨道耦合和电子磁矩耦合。通过测量石墨烯中空位缺陷的散射作用，我们证明了长程库伦势对短程势的屏蔽作用。由此，选择电荷转移弱的磁性有机分子修饰，在石墨烯中实现了载流子和局域磁矩之间的近藤耦合。针对孤立原子吸附对石墨烯影响弱的问题，我们提出通过对多层二维材料进行插层的方案。插层原子形成周期分布，在布拉格散射条件下，插层原子的作用形成相干叠加，在特定动量范围对二维材料的能带结构产生强烈影响。基于这一思路，我们在范德瓦尔斯层状材料VS2的插层化合物V5S8薄层中实现了铁磁有序，并借助反常霍尔效应确定了起居里-外斯温度和厚度的磁相图。我们也研究了一些基于二维原子晶体的二维拓扑绝缘体材料,如ZrTe5和WTe2，的电输运和热电输运性质。