二维过渡金属硫族化合物的磁性异质结中能谷电子学的第一性原理研究

Valley is a brand-new quantum degree of freedom of electrons, which will have important applications in future information and computational sciences. The valley spontaneous polarization is a key issue for information storage and logic operations. However, the current problem is that the valleys of most valleytronic materials found to date are not valley spontaneously polarized. Recently, it is found that forming heterostructures with magnetic substrates is an effective way to break the valley energy degeneracy and realize valley spontaneous polarization. However, there are still problems such as very low Curie temperature, the lack of global energy gap of the heterostructures and the realization of effective manipulation of valley spontaneous polarization. By means of first-principles calculations, our project plans to study the valleytronics in heterostructures formed by valleytronic materials (transition metal dichalcogenides) and 2D magnetic semiconductors: (1) Via optimizing the materials combination and the structure of the heterostructures, diverse and novel valleytronic magnetic heterostructures will be constructed; (2) Focus will be put on the important issues such as the stability of the magnetic heterostructures, the effects of stacking and magnetic substrate on the valleys spontaneous polarization; (3) We will explore the synergistic effects of external strain, electric, magnetic fields and magnetic substrates on the energy gap, exchange energy and the modulation of valleys. We will study the new effects resulted from the coupling of charge, spin and valley degrees of freedom that are tuned by these multi-fields. The goal of our research project is to realize the valley spontaneous polarization and valley control, and to find valleytronic magnetic heterostructures with high Curie temperature and semiconducting properties, providing good basis for the applications of valleytronics in the future.

能谷是电子的全新量子自由度，在未来信息和计算科学中有着重要应用。能谷自发极化是实现信息存储和逻辑运算的关键，目前的困难是大多数材料的能谷是非自发极化的。最近发现通过与磁性材料形成异质结打破能谷能量简并是实现能谷自发极化的有效途径，但是还存在居里温度低、异质结整体带隙为零和如何有效实现能谷自发极化的操控等问题。本项目将基于第一性原理方法研究能谷材料(过渡金属硫族化合物)与二维层状磁性半导体形成的异质结: (1)优化材料的匹配和异质结的结构，构筑丰富的新型磁性异质结能谷材料；(2)研究磁性异质结的稳定性以及堆垛和磁性基底对能谷自发极化的影响；(3)探索外加应变、磁场和电场与磁性基底的协同作用对带隙、交换能和能谷极化的调控，在多场调制下电荷、自旋和能谷自由度相互耦合产生的新效应。目的是实现能谷的自发极化和操控，发现居里温度高且具有半导体性质的能谷材料磁性异质结，为能谷电子学的应用打下基础。

基于对电子电荷自由度操控的传统电子器件已得到广泛应用，但它们已经接近热力学和量子力学极限。为克服这些极限，基于电子自旋自由度处理信息的自旋电子学应运而生并迅速发展。近年来研究发现，二维材料的价带和导带的不等价能谷附近的电子具有相反的电学、自旋和光学性质，由此提供了一个新的自由度-能谷，它与电子的电荷、自旋等其它自由度紧密关联，提供了更丰富、稳定和高效的信息处理方案。近年来，随着新型的二维材料的发现，能谷电子学得到迅速发展，发现了能谷光选择定则和能谷霍尔效应等新奇量子现象，正成为凝聚态物理中一个十分引人瞩目的研究领域。但是大多数材料的能谷是非自发极化的，而能谷自发极化是实现信息存储和逻辑运算的关键。最近发现磁场和磁性材料基底可以打破能谷能量简并，实现能谷自发极化。本项目基于第一性原理方法，计算了不同材料的能谷特性及其自发极化，研究了电场、磁场、应变和掺杂对能带、贝里曲率的影响，揭示了在二维能谷材料中能谷自发极化的产生和调控的规律。我们研究了具有Janus结构的VSSe中能谷的自发极化特性以及光学和输运性质，研究了WSe2在外加磁场中的能谷劈裂和贝里曲率的变化，研究了基底Tl2S对锗烯的能谷效应的提升作用，研究了在硅石墨烯中掺杂对其能带和能谷的影响。我们的研究成果为能谷电子学的应用和器件设计提供了理论依据。在本项目的资助下，在Phys.Rev.B、J. Phys.:Cond. Matt.、AIP Advances和RSC Advances等期刊上发表SCI论文11篇。