中心反演对称材料中隐藏自旋极化现象的形成与调控

FORMATION AND REGULATION OF HIDDEN SPIN POLARIZATION IN CENTRO-SYMMETRIC MATERIALS

赵世轩

11849400

硕士

物理学

刘奇航

2021-05-19

2021-06-11

哈尔滨工业大学

深圳

长久以来，非磁材料的自旋极化效应被认为需要空间反演对称性破缺以及自旋轨道耦合效应的共同存在。然而，随着隐藏自旋极化概念的提出，人们认识到局域的自旋极化仍可以在空间反演对称体系中存在。而这一现象是由于体系局域的空间反演对称性的破缺所导致。隐藏自旋极化这一全新的物理将给自旋电子学的理论与应用等方面注入新的活力，例如基于隐藏自旋极化效应的自旋场效应晶体管，可以通过电场选取两条方向不同的自旋通道作为“开”、“关”等状态， 具有比 Datta-Das 型更短的沟道长度、更低的耗散以及更高的实现可能性。同时，寻找较大的隐藏自旋极化体系亦是这一方向中的热点。而来自实空间中，两个反演对称的区块间的相互作用是降低隐藏自旋极化的主要原因。因此，本文设计了一种通过“非点式对称性（Non-Symmorphic Symmetry）”以最小化区块间相互作用，并增强隐藏自旋极化的方法，并在紧束缚模型的框架下,在某些特定的波矢处得到了强的隐藏自旋极化。相比于传统的通过增加区块间距、减 小相互作用进而增强隐藏自旋极化的方法显得更加“非平庸”。这一效应亦得到了密度泛函理论计算以及实验的验证。 然而，在那些不具备非点式对称性的体系中，隐藏自旋极化所满足的规律仍未揭晓。故本文通过紧束缚模型及其微扰展开，对具有代表性的二维六角晶格的隐藏自旋极化进行了详细的研究。结果表明，不同高对称点处具有不同隐藏自旋极化的原因有二：1.不同对称性诱导的区块相互作用强度的差异。2.不同方向上自旋的竞争。本文同时还研究了外电场对体系初始自旋结构的破坏作用，并定量地提出了衡量这一效应的参数，而这一参数可以从能带结构中直接获取。此外，锡烯的密度泛函理论的计算与紧束缚模型的结论完美吻合，证实了这一模型的有效性。

The spin polarization effect was considered to exist only under the cooperation of inversion symmetry breaking and spin orbital coupling. However, it is the proposal of Hidden Spin Polarization that reveals the local spin polarization also exist in centro symmetric system. This phenomenon is caused by the violation of local space inversion symmetry. Besides, as a new concept, Hidden Spin Polarization also brought vitality to spintronics. For example, one can pick one spin branch out of two as “open” or “close” states by using an electric field in the spin field effect transistor base on Hidden Spin Polarization system. These devices have a shorter channel length, lower spin dissipation, and a higher possibility of realizing compare with Datta-Das one.At the meantime, seeking a large Hidden Spin Polarization is also one of the aims in this direction. One of the crucial reasons for limited Hidden Spin Polarization is the interaction between two sectors connected by space inversion operation. Following this idea, we have proposed a method to minimize the interaction and further enhance the Hidden Spin Polarization by using “Non-Symmorphic Symmetry”, and by using Tight Binding method, a strong Hidden Spin Polarization is found in some specific position ink-space. This method is a non-trival way compared with regulating the interaction by modifying the distance between sectors. Also, this effect was proved by both Density Functional Theory calculation and experimental observation. However, in those systems without Non-Symmorphic Symmetry, the mechanics of Hidden Spin Polarization are still unrevealed. In this thesis, the physics of Hidden Spin Polarization in representative 2D hexagonal lattice was fully investigated using the Tight-Binding method and its perturbation in the vicinity of high symmetry points. The results show that the difference in Hidden Spin Polarization was caused by 1: Difference in symmetry induced interaction strength, 2: Competition between spin with different orientations. Meanwhile, this thesis has also explored the spin structure breaking effect brought by external electric field, and propose a quantitative index to describe this effect, which could be read directly in the band structure. Further, all these results were perfectly verified by using the Density Functional Theory calculation base on stanene.

隐藏自旋极化 紧束缚模型 密度泛函理论 非点式对称性 Rashba 效应 电场调控

Hidden Spin Polarization Tight-Binding method Density Functional Theory Non-Symmorphic Symmetry Rashba effect Electric field

中文

联合培养

南方科技大学

赵世轩. 中心反演对称材料中隐藏自旋极化现象的形成与调控[D]. 深圳. 哈尔滨工业大学,2021.