拓扑绝缘体输运性质与拓扑自旋波的研究

发布时间：2018-04-15 22:29

本文选题：拓扑绝缘体 + 非局域Andreev反射　；参考：《中国科学院大学(中国科学院物理研究所)》2017年博士论文

【摘要】：本文的研究内容之一是二维拓扑绝缘体与超导体异质结的输运性质。在普通异质结界面,入射电子可以发生反射与透射。在材料与超导体异质结界面,由于超导体中库伯对凝聚体的存在,入射电子除了反射与透射外,还能发生局域Andreev反射和非局域Andreev反射(交叉Andreev反射)。其中局域Andreev反射的基本过程为,入射电子与入射一侧某一个自旋相反的电子一同进入超导凝聚体成为库伯对,从而在入射一侧反射一个与入射电子自旋相反的带正电的空穴。而非局域Andreev反射的基本过程为,入射电子与异质结另一侧的某个自旋相反的电子一同进入超导凝聚体成为库伯对,从而在入射电子的另一侧反射一个与入射电子自旋相反的带正电的空穴。二维拓扑绝缘体的边缘态具有自旋动量锁定的性质,也就是说边缘态输运通道的电子传播方向是由电子自旋取向决定的,比如在同一个边缘,自旋向上的电子向右传播,那么自旋向下的电子只能向左传播,或者两者相反。这种性质会导致二维拓扑绝缘体与超导体异质结中产生完美的局域Andreev反射,也就是说所有的入射电子都发生Andreev反射。非局域的Andreev反射也具有重要的物理意义,比如它的逆过程可以用来制造纠缠电子对。基于前人在二维拓扑绝缘体与超导体异质结中实现的完美局域Andreev反射,我们在同样的体系中提出了新的可以实现可观的甚至完全的非局域Andreev反射的机制。首先,我们利用交换场在原来完美的Andreev反射通道中开辟出了自旋分辨的透射通道。再次,我们提出了不同的机制将透射通道的一部分转化为非局域Andreev反射通道。这样,由于自旋相反的电子对交换场在能量响应上的不同,最终我们实现了自旋分辨的各种输运行为,具体来说就是自旋向下的电子发生完全的局域Andreev反射时,自旋向上的电子则发生透射或者非局域的Andreev反射。更且,由于二维拓扑绝缘体自旋动量锁定的性质,各个输运通道在空间上都是分离的,这极大地有利于实验上的实现,探测及应用。本文的研究内容之二是磁有序体系中自旋波激发谱的拓扑性质。空间平移对称性保证了磁有序体系中的自旋波激发可以用谱能带很好的描述,而能带中的拓扑性质取决于所对应的波函数而与所描述的粒子的统计性质无关,因此拓扑能带理论也同样适用于自旋波。自旋波能谱描述的是自旋波量子磁振子的行为,因为磁振子遵守玻色统计,以及自旋体系特有的一些性质,使得自旋波能谱的拓扑性质具有一些与电子体系不一样的特性。我们研究了大量自旋体系的自旋波激发谱的拓扑方面的性质,发现了两类有趣的体系。第一类是三维蜂窝格子。我们研究了三维蜂窝格子上铁磁基态的最近邻海森堡模型的自旋波激发谱,发现所有的这类三维蜂窝格子都具有环接触零能隙的能谱,对应有鼓面状无色散的表面态。当我们再考虑次近邻Dzyaloshinskii-Moriya相互作用后,每个零能隙环则被打开为两个具有相反拓扑荷的外尔点,对应有连接两个外尔点的投影点的弧状表面态。第二类是具有中心反演对称性的格子。具有中心反演对称性格子上反铁磁基态的自旋体系具有空间反演与时间反演的联合对称性,以及反铁磁序所蕴含的U(1)对称性。我们证明了所有这样的体系的自旋波哈密顿量可以解耦成相互独立的两块,而且两块产生的自旋波能谱是完全一致的,当我们在某一子块中找到外尔点时,那么在另一子块也相应地有外尔点,而且空间反演及时间反演联合对称性保证了这样的两个外尔点的拓扑荷是相反的,因而它们共同构成了整个体系的一个狄拉克点。相应地,在系统表面则有双重的弧状表面态。
[Abstract]:One of the content of this paper is to study the 2D topological insulator superconductor heterostructures and transport properties. In general the heterojunction interface can occur, the incident electron reflection and transmission at the interface of materials and superconductor heterostructures, due to the existence of Kolb condensates in superconductors, the incident electron in addition to reflection and transmission, but also the local Andreev reflection and nonlocal Andreev reflection (crossed Andreev reflection). The basic process of local Andreev reflection, the incident electron and incident electron one side opposite spin together into the superconducting condensate of Kolb to become, in the reflection of the incident on one side opposite to the incident electron spin of positively charged holes and non basic. The process of local Andreev reflection, the incident electrons and a heterojunction electron spin on the other side opposite together into the superconducting condensate to become Kolb, resulting in the incident electron The other side of a reflection of incident electron spin opposite positively charged holes. The edge states in two-dimensional topological insulator has the property of spin momentum locked, that is to say the propagation direction of edge state transport channel is determined by electron spin orientation, for example in the same edge, electronic spin to spread to the right and then spin down can spread to the left, or vice versa. This nature will lead to local Andreev reflection perfect two-dimensional topological insulators and superconductors in a heterostructure, that is to say the incident all Andreev reflection. Nonlocal Andreev reflection also has important physical meaning, such as the inverse process of it can be used to produce entangled electrons. The perfect reflection in two previous local Andreev topological insulators and superconductors based on heterojunction, we put in the same system The new mechanism can achieve considerable or even completely nonlocal Andreev reflection. First, we use the exchange field in the Andreev reflection channel opened in the original perfect transmission channel spin resolved. Thirdly, we propose a different mechanism of transmission channel part into the nonlocal Andreev reflection channel. In this way, due to the electron spin opposite to the exchange field in different energy response, we finally achieve a variety of spin polarized transport behavior, specifically the spin down electrons localized Andreev reflection completely, since the electronic spin to the occurrence of the transmission or nonlocal Andreev reflection. Moreover, due to the nature of two-dimensional topological insulator spin momentum locked, all transport channels are separated in space, which greatly helps to realize the experiment, detection and application. The research content of Two is the topological properties of spectrum excitation of spin wave magnetic ordering system. Space translational symmetry ensures the spin wave excitation in the magnetic ordering system can use the spectrum with a very good description, and depends on the topological properties in the wave function in the corresponding statistical properties and the description of the particle. The topology of the band theory also applies to the spin wave. Spin wave spectrum is described quantum spin wave magnon behavior, because the magnon obey Bose statistics, as well as some unique properties of spin system, the topological properties of spin wave energy spectrum has some properties and the electronic system is not the same. We study on the spin wave excitation spectrum of the large spin system topology properties, found two kind of interesting system. The first is the three-dimensional honeycomb lattice. We study three-dimensional honeycomb lattice on the ferromagnetic ground state of the nearest neighbor Heisenberg model The excitation spectrum, found that this type of 3D honeycomb lattice all have zero ring contact energy spectral gap, corresponding to the surface state of drum surface Dispersionless. When we consider the next nearest neighbor interaction Dzyaloshinskii-Moriya, each zero band gap ring is opened for two with opposite topological charge outside Er, corresponding to the curved surface state projection point connecting two Weyl points. The second type is a center of inversion symmetry. Symmetry lattice with joint inversion center of symmetry lattice antiferromagnetic spin system with space inversion and time reversal, and the antiferromagnetic order contains U (1) symmetry. We prove that the spin Hamiltonian wave all such system can be decoupled into two independent, and two pieces of the spin wave energy spectrum is exactly the same, when we find out in a sub block in Seoul point, then In another block a corresponding Weyl point, but also the space inversion and time reversal symmetry and ensure that the topology of two points this Weyl charge is on the contrary, they constitute a Dirac point of the whole system. Accordingly, the system has surface surface state of double arc.

【学位授予单位】：中国科学院大学(中国科学院物理研究所)
【学位级别】：博士
【学位授予年份】：2017
【分类号】：O469

【相似文献】