1.Key Laboratory of Interface Science and Engineering in Advanced Materials of Ministry of Education, Taiyuan University of Technology, Taiyuan 030024, China 2.College of Physics and Optoelectronics, Taiyuan University of Technology, Taiyuan 030024, China
Fund Project:Project supported by the National Natural Science Foundation of China (Grant Nos. 11504258, 11805140), the Natural Science Foundation of Shanxi Province, China (Grant Nos. 201601D011015, 201801D221021, 201801D221031), and the Program for the Outstanding Innovative Teams of Higher Learning Institutions of Shanxi Province, China (Grant No. 163220120-S)
Received Date:20 October 2020
Accepted Date:07 December 2020
Available Online:05 April 2021
Published Online:20 April 2021
Abstract:In the Su-Schrieffer-Heeger (SSH) chain, the nontrivial topological edge states will have different winding numbers when the intra-cell and inter-cell hopping amplitudes are spin-dependent ones. Consequently, how to detect the edge states with different winding numbers theoretically and experimentally has become one of important topics in condensed matter physics. In this paper, in the framework of the tight-binding approximation, we study the topological properties and the electron transport properties of the edge states of the SSH chain with the spin-orbit coupling. It is demonstrated that the winding numbers of the quadruple-degenerate and twofold-degenerate edge states are two and one, respectively. Importantly, the electron transport properties in the vicinity of the zero energy can characterize the energy spectra of the edge states, when the spin-polarized electrons tunnel into the SSH chain from the source lead, namely, the source lead is a ferromagnetic one. With increasing the tunneling coupling strengths between the SSH chain and the two leads from the weak coupling regime to the strong coupling one, the number of transmission resonance peaks of the quadruple-degenerate with the winding numbers being two and twofold-degenerate edge states with the winding numbers being one will be reduced by four and two, respectively. In other words, the transmission resonance peaks related to the edge states will disappear when the SSH chain is strongly coupled to the two leads. Therefore, these results suggest an alternative way of detecting the nontrivial topological ones with different winding numbers by changing the number of transmission resonance peaks of edge states. Keywords:edge states/ Su-Schrieffer-Heeger chain/ spin-orbit coupling/ transmission probability
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2.1.耦合导线的自旋轨道耦合SSH原子链
本文考虑自旋轨道耦合SSH原子链与左、右导线耦合的系统, 如图1所示. 该系统在紧束缚近似下的哈密顿量可表示为 图 1 自旋轨道耦合SSH原子链与左、右导线耦合系统的示意图. 其中, 红色实心圆表示A原子, 蓝色实心圆表示B原子, 黑色空心圆表示导线上的原子. ${t_0}$表示导线上相邻原子之间的跳跃振幅, ${t_{{\rm{L}}, \sigma }}$和${t_{{\rm{R}}, \sigma }}$表示自旋轨道耦合SSH原子链与左、右电极之间自旋依赖的隧穿耦合强度. $\upsilon $和w分别表示胞内和胞间自旋守恒的跳跃振幅, 而${\lambda _{\upsilon} }$和${\lambda _w}$则分别表示胞内和胞间自旋翻转的跳跃振幅 Figure1. The schematic diagram of the SSH chain with spin-orbit coupling coupled to the left and right leads. The red filled circles denote the A atoms, the blue filled circles denote the B atoms, the black unfilled circles denote atoms on the leads. ${t_0}$ describes the hopping amplitude between two adjacent atoms on the leads. ${t_{{\rm{L}}, \sigma }}$ and ${t_{{\rm{R}}, \sigma }}$ characterize the spin-dependent tunnel coupling strengths between the SSH chain with spin-orbit coupling and the left lead, and that between the SSH chain with spin-orbit coupling and the right lead, respectively. $\upsilon $ and w are the intra-cell and inter-cell hopping amplitudes with the spin-conserving processes, respectively. Whereas ${\lambda _{\upsilon} }$ and ${\lambda _w}$ are the intra-cell and inter-cell hopping amplitudes with the spin-flip processes, respectively
由(18)式可知, 缠绕数${W_{{\rm{SSH}} \text{-} {\rm{SOC}}}}$从2到1和从1到0的相变分别发生在$\upsilon = 1 - \left| {{\lambda _{\upsilon} } - {\lambda _w}} \right|$和$\upsilon = 1 + $$ \left| {{\lambda _{\upsilon} } - {\lambda _w}} \right|$处. 对于胞内和胞间无自旋翻转跳跃过程的情形, 即${\lambda _{\upsilon} } = {\lambda _w} = 0$, 相应的缠绕数${W_{{\rm{SSH}}}}$仅可能取1和0. 因此, 当胞内和胞间的电子跳跃含有自旋翻转过程时, 即${\lambda _{\upsilon} } \ne 0$和${\lambda _w} \ne 0$, 其系统的非平庸拓扑边缘态类型会更加丰富[22-25]. 下面, 讨论自旋轨道耦合SSH原子链的缠绕数${W_{{\rm{SSH}} \text- {\rm{SOC}}}}$与其非平庸拓扑边缘态的关系. 为方便讨论, 在本文中, 将胞间自旋守恒的跳跃振幅选取为能量单位, 即$w = 1.0$, 自旋轨道耦合SSH原子链的其他参数选取为: ${\lambda _{\upsilon} } = 0.1$, ${\lambda _w} = 0.5$. 在图2(a), (b)中, 给出了原胞数$N = 10$和$N = 50$的能谱图, 发现缠绕数${W_{{\rm{SSH}} \text- {\rm{SOC}}}} = 2$的区域对应于自旋轨道耦合SSH原子链具有四重简并的零能本征态; 而${W_{{\rm{SSH}} \text- {\rm{SOC}}}} = 1$的区域对应于该系统具有二重简并的零能本征态. 尤其是, 原胞数越大, 其四重、二重简并的零能本征态区域($\upsilon $的取值范围)越接近于(18)式给出的范围, 如图2(c)所示. 但是当${W_{{\rm{SSH}} \text- {\rm{SOC}}}} = 0$时, 自旋轨道耦合SSH原子链没有零能本征态. 图 2 (a) 原胞数目为10的自旋轨道耦合SSH原子链的能谱图; (b) 原胞数目为50的自旋轨道耦合SSH原子链的能谱图; (c) 自旋轨道耦合SSH原子链的缠绕数随着胞内自旋守恒跳跃振幅$\upsilon $的变化图. 自旋轨道耦合SSH原子链的参数选取为: $w = 1.0$, ${\lambda _{\upsilon} } = 0.1$和${\lambda _w} = 0.5$ Figure2. (a), (b) The energy spectrum of the SSH chain with spin-orbit coupling for $N = 10$ and $N = 50$, respectively; (c) the winding number of the SSH chain with spin-orbit coupling as a function of the intra-cell hopping amplitude with the spin-conserving process $\upsilon $. The parameters of the SSH chain with spin-orbit coupling are chosen as $w = 1.0$, ${\lambda _{\upsilon} } = 0.1$ and ${\lambda _w} = 0.5$.
为进一步确定零能本征态就是零能边缘态, 这里, 以原胞数$N = 10$的自旋轨道耦合SSH原子链为例说明. 图3给出了最靠近零能的4个本征态波函数在每个原子上的几率幅分布情况. 对于四重简并的零能本征态, 例如, $\upsilon = 0.3$, 4个零能本征态的波函数${\psi _{4, 1}}$, ${\psi _{4, 2}}$, ${\psi _{4, 3}}$, ${\psi _{4, 4}}$在自旋轨道耦合SSH原子链最左边(第1个)和最右边(最后1个)的几率幅(绝对值)最大, 并且其几率幅从两端向中间的原子位置快速衰减, 此即边缘态的典型特征, 如图3(a)—图3(d)所示. 另外, 对于二重简并的零能本征态, 例如, $\upsilon = 0.6$, 2个零能本征态的波函数${\psi _{2, 1}}$, ${\psi _{2, 2}}$在各原子上的几率幅分布同样具有边缘态的特性, 如图3(f)和图3(g)所示. 因此, 缠绕数${W_{{\rm{SSH}} \text- {\rm{SOC}}}} = 2$的区域对应于自旋轨道耦合SSH原子链的四重简并边缘态; 而${W_{{\rm{SSH}} \text- {\rm{SOC}}}} = 1$的区域对应于该系统的二重简并边缘态[25,28]. 下面, 从电子输运的角度, 讨论如何区分自旋轨道耦合SSH原子链不同缠绕数的边缘态. 图 3 自旋轨道耦合SSH原子链的本征值在4个零能附近的本征态波函数在每个原子上的几率幅分布图 (a)—(d) $ \upsilon = 0.3$; (e)—(h) $ \upsilon = 0.6$, 自旋轨道耦合SSH原子链的其他参数选取为$ w = 1.0$, $ {\lambda _\upsilon } = 0.1$, $ {\lambda _w} = 0.5$, $ N = 10$ Figure3. (a)–(d) The distribution of probability amplitudes of the wave functions of the four nearly zero-energy eigenstates of the SSH chain with spin-orbit coupling: (a)–(d) $\upsilon = 0.3$; (e)–(h) $\upsilon = 0.6$. The other parameters of the SSH chain with spin-orbit coupling are chosen as $w = 1.0$, ${\lambda _{\upsilon} } = 0.1$, ${\lambda _w} = 0.5$and $N = 10$.
23.2.入射电子的自旋极化率对电子透射率的影响 -->
3.2.入射电子的自旋极化率对电子透射率的影响
为了探寻自旋轨道耦合SSH原子链不同缠绕数边缘态对其电子输运的依赖关系, 首先, 研究入射电子的自旋极化率对零能附近电子输运特性的影响. 为方便讨论, 假设左、右导线与自旋轨道耦合SSH原子链之间的隧穿耦合仅依赖于传导电子的自旋极化率并且强度相同, 即${t_{{\rm{L}}, \uparrow }} = {t_{{\rm{L}}, \downarrow }} = {t_{\rm{L}}}$, ${t_{{\rm{R}}, \uparrow }} = {t_{{\rm{R}}, \downarrow }} = {t_{\rm{R}}}$, ${t_{\rm{L}}} = {t_{\rm{R}}}$. 考虑3种情况: 1) 自旋极化率为零, 即${\left| {{c_ \uparrow }} \right|^2} = {\left| {{c_ \downarrow }} \right|^2} = 0.50$; 2)自旋极化率为0.50, 即${\left| {{c_ \uparrow }} \right|^2} = 0.75$, ${\left| {{c_ \downarrow }} \right|^2} = 0.25$; 3) 纯自旋流, 即${\left| {{c_ \uparrow }} \right|^2} = 1.00$, ${\left| {{c_ \downarrow }} \right|^2} = 0$. 当入射电子的自旋没有被极化时, 对于缠绕数${W_{{\rm{SSH}}\text-{\rm{SOC}}}} = 2$的四重简并边缘态情形, 例如, $\upsilon = $$ 0.3$, ${t_{\rm{L}}} = {t_{\rm{R}}} = 0.0005$, 和缠绕数${W_{{\rm{SSH}}\text-{\rm{SOC}}}} = 1$的二重简并边缘态情形, 例如, $\upsilon = 0.6$, ${t_{\rm{L}}} = {t_{\rm{R}}} = 0.005$, 在零能附近, 均观察到2个电子共振透射峰, 如图4(a)和图4(b)的实线所示. 虽然这2个电子透射峰对应的能量位置能够与自旋轨道耦合SSH原子链最靠近零能的2个能级一一对应, 如图5(a)和图5(b)所示. 但是, 对于有限长的自旋轨道耦合SSH原子链, 其缠绕数${W_{{\rm{SSH}} \text- {\rm{SOC}}}} = 2$的四重简并边缘态对应于零能附近的4条能级, 如图5(a)所示. 因此, 当左导线入射电子的自旋没有被极化时, 自旋轨道耦合SSH原子链在零能附近的电子输运特性不能用于分辨其不同缠绕数的边缘态. 图 4 自旋轨道耦合SSH原子链的电子透射率在不同自旋极化率情形下随入射电子能量的变化 (a) $\upsilon = 0.3$; (b) $\upsilon = $$ 0.6$, 其他参数与图3相同 Figure4. The transmission probabilities of the SSH chain with spin-orbit coupling as a function of the energy of incident electron for the different spin polarizations of left lead: (a) $\upsilon = $$ 0.3$; (b) $\upsilon = 0.6$. The other parameters are the same as Fig. 3.
图 5 (a), (b) 自旋轨道耦合SSH原子链在零能级附近的能谱图; (c) 自旋轨道耦合SSH原子链与左导线原子$j = - 1$, 右导线原子$j = 1$耦合的系统在零能级附近的能谱图, ${t_{\rm{L}}} = {t_{\rm{R}}} = 0.1$, 其他参数与图3相同. Figure5. (a) and (b) Energy spectrum of the SSH chain with spin-orbit coupling in the vicinity of the zero energy; (c) energy spectrum of the SSH chain with spin-orbit coupling coupled to the atom of the left lead $j = - 1$ and that of the right lead $j = 1$ in the vicinity of the zero energy, where ${t_{\rm{L}}} = {t_{\rm{R}}} = 0.1$. The other parameters are the same as Fig. 3
基于电子输运性质探测自旋轨道耦合SSH原子链的不同缠绕数边缘态, 需要研究与其边缘态关联的电子输运特性随着外界可调物理量的变化. 这里, 选取自旋轨道耦合SSH原子链与左、右导线之间的隧穿耦合强度${t_{\rm{L}}}$和${t_{\rm{R}}}$为可调变量, 研究与自旋轨道耦合SSH原子链不同缠绕数边缘态相关联的电子透射率特性. 当自旋轨道耦合SSH原子链具有缠绕数${W_{{\rm{SSH}} - {\rm{SOC}}}} = 2$的四重简并边缘态($\upsilon = 0.3$)时, 对于自旋轨道耦合SSH原子链与左、右导线之间的弱耦合情形, 例如, ${t_{\rm{L}}} = {t_{\rm{R}}} = 0.0002$, 在零能附近可以观察到4个电子透射峰, 如图6(a)的实线所示. 随着${t_{\rm{L}}}$和${t_{\rm{R}}}$数值的逐渐增大, 最靠近零能的2个峰值较高的透射峰先被展宽, 如图6(a)的点线所示; 然后, 演化为1个较宽的透射峰, 如图6(b)的实线所示. 但是, 其他2个透射峰的峰值几乎不变, 如图6(a)所示. 当${t_{\rm{L}}}$和${t_{\rm{R}}}$数值继续增大时, 这个较宽的透射峰将被继续展宽, 最后与外侧2个透射峰一起, 演变成1个更大峰宽的透射峰, 直至完全消失, 如图6(b)所示. 对于缠绕数${W_{{\rm{SSH}} - {\rm{SOC}}}} = 1$的二重简并边缘态($\upsilon = 0.6$)的情形, 当${t_{\rm{L}}}$和${t_{\rm{R}}}$的数值较小时, 例如, ${t_{\rm{L}}} = {t_{\rm{R}}} = 0.002$, 入射电子在零能附近出现2个透射峰, 如图7(a)的实线所示. 同样, 这2个透射峰将随着${t_{\rm{L}}}$和${t_{\rm{R}}}$数值的增大, 先由2个峰逐步演化为1个较宽的透射峰, 如图7(b)的点画线和图7(c)的实线所示. 然后, 这个较宽的透射峰在${t_{\rm{L}}}$和${t_{\rm{R}}}$数值增大到某一临界值时消失, 如图7(c)的点画线所示. 下面, 讨论自旋轨道耦合SSH原子链在零能附近电子输运特性的物理机制. 图 6 自旋轨道耦合SSH原子链的电子透射率在不同隧穿耦合强度下随入射电子能量的变化, $\upsilon = 0.3$, 其他参数与图3相同 Figure6. The transmission probabilities of the SSH chain with spin-orbit coupling as a function of the energy of incident electron for different strengths of tunneling coupling, $\upsilon = 0.3$. The other parameters are the same as Fig. 3.
图 7 自旋轨道耦合SSH原子链的电子透射率在不同隧穿耦合强度下随入射电子能量的变化, $\upsilon = 0.6$, 其他参数与图3相同 Figure7. The transmission probabilities of the SSH chain with spin-orbit coupling as a function of the energy of incident electron for different strengths of tunneling coupling, $\upsilon = 0.6$. The other parameters are the same as Fig. 3.