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
Abstract:The Su-Schrieffer-Heeger (SSH) is a typical one-dimensional system with topological edge states, which has been experimentally realized in the photon and cold atom systems.Therefore, how to confirm the existence of the edge states from theoretical and experimental has become one of the most important topics in condensed matter physics. In this paper, using the tight-binding approximation and transfer-matrix method, we have studied the transport signatures of electron through a quantum dot-SSH chain hybrid system. Here,the two quantum dots play a role in modulating the tunneling coupling strength between the SSH chain and the two electrodes.When the quantum dots are weakly coupled to the SSH chain, the quadruple-degenerate edge states of the quantum dot-SSH chain hybrid system correspond to that the SSH chain has two degenerate zero-energy edge states; whereas the twofold-degenerate ones correspond to that the SSH chain has no edge states. While the quantum dots are strongly coupled to the SSH chain, the edge states only exist when the intra-cell hopping amplitude is larger than the inter-cell hopping amplitude. In this situation, however, there is no edge states in the SSH chain. In particular, when the quantum dot-SSH chain hybrid system is strongly coupled to the two external electrodes, the number of transmission resonance peaks of the edge states of the quantum dot-SSH chain hybrid system will be reduced by 2. For example, in the case of the quadruple-degenerate edge states, the number of transmission resonance peaks will be two; whereas in the case of twofold-degenerate ones, that will disappear. Therefore, by modulating the tunneling coupling strength between the quantum dots and the SSH chain and that between the quantum dots and the two external electrodes, we can observe the variation of the number of transmission resonance peaks of edge states to detect whether the SSH chain is in the nontrivial topological state or not. Keywords:edge states/ Su-Schrieffer-Heeger chain/ transmission probability
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2.1.物理模型
本文考虑的物理模型为量子点-SSH原子链系统和两个外加电极耦合, 如图1所示, 整个系统的哈密顿量可以表示为 图 1 量子点-SSH原子链系统的示意图. 其中, 空心圆为电极上的原子, 阴影圆表示量子点, 红色圆表示A原子, 蓝色圆表示B原子. ${t_0}$是电极上最近邻两个原子之间的跳跃振幅, ${t_\eta }(\eta = {\rm{L, R}})$表示导线与量子点之间的隧穿耦合强度, $\tau $为量子点与SSH原子链之间的隧穿耦合强度, $\upsilon $为胞内的跳跃振幅, $\omega $为胞间的跳跃振幅, N为原胞数目 Figure1. Schematic of the considered quantum dot-SSH chain hybrid system. The hollow circles denote atoms on the leads, the shadow circles are the quantum dots, red circles are the A atoms, the blue circles represent the B atoms.${t_0}$ is the hopping amplitude between the two nearest-neighbor atoms on the leads. ${t_\eta }~(\eta = {\rm{L, R}})$ describes the strength of tunneling coupling between the lead-η and quantum dot-η, $\tau $ is the strength of tunneling coupling between quantum dot and SSH chain, $\upsilon $ and $\omega $ denote the intra-cell and inter-cell hopping amplitudes, respectively. N is the number of unit cells.
对于SSH原子链, 其边缘态在胞内跳跃振幅$\upsilon $小于胞间跳跃振幅$\omega $情形下, 即$\upsilon < \omega $情形下存在, 如图2(a)所示. 在量子点-SSH原子链系统中, 其边缘态性质依赖于量子点与SSH原子链的隧穿耦合强度. 当量子点与SSH原子链处于弱隧穿耦合区域时, 例如, $\tau = 0.01$, 边缘态在不同区域具有不同的简并度, 如图2(b)所示. 当量子点与SSH原子链处于强隧穿耦合区域时, 例如, $\tau = 1.00$, 边缘态仅在$\upsilon > \omega $情形下存在, 如图2(c)所示. 为了确定边缘态及其简并度, 图3给出了SSH原子链和量子点-SSH原子链系统的边缘态, 即零能本征值对应的波函数在每个格点位置上的几率分布, 这里, 零能本征态记为${\varphi _\mu }$, 其中$\mu $为边缘态的态指标. 图 2 (a) SSH原子链的能谱图; (b)和(c)量子点-SSH原子链系统的能谱图, 其中, (b) $\tau = 0.01$, (c) $\tau = 1.00$. 胞间跳跃振幅$\omega = 1.00$, 原胞数目$N = 10$ Figure2. (a) Energy spectrum of the SSH chain; (b) and (c) Energy spectrum of the quantum dot-SSH chain hybrid system, where (b) $\tau = 0.01$ and (c) $\tau = 1.00$. Here, $\omega = 1.00$ and $N = 10$.
图 3 (a) SSH原子链的零能模波函数在每个格点位置上的几率分布, 其中, $\upsilon = 0.50$; (b)?(d) 量子点-SSH原子链系统的零能模波函数在每个格点位置上的几率分布, 其中: (b) $\tau = 0.01$, $\upsilon = 0.50$; (c) $\tau = 0.01$, $\upsilon = 1.50$; (d) $\tau = 1.00$$\upsilon = 2.00$ Figure3. (a) The probability distributions of wave functions of the zero-energy modes at each sites in the SSH chain with $\upsilon = 0.50$; (b)?(d) The probability distributions of wave functions of the zero-energy modes at each sites in the quantum dot-SSH chain hybrid system, where (b) $\tau = 0.01$, $\upsilon = 0.50$, (c)$\tau = 0.01$, $\upsilon = 1.50$, (d)$\tau = 1.00$$\upsilon = 2.00$.
首先, 分析量子点-SSH原子链系统四重简并边缘态的电子透射率特性, 即图2(b)中$\upsilon $小于0.8的情形. 当量子点-SSH原子链系统与外加电极耦合时, 量子点-SSH原子链系统与左、右电极的耦合强度${t_{\rm{L}}}$和${t_{\rm{R}}}$, 将影响量子点-SSH原子链系统的电子结构. 因而, ${t_{\rm{L}}}$和${t_{\rm{R}}}$的数值将影响其电子输运特性, 尤其是边缘态的电子输运特性. 为方便讨论, 这里选取${t_{\rm{L}}} = {t_{\rm{R}}}$. 当量子点-SSH原子链系统与电极之间处于弱耦合区域时, 外加电极对量子点-SSH原子链系统的电子结构影响很小. 对于有限长的SSH原子链, 量子点-SSH原子链系统的边缘态, 实际上是由四个能量不相等, 但其数值都接近于零的本征态组成, 如图4(a)所示. 此时, 入射电子将在${E_{{\rm{in}}}} = 0$附近, 出现四个共振透射峰, 例如, 当$\upsilon = 0.60$时, 在${t_{\rm{L}}} = {t_{\rm{R}}} = 0.010$的情形下, 入射电子能量在${E_{{\rm{in}}}} = \pm 0.006$和${E_{{\rm{in}}}} = \pm 0.010$附近, 出现了四个共振透射峰, 其峰值对应的入射电子能量与图4(a)的能量本征值定性一致, 如图5(a)中的实线所示. 图 4 (a), (c)和(e)量子点-SSH原子链系统在零能级附近的能谱图; (b), (d)和(f)量子点-SSH原子链系统与左、右电极第–1个和第1个原子耦合的系统在零能级附近的能谱图, 其中, ${t_{\rm{L}}} = {t_{\rm{R}}} = 1.00$ Figure4. (a), (c) and (e)Energy spectrum of the quantum dot-SSH chain hybrid system in the vicinity of the zero energy; (b), (d) and (f) Energy spectrum of the quantum dot-SSH chain hybrid system coupled to the first atom (–1) of the left lead and the first atom (1) of the right one in the vicinity of the zero energy at ${t_{\rm{L}}} = {t_{\rm{R}}} = 1.00$.
图 5 对于不同的隧穿耦合强度, 量子点-SSH原子链系统的电子透射率随入射电子能量的变化. 其中, $\tau = 0.01$, $\upsilon = 0.60$, $\omega = 1.00$, $N = 10$ Figure5. The transmission probability versus the energy of incident electron for different strengths of tunneling coupling at $\tau = 0.01$, $\upsilon = 0.60$, $\omega = 1.00$ and $N = 10$.
但是, 当量子点-SSH原子链系统与电极之间处于强耦合区域时, 外加电极将对量子点-SSH原子链系统的电子结构产生决定性的影响. 相应地, ${t_{\rm{L}}}$和${t_{\rm{R}}}$的数值将对入射电子在${E_{{\rm{in}}}} = 0$附近的电子输运特性起决定作用. 因而, 随着${t_{\rm L}}$和${t_{\rm R}}$的数值逐渐增大, 四个共振透射峰之间谷底的数值将逐渐增大, 如图5(a)所示, 并逐渐转变为两个较宽的透射峰, 如图5(b)中的虚线和点线所示. 之后, 这两个较宽的透射峰将随着${t_{\rm{L}}}$和${t_{\rm{R}}}$数值的继续增大而形成一个更宽的透射峰, 如图5(b)中的双点划线和5(c)中的实线所示. 若继续增大${t_L}$和${t_R}$的数值, 这个很宽的透射峰将劈裂为两个透射峰, 最后, 在Ein = $ \pm 0.004$附近形成两个共振透射峰, 如图5(c)所示. 为了解释此现象的物理机制, 在图4(b)中, 给出了量子点-SSH原子链系统与左、右电极第 –1个和第1个原子耦合的系统在零能级附近的能谱图, 这里, 选取${t_{\rm{L}}} = {t_{\rm{R}}} = 1.000$. 由图4(b)可知, 量子点-SSH原子链系统与左、右电极第 –1个和第1个原子耦合系统在$\upsilon = 0.60$时的能量本征值与这两个共振透射峰的位置${E_{{\rm{in}}}} = \pm 0.004$定性一致. 需要说明的是, 量子点-SSH原子链系统的零能本征态${\varphi _1}$和${\varphi _2}$的几率在SSH原子链最左边和最右边的两个原子上占据几率最大, 如图6所示. 因此, 当量子点-SSH原子链系统与电极之间的隧穿耦合强度从弱耦合区域变化到强耦合区域时, 在${E_{{\rm{in}}}} = 0$附近, 电子的共振透射峰将从四个减少为两个. 此特性可以用来判断SSH原子链是否处于非平庸拓扑态. 图 6 量子点-SSH原子链系统与左、右电极第–1个和第1个原子耦合系统的零能模波函数在每个格点位置上的几率分布. 其他参数与图5相同. Figure6. The probability distributions of wave functions of the zero-energy modes at each sites in the quantum dot-SSH chain hybrid system coupled to the first atom (–1) of the left lead and the first atom (1) of the right one. The other parameters are the same as in Fig. 5.
其次, 分析量子点-SSH原子链系统的二重简并边缘态的电子透射率特性, 即图2(b)中$\upsilon $大于0.8的情形. 当量子点-SSH原子链系统与电极之间处于弱耦合区域时, 外加电极对量子点-SSH原子链系统的电子结构影响很小. 对于有限长的SSH原子链, 量子点-SSH原子链系统的边缘态, 实际上是由两个能量不相等, 但其数值都接近于零的本征态组成, 如图4(c)所示. 此时, 入射电子将在${E_{{\rm{in}}}} = 0$附近, 出现两个共振透射峰, 例如, 对于$\upsilon = 1.50$的情形, 当${t_{\rm{L}}} = {t_{\rm{R}}} = 0.0001$时, 入射电子能量在${E_{{\rm{in}}}} = \pm 1.8 \times {10^{ - 6}}$附近, 出现了两个共振透射峰, 其峰值对应的入射电子能量与图4(c)的能量本征值定性一致, 如图7(a1)中的实线所示. 图 7 对于不同的隧穿耦合强度, 量子点-SSH原子链系统的电子透射率随入射电子能量的变化. 其中, $\tau = 0.01$, $\omega = 1.00$, $N = 10$. (a1)和(a2) $\upsilon = 1.50$; (b1)和(b2) $\upsilon = 2.00$ Figure7. The transmission probability versus the energy of incident electron for different strengths of tunneling coupling at $\tau = 0.01$, $\omega = 1.00$ and $N = 10$. (a1) and (a2) $\upsilon = 1.50$; (b1) and (b2) $\upsilon = 2.00$.