二维拓扑绝缘体中的光激发：量子自旋霍尔系统中电子波包的产生和控制
Photoexcitation in two-dimensional topological insulators: Generating and controlling electron wavepackets in Quantum Spin Hall systems
摘要：
物理学中最吸引人的挑战之一是实现量子光学的电子对应物，这需要产生和控制单电子波包的能力。量子自旋霍尔（QSH）系统的边缘态，即在HgTe/CdTe和InAs/GaSb量子阱中实现的二维（2D）拓扑绝缘体，可以扭转该领域的潮流，因为它们不需要限制基于量子霍尔效应的实现的磁场。在这里，我们表明，一个电脉冲，本地化的空间和/或时间，并施加在一个QSH边缘，可以光激发电子波包内的分支电跃迁，而不调用体状态或塞曼耦合。这样的波包是自旋极化的，并以相反的方向传播，其密度分布与初始平衡温度无关，并且由于光谱的线性和表征无质量狄拉克电子的手征异常而不表现出色散。我们还调查了光激发的能量分布，并显示如何在适当的情况下，最小的激发（Levitons）产生。此外，我们表明，Rashba自旋轨道耦合的存在下，可以利用定制的光激发波包的形状。可能的实验实现进行了讨论。
英文摘要：
One of the most fascinating challenges in Physics is the realization of an electron-based counterpart of quantum optics, which requires the capability to generate and control single electron wave packets. The edge states of quantum spin Hall (QSH) systems, i.e. two-dimensional (2D) topological insulators realized in HgTe/CdTe and InAs/GaSb quantum wells, may turn the tide in the field, as they do not require the magnetic field that limits the implementations based on quantum Hall effect. Here we show that an electric pulse, localized in space and/or time and applied at a QSH edge, can photoexcite electron wavepackets by intra-branch electrical transitions, without invoking the bulk states or the Zeeman coupling. Such wavepackets are spin-polarised and propagate in opposite directions, with a density profile that is independent of the initial equilibrium temperature and that does not exhibit dispersion, as a result of the linearity of the spectrum and of the chiral anomaly characterising massless Dirac electrons. We also investigate the photoexcited energy distribution and show how, under appropriate circumstances, minimal excitations (Levitons) are generated. Furthermore, we show that the presence of a Rashba spin-orbit coupling can be exploited to tailor the shape of photoexcited wavepackets. Possible experimental realizations are also discussed.
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