Lu Xin, Chen Xiaotong, Dubey Sudipta, Yao Qiang, Li Weijie, Wang Xingzhi, Xiong Qihua, Srivastava Ajit
Department of Physics, Emory University, Atlanta, GA, USA.
Division of Physics and Applied Physics, School of Physical and Mathematical Sciences, Nanyang Technological University, Singapore, Singapore.
Nat Nanotechnol. 2019 May;14(5):426-431. doi: 10.1038/s41565-019-0394-1. Epub 2019 Mar 4.
Control and manipulation of single charges and their internal degrees of freedom, such as spin, may enable applications in quantum information technology, spintronics and quantum sensing. Recently, atomically thin semiconductors with a direct bandgap such as group VI-B transition-metal dichalcogenide monolayers have emerged as a platform for valleytronics-the study of the valley degree of freedom of charge carriers to store and control information. They offer optical, magnetic and electrical control of the valley index, which, with the spin, is locked into a robust spin-valley index. However, because recombination lifetimes of photogenerated excitations in transition-metal dichalcogenides are of the order of a few picoseconds, optically generated valley excitons possess similar lifetimes. On the other hand, the valley polarization of free holes has a lifetime of microseconds. Whereas progress has been made in optical control of the valley index in ensembles of charge carriers, valley control of individual charges, which is crucial for valleytronics, remains unexplored. Here we provide unambiguous evidence for localized holes with a net spin in optically active WSe quantum dots and we initialize their spin-valley state with the helicity of the excitation laser under small magnetic fields. Under such conditions, we estimate a lower bound of the valley lifetime of a single charge in a quantum dot from the recombination time to be of the order of nanoseconds. Remarkably, neutral quantum dots do not exhibit such spin-valley initialization, which illustrates the role of the excess charge in prolonging the valley lifetime. Our work extends the field of two-dimensional valleytronics to the level of single spin- valleys, with implications for quantum information and sensing applications.
对单个电荷及其内部自由度（如自旋）的控制和操纵，可能会推动量子信息技术、自旋电子学和量子传感等领域的应用发展。最近，具有直接带隙的原子级薄半导体，如VI - B族过渡金属二硫属化物单层，已成为谷电子学的一个平台——谷电子学是研究电荷载流子的谷自由度以存储和控制信息的学科。它们提供了对谷指数的光学、磁学和电学控制，谷指数与自旋一起，被锁定为一个稳健的自旋 - 谷指数。然而，由于过渡金属二硫属化物中光生激发的复合寿命约为几皮秒，光生谷激子也具有类似的寿命。另一方面，自由空穴的谷极化寿命为微秒级。虽然在电荷载流子集合体的谷指数光学控制方面已经取得了进展，但对于谷电子学至关重要的单个电荷的谷控制仍未得到探索。在这里，我们提供了明确的证据，证明在光学活性的WSe量子点中存在具有净自旋的局域空穴，并且在小磁场下，我们用激发激光的螺旋度初始化了它们的自旋 - 谷状态。在这种条件下，我们根据复合时间估计量子点中单个电荷的谷寿命下限为纳秒级。值得注意的是，中性量子点不表现出这种自旋 - 谷初始化，这说明了多余电荷在延长谷寿命中的作用。我们的工作将二维谷电子学领域扩展到了单个自旋 - 谷的层面，对量子信息和传感应用具有重要意义。
