Lin Zuzhang, Liu Yizhou, Wang Zun, Xu Shengnan, Chen Siyu, Duan Wenhui, Monserrat Bartomeu
Institute for Advanced Study, Tsinghua University, Beijing 100084, China.
State Key Laboratory of Low Dimensional Quantum Physics and Department of Physics, Tsinghua University, Beijing 100084, China.
Phys Rev Lett. 2022 Jul 8;129(2):027401. doi: 10.1103/PhysRevLett.129.027401.
The ability to selectively photoexcite at different Brillouin zone valleys forms the basis of valleytronics and other valley-related physics. Symmetry arguments combined with static lattice first-principles calculations suggest an ideal 100% valley polarization in transition-metal dichalcogenides under circularly polarized light. However, experimental reports of the valley polarization range from 32% to almost 100%. Possible explanations for this discrepancy include phonon-mediated transitions, which would place a fundamental limit to valley polarization, and defect-mediated transitions, which could, in principle, be reduced with cleaner samples. We explore the phonon-mediated fundamental limit by performing calculations of phonon-mediated optical absorption for circularly polarized light entirely from the first principles. We also use group theory to reveal the microscopic mechanisms behind the phonon-mediated excitations, discovering contributions from several individual phonon modes and from multiphonon processes. Overall, our calculations show that the phonon-limited valley polarization is around 70% at room temperature for state-of-the-art valleytronic materials including MoSe_{2}, MoS_{2}, WS_{2}, WSe_{2}, and MoTe_{2}. This fundamental limit implies that sufficiently pure transition-metal dichalcogenides are ideal candidates for valleytronics applications.
在不同布里渊区谷进行选择性光激发的能力构成了谷电子学及其他与谷相关物理学的基础。对称性论证与静态晶格第一性原理计算表明，在圆偏振光下，过渡金属二硫属化物中理想的谷极化率为100%。然而，关于谷极化的实验报道范围从32%到几乎100%。这种差异的可能解释包括声子介导的跃迁，这会对谷极化设置一个基本限制，以及缺陷介导的跃迁，原则上，更纯净的样品可以减少这种跃迁。我们通过完全从第一性原理出发计算圆偏振光下声子介导的光吸收，来探索声子介导的基本限制。我们还运用群论揭示声子介导激发背后的微观机制，发现了几个单独声子模式以及多声子过程的贡献。总体而言，我们的计算表明，对于包括MoSe₂、MoS₂、WS₂、WSe₂和MoTe₂在内的最先进的谷电子学材料，室温下声子限制的谷极化率约为70%。这一基本限制意味着足够纯净的过渡金属二硫属化物是谷电子学应用的理想候选材料。
