Impact of indirect transitions on valley polarization in WS2 and WSe2,Nanoscale

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Impact of indirect transitions on valley polarization in WS2 and WSe2
Nanoscale ( IF 5.8 ) Pub Date : 2022-11-21 , DOI: 10.1039/d2nr04800k
Rasmus H Godiksen ₁ , Shaojun Wang _{1,

2} , T V Raziman ₁ , Jaime Gómez Rivas ₁ , Alberto G Curto _{1,

3,

4}

Affiliation

Controlling the momentum of carriers in semiconductors, known as valley polarization, is a new resource for optoelectronics and information technologies. Materials exhibiting high polarization are needed for valley-based devices. Few-layer WS₂ shows a remarkable spin-valley polarization above 90%, even at room temperature. In stark contrast, polarization is absent for few-layer WSe₂ despite the expected material similarities. Here, we explain the origin of valley polarization in both materials based on the interplay between two indirect optical transitions. We show that the relative energy minima at the Λ- and K-valleys in the conduction band determine the spin-valley polarization of the direct K–K transition. Polarization appears as the energy of the K-valley rises above the Λ-valley as a function of temperature and number of layers. Our results advance the understanding of the high spin-valley polarization in WS₂. This insight will impact the design of both passive and tunable valleytronic devices operating at room temperature.

中文翻译：

间接跃迁对 WS2 和 WSe2 中谷极化的影响

控制半导体中载流子的动量，称为谷极化，是光电子学和信息技术的新资源。基于谷的器件需要表现出高极化的材料。即使在室温下，少层 WS ₂也显示出超过 90% 的显着自旋谷极化。形成鲜明对比的是，少层 WSe _{2没有极化}尽管存在预期的物质相似性。在这里，我们基于两个间接光学跃迁之间的相互作用解释了两种材料中谷极化的起源。我们表明导带中 Λ 谷和 K 谷的相对能量最小值决定了直接 K-K 跃迁的自旋谷极化。作为温度和层数的函数，当 K 谷的能量上升到 Λ 谷之上时出现极化。_{我们的结果推进了对 WS 2}中高自旋谷极化的理解。这种洞察力将影响在室温下运行的无源和可调谐谷电子设备的设计。

更新日期：2022-11-25

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