石墨烯通过鞍点的谷极化
Valley polarization of graphene via the saddle point
AI总结：
本研究提出结合深紫外与太赫兹双频光脉冲，通过M鞍点选择定则及位移激发，在无带隙石墨烯中实现近乎完美的谷极化，为光波谷电子学开辟新路径。
摘要：
石墨烯以及单层Xene家族的其他成员，代表了“谷电子学”（即控制谷局域电荷激发）的理想材料平台。然而，这些半金属中缺乏带隙，阻碍了圆偏振光脉冲对谷的激发，从而严格限制了在这些材料中实现光波谷电子学的可能性。在此，我们展示了将深紫外线性偏振光脉冲与太赫兹包络相结合，可以在石墨烯中诱导高度谷极化的态。这种双频光形式通过以下机制运作：（i）深紫外脉冲在M鞍点激活选择定则，（ii）太赫兹脉冲将M点激发位移至低能K谷之一。利用紧束缚模型和先进的时间依赖密度泛函理论，我们证明了这种脉冲在石墨烯中产生近乎完美的谷极化激发，从而为无带隙Xene家族中的光波谷电子学提供了一条通过鞍点的途径。
英文摘要：
Graphene, and other members of the monolayer Xene family, represent an ideal materials platform for "valleytronics", the control of valley localized charge excitations. The absence of a gap in these semi-metals, however, precludes valley excitation by circularly polarized light pulses, sharply circumscribing the possibility of a lightwave valleytronics in these materials. Here we show that combining a deep ultraviolet linearly polarized light pulse with a THz envelope can induce highly valley polarized states in graphene. This dual frequency lightform operates by (i) the deep ultraviolet pulse activating a selection rule at the M saddle points and (ii) the THz pulse displacing the M point excitation to one of the low-energy K valleys. Employing both tight-binding and state-of-the-art time dependent density functional theory, we show that such a pulse results in a near perfect valley polarized excitation in graphene, thus providing a route via the saddle point to a lightwave valleytronics in the gapless Xene family.
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