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arXiv 2311.04565cond-mat.mes-hallcond-mat.str-el
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扭曲双层石墨烯中的光子吸收

Photon absorption in twisted bilayer graphene

Disha Arora, Deepanshu Aggarwal, Sankalpa Ghosh, Rohit Narula

备注：
23 pages including appendices, 18 captioned figures
发表信息：
Phys. Rev. B 110, 245418 (2024)
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AI总结：

本文研究了扭曲双层石墨烯的单光子和双光子吸收，发现小扭曲角下吸收系数因态密度增加而显著增强，且共振特征随扭曲角增大从红外向可见光区域转移，掺杂仅引起轻微红移。

摘要：

我们通过研究调节扭曲角 $ θ$ 和激发能 $ E_l $ 对其吸收系数 $ α_{i=1,2}$ 的影响，研究了扭曲双层石墨烯（TBLG）中的单光子和双光子吸收。我们发现，对于 TBLG，作为 $ E_l $ 函数的 $ α_1 $ 表现出与其 van Hove 奇点（vHs）相对应的明显峰值。对于小扭曲角，例如 $θ\sim 1.8^{\circ}$，$α_1$ 共振峰的幅值大约是双层石墨烯（BLG）的两倍。与 BLG 相比，这种增强的响应可归因于扭曲结构中态密度（DOS）的增加。然而，随着扭曲角的增加，共振峰的幅值趋近于两个解耦的单层石墨烯（SLG）片的幅值。另一方面，在低扭曲角下，TBLG 的双光子吸收系数 $ α_2 $ 在共振峰对应的能量处，与 SLG 相比显示出约一个数量级的增强，并且相对于 BLG 也有轻微但显著的提升。随着扭曲角从 $ 8^{\circ} $ 减小到 $ 2.5^{\circ} $，共振峰增强了三个数量级。有趣的是，随着 $θ$ 增加，$α_{i=1,2}$ 相对于 $ E_l $ 表现出的共振特征逐渐从红外区域转移到可见区域。在对 TBLG 进行掺杂时，$α_1 $ 和 $ α_2 $ 相对于 $ E_l $ 基本保持不变，但其共振峰有轻微的红移。此外，我们探讨了双光子吸收的各种偏振配置，并确定了 $α_2$ 达到极值的条件。

英文摘要：

We investigate one- and two-photon absorption in twisted bilayer graphene (TBLG) by examining the effects of tuning the twist angle $ θ$ and the excitation energy $ E_l $ on its absorption coefficients $ α_{i=1,2}$. We find that $ α_1 $ as a function of $ E_l $ for TBLG exhibits distinct peaks corresponding to its van Hove singularities (vHs). For small twist angles, such as $θ\sim 1.8^{\circ}$, the magnitude of the resonant peak for $α_1$ is roughly twice that of bilayer graphene (BLG). This enhanced response, compared to BLG, can be attributed to the increased density of states (DOS) in the twisted structure. However, as the twist angle increases the magnitude of the resonant peak approaches that of two decoupled single-layer graphene (SLG) sheets. On the other hand, the two-photon absorption coefficient $ α_2 $ for TBLG at low twist angles displays an enhancement of about one order of magnitude compared to SLG at the energies corresponding to the resonant peak, as well as a small but notable increase relative to BLG. As the twist angle decreases from $ 8^{\circ} $ to $ 2.5^{\circ} $, the resonant peak intensifies by three orders of magnitude. Interestingly, as $θ$ increases the resonant features exhibited by $α_{i=1,2}$ \textit{vs.} $ E_l $ shift progressively from the infrared to the visible. On doping TBLG, both $α_1 $ and $ α_2 $ \textit{vs.} $ E_l $ remain essentially unchanged but with a slight red-shift in their resonant peaks. Additionally, we explore various polarization configurations for two-photon absorption and determine the conditions under which $α_2$ becomes extremal.

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