[论文解读] Gate-tunable exciton-polaron Rydberg series with strong roton effect

Erfu Liu, Jeremiah van Baren+6 more|arXiv (Cornell University)|Jun 8, 2020

2D Materials and Applications参考文献 3被引用 5

一句话总结

本研究在超洁净单层MoSe₂中展示了门控可调的激子-极化子里德伯态，其中激子被费米海中极化的电子-空穴云所屏蔽。观察到的强罗顿（roton）样色散导致从1s到3s态的能量级逐渐抑制并急剧偏移，随着费米海扩大，吸收带与发射带之间的能隙持续增大，证实了超越三子物理的多体效应。

ABSTRACT

The electronic exciton polaron is a hypothetical many-body quasiparticle formed by an exciton dressed with a polarized electron-hole cloud in the Fermi sea (FS). It is predicted to display rich many-body physics and unusual roton-like dispersion. Exciton polarons were recently evoked to explain the excitonic spectra of doped monolayer transition metal dichalcogenides (TMDs), but these studies are limited to the ground state. Excited-state exciton polarons can exhibit richer many-body physics due to their larger spatial extent, but detection is challenging due to their inherently weak signals. Here we observe gate-tunable exciton polarons for the 1s - 3s excitonic Rydberg series in ultraclean monolayer MoSe$_2$ devices by optical spectroscopy. When the FS expands, we observe increasingly severe suppression and steep energy shift from low to high Rydberg states. Their gate-dependent energy shifts go beyond the trion description but match our exciton-polaron theory. Notably, the exciton-polaron absorption and emission bands are separated with an energy gap, which increases from ground to excited state. Such peculiar characteristics are attributed to the roton-like exciton-polaron dispersion, where energy minima occur at finite momenta. The roton effect increases from ground to excited state. Such exciton-polaron Rydberg series with progressively significant many-body and roton effect shall provide a new platform to explore complex many-body phenomena.

研究动机与目标

• 研究掺杂单层过渡金属二硫属化合物中激发态激子-极化子，超越基态行为。
• 解决在关联二维体系中探测弱信号激发态准粒子的挑战。
• 探索静电栅压下多体激子态中罗顿样色散的出现机制。
• 建立一个通过可调里德伯激子-极化子研究复杂多体现象的平台。

提出的方法

• 采用光学光谱技术探测超洁净单层MoSe₂器件中1s–3s激子里德伯系列。
• 施加静电栅压以调节费米海大小并控制多体环境。
• 通过测量激发态能级的门依赖能量位移，区分激子-极化子行为与三子模型。
• 基于激子-极化子理论的理论建模用于解释观测到的光谱特征与色散关系。
• 将吸收带与发射带之间的分离量化为罗顿样色散的特征。
• 从里德伯态间光谱抑制与能量位移的演化推断其空间扩展与多体屏蔽效应。

实验结果

研究问题

• RQ1 在单层MoSe₂中，门控调节下激发里德伯系列中的激子-极化子态如何行为？
• RQ2 色散关系中的罗顿样能谷在里德伯激子的能量与光谱演化中在多大程度上显现？
• RQ3 观测到的光谱特征能否由三子模型解释，还是需要多体激子-极化子描述？
• RQ4 作为罗顿效应特征的多体相互作用强度（即有限动量处的能量极小值）如何从基态到激发里德伯态变化？
• RQ5 费米海扩展在调节激子-极化子吸收带与发射带之间能隙中起什么作用？

主要发现

• 1s–3s激子-极化子态在门控调节下的能量位移超出三子模型预测，证实了多体屏蔽效应。
• 从基态到激发态，吸收带与发射带之间的能隙持续增大，表明罗顿样色散增强。
• 随着费米海扩大，1s到3s态的光谱抑制与陡峭能量位移增强，表明多体效应更强。
• 罗顿效应（即有限动量处的能量极小值）从1s态到3s态逐步增强。
• 观测到的光谱特征与费米海中激子-极化子准粒子的理论预测定量一致。
• 本研究建立了一个门控可调平台，可用于探索二维量子材料中复杂多体现象。