对称三层异质结构中的四极激子与偶极激子：来自第一性原理理论的洞见
Quadrupolar and dipolar excitons in symmetric trilayer heterostructures: Insights from first principles theory
AI总结：
本文通过第一性原理计算揭示对称三层 WS$_2$/MoS$_2$/WS$_2$ 异质结构中偶极层间激子耦合形成四极激子，并解释其 Stark 位移随电场由二次转为线性的机制。
摘要：
范德华异质结构中的激子具有许多不同形式。在双层结构中，电子和空穴可能局域在同一层上，也可能相互分离，形成具有有限面外偶极矩的层间激子。利用第一性原理计算，我们研究了在垂直电场存在下对称 WS$_2$/MoS$_2$/WS$_2$ 异质结构中的激子。这些激子在低场强下表现出二次 Stark 位移，而在较强场强下表现出线性 Stark 位移。该行为可追溯到具有相反偶极矩的层间激子之间的耦合，这种耦合导致在小电场下形成四极激子。四极激子的形成取决于电场诱导的偶极激子分裂与它们之间耦合的相对大小，而该耦合由空穴穿越 MoS$_2$ 层的隧穿给出。对于反转结构 MoS$_2$/WS$_2$/MoS$_2$，偶极激子通过电子穿越 WS$_2$ 层的隧穿发生耦合。由于这一效应弱得多，所得四极激子更加脆弱，并会在更弱的电场下破裂。
英文摘要：
Excitons in van der Waals heterostructures come in many different forms. In bilayer structures, the electron and hole may be localized on the same layer or they may be separated forming an interlayer exciton with a finite out-of-plane dipole moment. Using first principles calculations, we investigate the excitons in a symmetric WS$_2$/MoS$_2$/WS$_2$ heterostructure in the presence of a vertical electric field. The excitons exhibit a quadratic Stark shift for low field strengths and a linear Stark shift for stronger fields. This behaviour is traced to the coupling of interlayer excitons with opposite dipole moments, which lead to the formation of quadrupolar excitons at small fields. The formation of quadrupolar excitons is determined by the relative size of the electric field-induced splitting of the dipolar excitons and the coupling between them given by the hole tunneling across the MoS$_2$ layer. For the inverted structure, MoS$_2$/WS$_2$/MoS$_2$, the dipolar excitons are coupled by electron tunneling across the WS$_2$ layer. Because this effect is much weaker, the resulting quadrupolar excitons are more fragile and break at a weaker electric field.
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