二维半导体莫尔超晶格中随位置与动量变化的层间耦合

Position-and momentum-dependent interlayer couplings in two-dimensional semiconductor moirésuperlattices

近些年来引起广泛关注的二维半导体莫尔超晶格系统中存在着莫尔激子、强关联电子态和面外铁电性等新奇物理现象,电子的层间耦合对于理解这些现象至关重要.本文研究了二维半导体双层莫尔超晶格中的层间耦合随位置和动量的变化.外势场导致的局域布洛赫波包的层间耦合与波包宽度以及中心位置处的层间平移有着密切关系.同时,层间耦合随动量的变化使得基态S型波包和激发态P±型波包有着截然不同的随中心位置变化的层间耦合形式:在两个S型波包的层间耦合消失的位置,S和P+型(或S和P−型)波包之间的层间耦合达到最强.利用该性质,可以通过外加光电场来调控特定谷的基态波包的层间输运.此外,双层系统中发现的面外铁电性可以归结为不同层导带和价带间的耦合导致的电子在两层中的再分配现象.将本文得到的层间耦合形式与单层紧束缚模型相结合,可计算出垂直平面的电偶极密度,其随层间平移的变化形式和数量级与实验观测相符.

In recent years,various novel phenomena have been observed in two-dimensional semiconductor moirésystems,including the moiréexcitons,strongly-correlated electronic states and vertical ferroelectricity.To gain an insight into the underlying physical mechanisms of these intriguing phenomena,it is essential to understand the interlayer coupling form of the electrons in moirésystems.In this work,the position-and momentum-dependent interlayer coupling effects in two-dimensional semiconductor moirésuperlattices are investigated.Starting from the monolayer Bloch basis,the interlayer coupling between two Bloch states are treated as a perturbation,and the coupling matrix elements in commensurate and incommensurate bilayer structures are obtained,which are found to depend on the momentum and the interlayer translation between the two layers.Under the effect of an external potential,the Bloch states form localized wavepackets,and their interlayer couplings are found to depend on the wavepacket width as well as the interlayer translation at the wavepacket center position.Meanwhile the momentum-dependence results in very different interlayer coupling forms for the ground-state S-type and the excited-state P^(±)-type wavepackets.It is shown that at a position where the interlayer coupling between two S-type wavepackets vanishes,the coupling between an S-type wavepacket and a P^(+)-type wavepacket(or between an S-type wavepacket and a P^(-)-type wavepacket)reaches a maximum strength.This can be used to manipulate the valley-selective interlayer transport of the ground-state wavepackets through external electric and optical fields.Besides,the vertical ferroelectricity recently discovered in bilayer systems can be attributed to the charge redistribution induced by the coupling between conduction and valence bands in different layers.Using the obtained interlayer coupling form combined with a simplified tight-binding model for the monolayer,the vertical electric dipole density can be calculated whose form and order of magnitude accord with the experimental observations.