Theory of optical excitation spectra and depolarization dynamics in bilayer WS 2 from the viewpoint of excimers
Theory of optical excitation spectra and depolarization dynamics in bilayer WS 2 from the viewpoint of excimers
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DOI：
10.1103/physrevb.90.035437
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发表时间：
2014-06
影响因子：
3.7
通讯作者：
T. Yu;M. W. Wu
中科院分区：
文献类型：
--
作者：
T. Yu;M. W. Wu
我们研究了双层WS$_2$的光激发光谱和光致发光退偏振动力学。对Zhu等人最近光致发光实验中光激发谱的不同理解[arXiv：1403.6224]提出双层WS$_2$。在实验中，观察到4个激发（1.68，1.93，1.99和2.37 eV），分别为γ谷、三电子、A激子和B激子的间接激子激发，当样品由单层合成为双层时，A激子的能量红移为3050 meV.我们的研究表明，考虑到双层WS$_2$中同时存在层内激子和电荷转移激子，在空穴层间跳跃的情况下，存在由层内激子态和电荷转移激子态叠加而成的准分子态。相应地，我们证明了双层WS$2 $中的四种光激发分别是A电荷转移激子、${\rmA}'$激子、${\rmB}'$激子和B层内激子态，计算的共振能与实验符合得很好.在我们的图片中，推测的间接激子，这涉及一个高阶声子吸收/发射过程，是不必要的。此外，准分子态的结合能计算为40 meV，提供了合理的解释实验观察到的A激子的能量红移。基于准分子态，我们进一步导出了交换作用哈密顿量。在泵浦-探测系统中，利用自旋动力学Bloch方程研究了电子-空穴交换相互作用引起的光致发光退偏振动力学。今天我们发现......
We investigate the optical excitation spectra and the photoluminescence depolarization dynamics in bilayer WS$_2$. A different understanding of the optical excitation spectra in the recent photoluminescence experimentby Zhu {\em et al.} [arXiv:1403.6224] in bilayer WS$_2$ is proposed. In the experiment, four excitations (1.68, 1.93, 1.99 and 2.37 eV) are observed and identified to be indirect exciton for the $\Gamma$ valley, trion, A exciton and B exciton excitations, respectively, with the redshift for the A exciton energy measured to be 30$\sim$50 meV when the sample synthesized from monolayer to bilayer. According to our study, by considering there exist both the intra-layer and charge-transfer excitons in the bilayer WS$_2$, with inter-layer hopping of the hole, there exists excimer state composed by the superposition of the intra-layer and charge-transfer exciton states. Accordingly, we show that the four optical excitations in the bilayer WS$_2$ are the A charge-transfer exciton, ${\rm A}'$ excimer, ${\rm B}'$ excimer and B intra-layer exciton states, respectively, with the calculated resonance energies showing good agreement with the experiment. In our picture, the speculated indirect exciton, which involves a high-order phonon absorption/emission process, is not necessary. Furthermore, the binding energy for the excimer state is calculated to be 40 meV, providing reasonable explanation for the experimentally observed energy redshift of the A exciton. Based on the excimer states, we further derive the exchange interaction Hamiltonian. Then the photoluminescence depolarization dynamics due to the electron-hole exchange interaction is studied in the pump-probe setup by the kinetic spin Bloch equations. We find that ......