Quantum beats of coherent 1s 2s excitons in two dimensional transition metal

Abstract: Motivated by recent experimental measurement of the intrinsic excitonic wave-function in 2D Transition-metal dichalcogenides (TMDs) by angle-resolved photoemission spectroscopy (ARPES), we developed a theoretical study to resolve some characteristics of these excitons and some of the many open issues in these systems. The system is assumed to be embedded in an environment with average dielectric constant, below which electrostatic interactions in the corresponding TMD layer are screened. We adopt the long range approximation, which gives the electron-hole interaction in the Rytova - Keldysh form. Latter allows understanding the role of screening in TMDs structures. The bound state 1s 2s energy eigenvalues for the twodimensional are reformulated in momentum space leads to an integral form of the Wannier equation. The eigenfunctions are then expanded in terms of spherical harmonics. To evaluate the dynamic of the angle resolved photoemission spectrum arising from the dissociation of excitons given their steady states 1s 2s expressions, we follow the semi perturbative theoretical description developed by previous calculations. We discuss the dielectric environment effect on the dispersive features of the spectrum for different 1s and 2s exciton distributions. Quantum beat signatures in photoemission intensity demonstrate coherent coupling between 1s and 2s excitons. The beating contribution due to excitonic coherence is also discussed. The periodic oscillations arising from coherent superposition states, quantum beats, enable exploration of novel coherent phenomena.