Abstract
The physics of excitons, electron-hole pairs that are bound together by their mutual Coulomb attraction, can to great extent be understood in the framework of the quantum-mechanical hydrogen model. This model has recently been challenged by spectroscopic measurements on two-dimensional transition-metal dichalchogenides that unveil strong deviations from a hydrogenic spectrum. Here, we show that this deviation is due to the particular relativistic character of electrons in this class of materials. Indeed, their electrons are no longer described in terms of a Schrödinger but a massive Dirac equation that intimately links electrons to holes. Dirac excitons therefore inherit a relativistic quantum spin-1/2 that contributes to the angular momentum and thus the exciton spectrum. Most saliently, the level spacing is strongly reduced as compared to the hydrogen model, in agreement with spectroscopic measurements and ab-initio calculations.
Highlights
In this paper, we focus on the problem of excitonic spectra in two dimensional (2d) semiconducting transition metal dichalcogenides (2dTMDs)
The exciton spectrum recently observed in 2d TMDs [2, 3, 4] does not resemble the conventional Rydberg series (1)
We claim that the main mechanism responsible for the nonhydrogenic Rydberg series in 2dTMDs was overlooked until now
Summary
We focus on the problem of excitonic spectra in two dimensional (2d) semiconducting transition metal dichalcogenides (2dTMDs). The conventional 2d hydrogen-like exciton spectrum is given by e4μ. The exciton spectrum recently observed in 2d TMDs [2, 3, 4] does not resemble the conventional Rydberg series (1). A few very recent Letters [5, 6, 7] propose different explanations of the nonhydrogenic exciton spectra based on the Berry’s phase and non-local screening. We claim that the main mechanism responsible for the nonhydrogenic Rydberg series in 2dTMDs was overlooked until now.
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