Exciton Lifetime and Optical Line Width Profile via Exciton–Phonon Interactions: Theory and First-Principles Calculations for Monolayer MoS₂

Abstract

Exciton dynamics dictates the evolution of photoexcited carriers in photovoltaic and optoelectronic devices. However, interpreting their experimental signatures is a challenging theoretical problem due to the presence of both electron−phonon and many-electron interactions. We develop and apply here a firstprinciples approach to exciton dynamics resulting from exciton− phonon coupling in monolayer MoS 2 and reveal the highly selective nature of exciton−phonon coupling due to the internal spin structure of excitons, which leads to a surprisingly long lifetime of the lowest-energy bright A exciton. Moreover, we show that optical absorption processes rigorously require a second-order perturbation theory approach, with photon and phonon treated on an equal footing, as proposed by Toyozawa and Hopfield. Such a treatment, thus far neglected in first-principles studies, gives rise to offdiagonal exciton−phonon self-energy, which is critical for the description of dephasing mechanisms and yields exciton line widths in excellent agreement with experiment.

Ultrafast Nonadiabatic Phonon Renormalization in Photoexcited Single-Layer MoS2

“…MoS 2 is a semiconducting TMD with a direct band gap at the K point of the BZ and interesting multi-valley topology of VB and CBs. The latter is considered to be instrumental for various physical phenomena in MoS 2 , such as enhanced EPC of the A 1g phonon mode as observed with Raman spectroscopy, exciton-phonon coupling, , anisotropic electron–phonon scattering following laser excitation, and multi-valley superconductivity. ,− As it will be shown below, the rich phase space for electron–phonon scatterings come from the Γ and K valleys in the VB and the K and Q valleys in the CB.…”

“…Importantly, we show that the overall phonon scattering rate is significantly increased in nonequilibrium, opening a possibility for enhancing the total EPC strength. These findings demonstrate that the photo-induced nonequilibrium state is a promising route for tailoring vibrational properties of quantum matter, especially for MoS 2 where phonons play a primary role in the emergence of novel quantum phenomena, such as in exciton dynamics − as well as formation of Holstein polaron, CDW, and superconductivity. ,− …”

Giant self-driven exciton-Floquet signatures in time-resolved photoemission spectroscopy of MoS 2 from time-dependent GW approach

“…In contrast, excitons in bulk semiconductors or quantum wells often have a much smaller binding energy, which make it difficult to excite a single exciton level due to the inherent linewidth of the laser pulse. Second, the coherence lifetime in monolayer MoS 2 due to exciton–phonon couplings is about 100 fs at 300 K and can be as long as picoseconds at 100 K ( 62 ). The carrier-scattering-limited coherence time was estimated to be 50~150 fs depending on carrier densities ( 38 ).…”

Semiconductor electron-phonon equations: A rung above Boltzmann in the many-body ladder

“…In fact, the polarizations carry information on the electronic coherence and are expected to vanish after the photo-excitation. The polarization rates Γ pol kµν can be calculated by different means [71][72][73], although they are often treated as fitting parameters. A semi-empirical way to estimate them is based on the observation that the electronic scattering term in Eq.…”