Nonadiabatic exciton-phonon coupling in Raman spectroscopy of layered materials

Abstract

We present an ab initio computational approach for the calculation of resonant Raman intensities, including both excitonic and nonadiabatic effects. Our diagrammatic approach, which we apply to two prototype, semiconducting layered materials, allows a detailed analysis of the impact of phonon-mediated exciton-exciton scattering on the intensities. In the case of bulk hexagonal boron nitride, this scattering leads to strong quantum interference between different excitonic resonances, strongly redistributing oscillator strength with respect to optical absorption spectra. In the case of MoS2, we observe that quantum interference effects are suppressed by the spin-orbit splitting of the excitons.

Sub-10 fs Intervalley Exciton Coupling in Monolayer MoS2 Revealed by Helicity-Resolved Two-Dimensional Electronic Spectroscopy

“…76 Nonadiabatic coupling of excitons to phonons has also been shown to couple exciton states, although this effect is suppressed in MoS 2 because of the strong spin−orbit coupling compared with, for example, hexagonal boron nitride. 77 Phonon emission remains possible at cryogenic temperature, for example in intervalley scattering via the emission of acoustic phonons. 78 This mechanism requires an excess above-band-gap excitation of twice the phonon energy.…”

“…Phonon-mediated processes have been implicated previously in subpicosecond intervalley scattering as well as exciton formation on the ∼30 fs time scale and interlayer charge separation in TMD heterostructures . Nonadiabatic coupling of excitons to phonons has also been shown to couple exciton states, although this effect is suppressed in MoS 2 because of the strong spin–orbit coupling compared with, for example, hexagonal boron nitride . Phonon emission remains possible at cryogenic temperature, for example in intervalley scattering via the emission of acoustic phonons .…”

Interlayer exciton-polaron in atomically thin semiconductors

“…The polaron effects can also influence the transport of excitons, their diffusion, the phononrelated drag, and Raman scattering since the exciton is surrounded by a phonon cloud. These effects are now actively studied in TMDC-based and other layered nanosystems [52][53][54][55][56] .…”

Exciton-phonon interaction calls for a revision of the "exciton" concept

“…If the optical exciton is assumed to be a welldefined, boson-like particle, then the phonons can mediate the exciton-exciton interaction causing, for example, the dressing of the excitons and finite lifetimes 11,45 . This picture has been employed to describe phonon-assisted sidepeaks in absorption and luminescence spectra, 39,42,44,[46][47][48][49] as well as the linewidths of ex-citon peaks 12,38,41,50 . The methodologies employed in the above references range from parametrised simple models to tight-binding treatments and fully first-principles descriptions.…”