Quantum Interference Effect on Exciton Transport in Monolayer Semiconductors

Abstract

We study theoretically weak localization of excitons in atomically-thin transition metal dichalcogenides. The constructive interference of excitonic de Broglie waves on the trajectories forming closed loops results in a decrease of the exciton diffusion coefficient. We calculate the interference contribution to the diffusion coefficient for the experimentally relevant situation of exciton scattering by acoustic phonons and static disorder. For the acoustic phonon scattering, the quantum interference becomes more and more important with increasing the temperature. Our estimates show that the quantum contribution to the diffusion coefficient is considerable for the state-of-the-art monolayer and bilayer transition metal dichalcogenides.

Nonclassical Exciton Diffusion in Monolayer WSe2

“…They uniquely combine the characteristics of the two descriptions, with wave functions being delocalized across many unit cells but also exhibiting high binding energies [17]. This duality is expected to manifest itself prominently in the exciton transport behavior, including potential emergence of quantum interference phenomena [48][49][50][51][52]. Despite much progress, however, only little is known regarding the appropriate picture for the exciton propagation in monolayer semiconductors, currently based on the assumption of a purely semiclassical framework [22,24,28,30,32,33,36,37,53].…”

“…These conclusions are further supported by quantitative analysis involving experimentally determined scattering rates as well as many-body calculations of the spatiotemporal exciton dynamics. In view of the recently predicted quantum interference phenomena for TMDC monolayers [52], it allows us to experimentally demonstrate an interesting scenario that requires nonclassical effects in the exciton transport.…”

“…Ideally, the primary mechanism limiting the diffusion at low temperatures is the quasielastic exciton scattering with longwavelength acoustic phonons. The corresponding rate scales linearly with the temperature and is expected to yield a temperature-independent, constant diffusivity for the purely semi-classical behavior [52]. To elucidate the nature of the exciton transport it is thus necessary to gain independent access to both diffusion coefficient and scattering rate τ −1 s as functions of the exciton temperature.…”

“…3(e). Recently developed for 2D TMDCs [52], the calculations are adapted for the WSe 2 monolayer by using the exciton mass and the sound velocity of 0.75m 0 and 3.3 km=s, respectively. The model accounts perturbatively for quantum interference effects in the exciton transport.…”

“…Constructive interference can arise between clock-and counterclockwise propagation of exciton wave packets through closed loops, leading to an effective localization of excitons (also see Supplemental Material [65]). For quasielastic exciton-phonon interaction [52], the specific interplay between the loss of the relative phase in the loop and momentum scattering results in an initial decrease of the effective diffusivity with increasing temperature. Interestingly, the functional form of the quantum corrections and their temperature dependence indeed resemble experimental observations.…”