Nonclassical Exciton Diffusion in Monolayer WSe2

Abstract

We experimentally demonstrate time-resolved exciton propagation in a monolayer semiconductor at cryogenic temperatures. Monitoring phonon-assisted recombination of dark states, we find a highly unusual case of exciton diffusion. While at 5 K the diffusivity is intrinsically limited by acoustic phonon scattering, we observe a pronounced decrease of the diffusion coefficient with increasing temperature, far below the activation threshold of higher-energy phonon modes. This behavior corresponds neither to wellknown regimes of semiclassical free-particle transport nor to the thermally activated hopping in systems with strong localization. Its origin is discussed in the framework of both microscopic numerical and semiphenomenological analytical models illustrating the observed characteristics of nonclassical propagation. Challenging the established description of mobile excitons in monolayer semiconductors, these results open up avenues to study quantum transport phenomena for excitonic quasiparticles in atomically thin van der Waals materials and their heterostructures.

Electron Dynamics in a Two-Dimensional Nanobubble: A Two-Level System Based on Spatial Density

“…We note that for the traveling wave packet we consider coherent dynamics in contrast to diffusion dynamics already observed in experiments. 8,30,47−52 Here, in fact we address the dynamics in the very first few hundreds of femtoseconds at very low temperature, where TMDCs show scattering times of several picoseconds 53 and energy-thermalization time scales of tens of picoseconds, as recently experimentally observed for related excitons 54 via phononassisted photoluminescence, cf. ref 55.…”

“…In particular, here we account for the electron–phonon interaction with longitudinal optical (LO) phonons of energy ℏω LO = 46.3 meV via the Fröhlich coupling. , We use a Lindblad formalism including all nondiagonal density matrix elements accounting for the spatial inhomogenity in our system, capturing most effects found also in quantum kinetics calculations (for more details see refs , , , and and Supporting Information). We note that for the traveling wave packet we consider coherent dynamics in contrast to diffusion dynamics already observed in experiments. ,,− Here, in fact we address the dynamics in the very first few hundreds of femtoseconds at very low temperature, where TMDCs show scattering times of several picoseconds and energy-thermalization time scales of tens of picoseconds, as recently experimentally observed for related excitons via phonon-assisted photoluminescence, cf. ref .…”

Microscopic modelling of exciton-polariton diffusion coefficients in atomically thin semiconductors

“…Previous studies have shown that TMD monolayers with their rich exciton landscape, including dark and bright exciton states [30][31][32], exhibit an interesting spatio-temporal exciton dynamics resulting in an intriguing exciton diffusion behaviour. This includes non-classical diffusion [33], transient negative diffusion [34], accelerated hot-exciton diffusion [35] or formation of spatial rings (halos) [36][37][38] and unconventional exciton funneling effects [39]. In view of their light component, polaritons show an interesting transport behaviour resulting in a fast propagation in the ballistic regime [27,29].…”

Electrical control of excitons in GaN/(Al,Ga)N quantum wells

“…Indirect excitons (IXs) are bound pairs of an electron and a hole confined in separated quantum layers, either semiconductor quantum wells (QWs) 1,2 , or various transition metal dichalcogenides (TMD) monolayers [3][4][5][6] . Due to their permanent dipole moment, IXs can be controlled in-situ by voltage [6][7][8] , can travel over large distances 5,[9][10][11][12][13] , and can cool below the temperature of quantum degeneracy before recombination [14][15][16][17][18][19][20][21][22][23][24] . Due to these properties, IXs are considered as a promising platform for the development of excitonic devices 25,26 .…”