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Ab Initio Theory of Exciton Transport from the Ballistic to Diffusive Regimes

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In low-dimensional and nanostructured materials, the optical response is dominated by correlated electron-hole pairs—or excitons—bound together by the Coulomb interaction. By now, it is well-established that these large excitonic effects are a combined consequence of quantum confinement and reduced screening in low dimensions. However, many challenges remain in understanding the transporting of these excitons, especially when it comes to correlating complex experimental signatures with underlying physical phenomena through the use of quantitatively predictive theories.

In this talk, I will present recent work in which we derive how signatures of dimensionality and symmetry manifest in the exciton bandstructure. Remarkably, we find that the exciton bandstructure contains non-analytical discontinuities in the long-range limit in materials of all dimensions. These discontinuities are quantum phenomena, arising from the exchange scattering of electron-hole pairs, and give rise to unexpected structure in the time-evolution of an exciton wavepacket, resulting in good agreement with experimental diffusion coefficients in monolayer MoS2, acene molecular crystals, and carbon nanotubes. Then, we develop and apply a first-principles approach to study exciton-phonon coupling in monolayer MoS2 and reveal the highly selective nature of exciton-phonon coupling due to the internal spin structure of the excitons. Moreover, we show that interference terms due to off-diagonal exciton-phonon matrix elements, which have thus far been neglected in first-principles studies, are critical for the description of dephasing mechanisms, and once accounted for, yield exciton linewidths in excellent agreement with experiment.

This talk is part of the Lennard-Jones Centre series.

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