Two Condensed Matter Physics Talks - Livestream

Superexchange-induced valley splitting in two-dimensional transition metal dichalcogenides

Breaking time-reversal symmetry via an external magnetic field or supporting magnetic substrate has been demonstrated to lift the degeneracy of the band gaps at the inequivalent K and K’ valleys in monolayer transition metal dichalcogenides (TMDs), a phenomenon known as valley splitting. However, reported valley splittings thus far are modest, and a detailed structural and chemical understanding of valley splitting via magnetic substrates is lacking. In this talk I will present results from my density functional theory (DFT) investigation of magnetic atoms in proximity to monolayer WSe2 and MoS2 TMDs to demonstrate the sensitivity of this phenomenon to the overlap of TMD Bloch states at the valley extrema with the localized d states of the magnetic atom. I will rationalize these results with a model Hamiltonian with second-order spin-dependent exchange coupling to demonstrate that valley splitting via magnetic substrates is driven by a superexchange mechanism. Finally, I will use these results to offer general design principles and propose optimal magnetic substrates for large valley splitting.

Speaker: Liz Peterson, UC Berkeley

A unified ab-initio framework for studying phonon mediated and limited exciton diffusion in molecular crystals

Developing a predictive first principles framework to accurately describe exciton transport in complex materials remains an open challenge. In organic semiconductors - optoelectronic materials with strong light-matter interactions and chemical tunability - understanding exciton transport is further complicated by the fact that exciton bandwidths and exciton-phonon coupling strengths are similar in magnitude. For these systems, it is unclear a priori whether exciton diffusion is best described by phonon-limited Boltzmann-like or phonon-mediated thermally activated hopping theories. Several computational approaches have been put forward to understand exciton dynamics in the hopping or band-like regime separately; however to date, few approaches exist which are general enough to be applied to both regimes in solids. In this talk, using state-of-the-art density functional perturbation theory and the ab initio GW plus Bethe-Salpeter equation approach, we develop a self-contained framework for computing exciton diffusion coefficients for solids in both the band-like and hopping exciton-polaron regimes. We apply our method to a select set of acene crystals, comparing our results in the two limits with experiments and elucidating microscopic origins of exciton diffusion in these and related materials.

Speaker: Jonah Haber, UC Berkeley

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