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SUMMARY:Ab Initio Theory of Exciton Transport from the Ballistic to Diffus
 ive Regimes - Diana Qiu\, Yale University
DTSTART:20211025T153000Z
DTEND:20211025T160000Z
UID:TALK162307@talks.cam.ac.uk
CONTACT:Dr Christoph Schran
DESCRIPTION:In low-dimensional and nanostructured materials\, the optical 
 response is dominated by correlated electron-hole pairs--or excitons--boun
 d together by the Coulomb interaction. By now\, it is well-established tha
 t these large excitonic effects are a combined consequence of quantum conf
 inement and reduced screening in low dimensions. However\, many challenges
  remain in understanding the transporting of these excitons\, especially w
 hen it comes to correlating complex experimental signatures with underlyin
 g physical phenomena through the use of quantitatively predictive theories
 . \n\nIn this talk\, I will present recent work in which we derive how sig
 natures of dimensionality and symmetry manifest in the exciton bandstructu
 re. Remarkably\, we find that the exciton bandstructure contains non-analy
 tical discontinuities in the long-range limit in materials  of all dimensi
 ons. These discontinuities are quantum phenomena\, arising from the exchan
 ge scattering of electron-hole pairs\, and give rise to unexpected structu
 re in the time-evolution of an exciton wavepacket\, resulting in good agre
 ement with experimental diffusion coefficients in monolayer MoS2\, acene m
 olecular crystals\, and carbon nanotubes. Then\, we develop and apply a fi
 rst-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 inter
 ference terms due to off-diagonal exciton-phonon matrix elements\, which h
 ave 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.
LOCATION:https://zoom.us/j/92447982065?pwd=RkhaYkM5VTZPZ3pYSHptUXlRSkppQT0
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