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arXiv · 1401.0667

Bright excitons in monolayer transition metal dichalcogenides: from Dirac cones to Dirac saddle points

Hongyi Yu·Guibin Liu·Pu Gong·Xiaodong Xu·Wang Yao
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Abstract

In monolayer transition metal dichalcogenides, tightly bound excitons have been discovered with a valley pseudospin that can be optically addressed through polarization selection rules. Here, we show that this valley pseudospin is strongly coupled to the exciton center-of-mass motion through electron-hole exchange. This coupling realizes a massless Dirac cone with chirality index I=2 for excitons inside the light cone, i.e. bright excitons. Under moderate strain, the I=2 Dirac cone splits into two degenerate I=1 Dirac cones, and saddle points with a linear Dirac spectrum emerge in the bright exciton dispersion. Interestingly, after binding an extra electron, the charged exciton becomes a massive Dirac particle associated with a large valley Hall effect protected from intervalley scattering. Our results point to unique opportunities to study Dirac physics, with exciton's optical addressability at specifiable momentum, energy and pseudospin. The strain-tunable valley-orbit coupling also implies new structures of exciton condensates, new functionalities of excitonic circuits, and possibilities for mechanical control of valley pseudospin.

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Hongyi Yu, Guibin Liu, Pu Gong, Xiaodong Xu, Wang Yao. 2014-01-03. Bright excitons in monolayer transition metal dichalcogenides: from Dirac cones to Dirac saddle points. https://doi.org/10.1038/ncomms4876

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