Dirac cones and Dirac saddle points of bright excitons in monolayer transition metal dichalcogenides

Hongyi Yu, Gui-Bin Liu, Pu Gong, Xiaodong Xu and Wang Yao ()
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Hongyi Yu: The University of Hong Kong
Gui-Bin Liu: The University of Hong Kong
Pu Gong: The University of Hong Kong
Xiaodong Xu: University of Washington
Wang Yao: The University of Hong Kong

Nature Communications, 2014, vol. 5, issue 1, 1-7

Abstract: Abstract In monolayer transition metal dichalcogenides, tightly bound excitons have been discovered with a valley pseudospin optically addressable through polarization selection rules. Here, we show that this valley pseudospin is strongly coupled to the exciton centre-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, that is, 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. 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 mechanical control of valley pseudospin.

Date: 2014
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DOI: 10.1038/ncomms4876

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