Journal
NATURE COMMUNICATIONS
Volume 5, Issue -, Pages -Publisher
NATURE RESEARCH
DOI: 10.1038/ncomms4876
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Funding
- Croucher Foundation under the Croucher Innovation Award
- Research Grant Council of Hong Kong [HKU705513P, HKU8/CRF/11G]
- NFSC [11304014]
- 973 Programme of China [2013CB934500]
- BIT Basic Research Funds [20131842001, 20121842003]
- US DoE
- BES
- Materials Sciences and Engineering Division [DE-SC0008145]
- NSF [DMR-1150719]
- Division Of Materials Research
- Direct For Mathematical & Physical Scien [1150719] Funding Source: National Science Foundation
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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.
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