Momentum space valleys in WS2 monolayer, schematic and observation (IMAGE)
Caption
The atomic structure of ultrathin semiconductors like TMDs is hexagonal, and this symmetry is reflected in momentum space, where the conduction (top) and valence (bottom) bands each have local energy minima and maxima at specific points (K), which can be visualized as valleys in a momentum landscape. Time-reversal symmetry in quantum mechanics dictates that what happens in one valley is mirrored in the opposite valley: if the conduction band at K has spin-down (red), then K’ must have spin-up (blue), leading to an alternating pattern along the edge of the hexagon. Bright excitons form when the electron rests in the same valley and has the same spin as the corresponding hole. By using either left- or right-circularly polarized light, one can selectively populate bright exciton in a specific valley. The insert shows energy measurements of bright excitons, showing the contrast in valleys K and K’.
Credit
Momentum landscape figure adapted Bussolotti et al., (2018) Nano Futures 2 032001. Insert adapted from Zhu et al., (2025) Nature Communications 16 6385
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