Exchange interaction

Spectroscopy

Quantum mechanics

Catalysis

Mixing (physics)

Spin (aerodynamics)

Materials science

Ferromagnetism

Atomic physics

Transition metal

Biochemistry

Spins

Thermodynamics

Physics

Nanotechnology

Exciton

Condensed matter physics

Biexciton

Excited state

Monolayer

Chemistry

Nature Physics

Monolayer transition metal dichalcogenides (TMDCs) are promising two-dimensional (2D) semiconductors for application in optoelectronics. Their optical properties are dominated by two series of photo-excited exciton states—A (XA) and B (XB)1,2—that are derived from direct interband transitions near the band extrema. These exciton states have large binding energies and strong optical absorption3–6, and form an ideal system to investigate many-body effects in low dimensions. Because spin–orbit coupling causes a large splitting between bands of opposite spins, XA and XB are usually treated as spin-polarized Ising excitons, each arising from interactions within a specific set of states induced by interband transitions between pairs of either spin-up or spin-down bands (TA or TB). Here, by using monolayer MoS2 as a prototypical system and solving the first-principles Bethe–Salpeter equations, we demonstrate a strong intravalley exchange interaction between TA and TB, indicating that XA and XB are mixed states instead of pure Ising excitons. Using 2D electronic spectroscopy, we observe that an optical excitation of the lower-energy TA induces a population of the higher-energy TB, manifesting the intravalley exchange interaction. This work elucidates the dynamics of exciton formation in monolayer TMDCs, and sheds light on many-body effects in 2D materials. Two-dimensional electronic spectroscopy experiments and first-principles many-electron calculations demonstrate the quantum mixing of different exciton states in monolayer MoS2. This reveals the many-body effects and dynamics of exciton formation in 2D materials.

Exchange-driven intravalley mixing of excitons in monolayer transition metal dichalcogenides