Valley Dynamics in Transition Metal Dichalcogenides

Transition metal dichalcogenide monolayers exhibit a pair of inequivalent energy extrema, or valleys, in their electronic band structure. The valley degree of freedom, often termed valley pseudospin, can be selectively addressed by circularly polarised light owing to chiral optical selection rules. Strong spin–orbit coupling locks spin and valley indices, giving rise to spin–valley-locked excitons that underpin potential valleytronic and quantum information applications. Valley dynamics are governed by intervalley scattering—mediated by electron–hole exchange, phonons and defects—which leads to depolarisation on timescales ranging from femtoseconds to nanoseconds. Dielectric screening, heterostructure design and electrostatic gating have been shown to modulate exciton lifetimes, coherence times and exchange interactions, thereby extending valley polarisation retention. Emerging control techniques include Floquet engineering via periodic optical or cavity fields and strain or magnetic-field tuning to break time-reversal symmetry. These advances not only deepen our fundamental understanding of nonequilibrium carrier dynamics in two-dimensional semiconductors but also pave the way for ultrathin valleytronic devices operable at ambient conditions.

Research from Nature Portfolio

Recent studies demonstrate that cavity-enhanced periodic driving can induce large spin and valley splittings in WSe₂ excitons under low fluence, effectively generating an optically controlled magnetic field exceeding 200 T and enabling ultrafast valley logic operations. Ab initio simulations have revealed that optical phonon pumping in MoS₂ breaks lattice symmetry and yields spin-valley Floquet bands with net out-of-plane magnetisation, offering a route to infrared-controlled magneto-valleytronics. Experiments on WSe₂ monolayers have further established that exciton valley relaxation is dominated by momentum-dependent long-range electron–hole exchange interactions, whose strength can be tuned by carrier density and exciton linewidth, providing a unified framework for engineering valley lifetimes.

Valley Dynamics in Transition Metal Dichalcogenides publication trend

The graph below shows the total number of articles in valley dynamics in transition metal dichalcogenides across all publications each year (not limited to Nature Index journals).

Technical terms

Valley pseudospin: A quantum index labelling distinct energy minima (valleys) in momentum space.

Exciton: A bound state of an electron and a hole attracted by Coulomb interaction.

Intervalley scattering: A process by which carriers transfer between inequivalent valleys, leading to depolarisation.

Spin–orbit coupling: An interaction coupling an electron’s spin with its orbital motion in a crystal lattice.

Floquet engineering: The modulation of electronic band structures via periodic driving fields, such as optical or cavity modes.

References

1. Observation of ~100% valley-coherent excitons in monolayer MoS2 through giant enhancement of valley coherence time. Light: Science & Applications (2023).
2. Cavity Floquet engineering. Nature Communications (2024).
3. Phonon-driven spin-Floquet magneto-valleytronics in MoS2. Nature Communications (2018).
4. Evidence for line width and carrier screening effects on excitonic valley relaxation in 2D semiconductors. Nature Communications (2018).
5. Dramatically Enhanced Valley‐Polarized Emission by Alloying and Electrical Tuning of Monolayer WTe2xS2(1‐x) Alloys at Room Temperature with 1T′‐WTe2‐Contact. Advanced Science (2023).
6. Real-time GW-BSE investigations on spin-valley exciton dynamics in monolayer transition metal dichalcogenide. Science Advances (2021).
7. Optical polarization and intervalley scattering in single layers of MoS2 and MoSe2. Scientific Reports (2016).
8. Valley excitons in two-dimensional semiconductors. National Science Review (2015).