Valleytronics and the Role of Dark Excitons

In the world of electronics, electron charge is used to process information. In the field of spintronics, we utilize electron spin to transfer information. However, valleytronics introduces a new dimension by using the unique crystal structures of materials to encode information in distinct momentum states of electrons, known as valleys.

Definition of Bright and Dark Excitons

Over the past decade, progress has been made in developing a class of ultra-thin semiconductor materials known as transition metal dichalcogenides (TMDs). These materials feature an arrangement of atoms in a crystal lattice that confines electrons at a specific energy level, such as the valence band. When exposed to light, electrons are excited to a higher energy level, the conduction band, leaving behind a positively charged hole in the valence band. These electrons and holes are bound by electrostatic attraction to form hydrogen-like quasiparticles known as excitons.

If the quantum properties of the electron and the hole match, they recombine within a picosecond, emitting light in the process. These are known as bright excitons. However, if the quantum properties do not match, the electron and hole do not recombine spontaneously, and no light is emitted; these are known as dark excitons.

Dark Excitons as Candidates for Quantum Technologies

Dark excitons are inherently more resistant to environmental factors like thermal background compared to the current generation of qubits, requiring less extreme cooling and making them less prone to decoherence, where the unique quantum state collapses. The ability to use the valley degree of freedom of dark excitons to transfer information makes them a promising candidate for quantum technologies.

The Role of Dark Excitons in Valleytronics

The unique atomic symmetry of TMDs means that when exposed to circularly polarized light, bright excitons can be created in a specific valley. This is the fundamental principle of valleytronics. However, bright excitons quickly convert into a large number of dark excitons that can retain valley-related information.

Observing Electrons on the Femtosecond Scale

Utilizing the world-leading TR-ARPES system at OIST, which includes a proprietary and portable XUV source, the team was able to track the properties of all excitons after the creation of bright excitons in a specific valley in a TMD semiconductor over time by measuring momentum, spin state, and population levels of electrons and holes simultaneously.

The results found that within a picosecond, some bright excitons are scattered by phonons into different momentum valleys, making them momentum-dark. Subsequently, spin-dark excitons dominate, where spins flip within the same valley, persisting on nanosecond scales.

Conclusion

Thanks to the advanced TR-ARPES system at OIST, the team gained direct insights into how dark excitons retain long-term valley information. Future developments in reading the valley properties of dark excitons will open up wide applications in the field of dark valleytronics across information systems.