Quantum 'alchemy' Made Feasible With Excitons
Okinawa Institute of Science and Technology Graduate University
Imagine a world where materials can be transformed by simply shining a light on them. This concept, once confined to the realms of science fiction and alchemy, is now becoming a reality for physicists exploring the emerging field of Floquet engineering. By using a periodic drive, such as light, scientists can alter the electronic structure of materials, potentially turning a simple semiconductor into a superconductor. While the theory of Floquet physics has been around since 2009, it's only recently that experiments have demonstrated its feasibility. However, these experiments have been limited by the reliance on light, which requires extremely high intensities that can damage the material while only achieving moderate results.
But a groundbreaking study led by researchers from the Okinawa Institute of Science and Technology (OIST) and Stanford University has introduced a game-changing approach. They've shown that excitons can produce Floquet effects much more efficiently than light. Excitons, formed in semiconductors when electrons are excited to a higher energy level, have a stronger coupling to the material due to the Coulomb interaction. This allows them to achieve strong Floquet effects while avoiding the challenges posed by light. The team's findings, published in Nature Physics, open up a new pathway to creating exotic quantum devices and materials that Floquet engineering promises.
The key to this discovery lies in the world-class time- and angle-resolved photoemission spectroscopy (TR-ARPES) setup at OIST. This setup, featuring a table-top extreme-UV source, captured the first real images of excitons and helped sketch out the evolution of dark excitons. By exciting an atomically thin semiconductor with an optical drive and recording the energy levels of the electrons, the team was able to observe Floquet effects directly and then capture the excitonic Floquet effects separately from the optical drive. The experiments demonstrated that Floquet effects can be achieved not only with light but also with other bosons, such as excitons, which are significantly less energetic.
This breakthrough is the culmination of OIST's history of exciton research and the world-class TR-ARPES setup. The team has conclusively proven that Floquet effects can be reliably generated with other bosons, opening up the field to a wide variety of possibilities. Excitonic Floquet engineering holds great promise for creating and directly manipulating quantum materials, and the researchers have laid the foundation for practical Floquet engineering. As study co-first author Dr. David Bacon puts it, 'We've opened the gates to applied Floquet physics to a wide variety of bosons. This is very exciting, given its strong potential for creating and directly manipulating quantum materials. We don't have the recipe for this just yet - but we now have the spectral signature necessary for the first, practical steps.'
But here's where it gets controversial... The study raises questions about the future of Floquet engineering. While it has been traditionally associated with light drives, the discovery of excitonic Floquet engineering challenges this notion. Will this new approach become the norm, or will light drives remain the dominant method? The answer lies in further research and experimentation, and the scientific community eagerly awaits the next chapter in this exciting field.
