What if the creation of new materials could be as simple as shining a light on them? While this notion might sound like science fiction or modern-day alchemy, it is the ambitious goal of physicists exploring the rapidly advancing field of Floquet engineering. By using a periodic drive, such as light, scientists can modify the electronic structure of materials, potentially transforming a basic semiconductor into a superconductor. Despite the theoretical foundation laid by Oka and Aoki in 2009, practical demonstrations of Floquet effects have been scarce, primarily due to the high-intensity light required, which risks damaging the material.
In a groundbreaking development, an international team of researchers, led by the Okinawa Institute of Science and Technology (OIST) and Stanford University, has unveiled a novel approach to Floquet engineering. Their research, published in Nature Physics, reveals that excitons can induce Floquet effects more efficiently than light. According to Professor Keshav Dani from OIST, “Excitons couple much stronger to the material than photons due to the strong Coulomb interaction, particularly in 2D materials,” allowing for significant Floquet effects without the challenges associated with light.
Exploring the Potential of Floquet Engineering
Floquet engineering has long been considered a promising avenue for creating quantum materials on demand from ordinary semiconductors. The principle is based on subjecting a system to a periodic drive, such as a repeating external force, which can lead to complex behaviors beyond simple repetitions. This concept is akin to pushing a swing periodically to achieve greater heights.
In the quantum realm, Floquet engineering applies this principle to alter the energy bands of electrons in materials like semiconductors. When light of a specific frequency interacts with a crystal, it introduces a second periodic drive, shifting the energy bands and altering the material’s properties. This process is akin to musical notes harmonizing to create a new sound. However, once the light is turned off, the material reverts to its original state.
Challenges and Innovations
Traditionally, Floquet engineering has relied on light drives, but these systems face limitations. As Xing Zhu, a PhD student at OIST, explains, “Light couples weakly to matter, requiring very high frequencies to achieve hybridization. Such high energy levels can vaporize the material, and the effects are short-lived.” In contrast, excitonic Floquet engineering requires much lower intensities.
Excitons are formed in semiconductors when electrons are excited from their resting state to a higher energy level, creating a bosonic quasiparticle. Professor Gianluca Stefanucci of the University of Rome Tor Vergata elaborates, “Excitons carry self-oscillating energy, impacting surrounding electrons at tunable frequencies. They couple more strongly with the material than light, requiring significantly less light to create a dense exciton population for effective hybridization.”
Breakthrough with TR-ARPES Technology
This breakthrough stems from the OIST unit’s extensive exciton research and their advanced TR-ARPES (time- and angle-resolved photoemission spectroscopy) setup. The team excited an atomically thin semiconductor with an optical drive, recording the electrons’ energy levels. They first observed the Floquet effect using a strong optical drive, then reduced the intensity to capture excitonic Floquet effects separately.
“The experiments spoke for themselves,” says Dr. Vivek Pareek, an OIST graduate now at Caltech. “It took tens of hours to observe Floquet replicas with light, but only around two hours to achieve excitonic Floquet with a much stronger effect.”
This research conclusively demonstrates that Floquet effects can be achieved with bosons other than photons, paving the way for practical Floquet engineering. Excitonic Floquet engineering is less energy-intensive than optical methods, and similar effects could be achieved with other bosons, such as phonons, plasmons, and magnons.
Implications for Quantum Material Development
The successful demonstration of excitonic Floquet engineering opens new avenues for creating and manipulating quantum materials. “We’ve opened the gates to applied Floquet physics,” concludes Dr. David Bacon, co-first author and former OIST researcher now at University College London. “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.”
As researchers continue to explore the potential of excitonic Floquet engineering, the field promises to revolutionize the development of exotic quantum devices and materials, bringing science fiction closer to reality.
