Introduction
Researchers from the Okinawa Institute of Science and Technology (OIST) and Stanford University have demonstrated a new method for Floquet engineering using excitons. This approach offers greater efficiency compared to traditional light-based methods.
Floquet engineering aims to modify the fundamental properties of materials, such as turning a semiconductor into a superconductor, by altering their electronic structure using a periodic drive.
Limitations of Light-Based Floquet Engineering
Previously, Floquet engineering primarily relied on light as the periodic drive. While this method demonstrated Floquet effects, it required very high light intensities. These intensities could damage the material and produced moderate, short-lived results, which limited broader application.
Exciton-Based Approach
The new approach utilizes excitons, which are quasiparticles formed in semiconductors when an electron is excited from its resting state. Professor Keshav Dani from OIST stated that excitons interact more strongly with materials than photons, particularly in 2D materials, due to strong Coulomb interaction. This stronger coupling allows excitons to achieve significant Floquet effects with much lower intensities, thereby avoiding the challenges associated with high-intensity light.
Principles of Floquet Engineering
In Floquet physics, a periodic external force—such as light or excitons—interacts with the electrons within a material's crystal lattice. This interaction shifts the permitted energy bands for electrons, causing them to inhabit new, hybrid bands. By adjusting the frequency and intensity of this periodic drive, the material's electron behavior and overall properties can be altered. The material returns to its original state once the drive is removed.
Experimental Validation
The research team used OIST's time- and angle-resolved photoemission spectroscopy (TR-ARPES) setup for their experiments. They initially confirmed Floquet effects using a strong optical drive. Subsequently, they significantly reduced the optical drive intensity and measured the electron signal to isolate and observe excitonic Floquet effects. The experiments showed that excitonic Floquet effects were achieved with significantly less data acquisition time and demonstrated a stronger effect compared to the optical method.
Future Implications
This research confirms that Floquet effects are achievable not only with light but also with other bosons, like excitons. This discovery establishes a foundation for practical Floquet engineering, opening pathways for the creation of novel quantum materials and devices. Researchers suggest that similar effects could potentially be achieved using other forms of excitation, such as phonons, plasmons, or magnons.