Imagine a world where you can create new materials simply by shining a light on them - it sounds like something straight out of a fantasy novel, right? Well, prepare to be amazed, because scientists are on the brink of making this 'quantum alchemy' a reality!
At the Okinawa Institute of Science and Technology Graduate University, researchers have been exploring the fascinating field of Floquet engineering. This emerging area of physics aims to manipulate the electronic structure of materials using periodic drives, such as light, to unlock their hidden potential. The goal? To transform ordinary semiconductors into extraordinary superconductors and more.
While the theory of Floquet physics has been around since 2009, experimental demonstrations have been few and far between. And those that have been successful have relied heavily on light, which poses significant challenges. The intense light required can almost vaporize the material, and the results are often moderate at best.
But a diverse team of researchers, led by OIST and Stanford University, have discovered a game-changing alternative approach. They've shown that excitons, rather than light, can produce Floquet effects more efficiently. This breakthrough is published in Nature Physics, and it opens up a whole new world of possibilities for quantum devices and materials.
Professor Keshav Dani from OIST's Femtosecond Spectroscopy Unit explains, "Excitons couple much stronger to the material than photons due to the strong Coulomb interaction, particularly in 2D materials. This means we can achieve strong Floquet effects without the challenges posed by light."
So, what exactly are these Floquet effects, and how do they work? Floquet engineering applies the principle of periodic drives to the quantum world, where time and space become intertwined. In crystals like semiconductors, electrons are already subject to a periodic potential in space due to the tight lattice formation of atoms. When light is introduced at a specific frequency, a second periodic drive is created in time, as the photons interact rhythmically with the electrons. This shifts the permitted energy bands of the electrons, allowing them to inhabit new, hybrid bands and exhibit novel behaviors.
"Until now, Floquet engineering has been synonymous with light drives," says Xing Zhu, a PhD student at OIST. "While these systems have proven the existence of Floquet effects, light couples weakly to matter, requiring very high frequencies and intense light. This can vaporize the material, and the effects are short-lived. Excitonic Floquet engineering, on the other hand, requires much lower intensities."
Excitons form when electrons in semiconductors are excited from their resting state to a higher energy level, usually by photons. The negatively charged electron leaves behind a positively charged hole, and together they form a bosonic quasiparticle. Excitons carry self-oscillating energy, impacting the surrounding electrons at tunable frequencies. Because they are created from the material's own electrons, excitons couple more strongly with the material than light.
The team's world-class TR-ARPES (time- and angle-resolved photoemission spectroscopy) setup played a crucial role in this breakthrough. They excited an atomically thin semiconductor with an optical drive and recorded the energy levels of the electrons. By dialing down the optical drive and measuring the electron signal later, they were able to observe the excitonic Floquet effects separately. The results were astonishing - the excitonic Floquet effects were achieved with much less effort and had a stronger impact.
This discovery proves that Floquet effects are not limited to light drives and can be generated using other bosons, such as phonons, plasmons, magnons, and more. Excitonic Floquet engineering is less energetic than optical methods, making it a more practical approach. The researchers have laid the foundation for applied Floquet physics, which holds immense promise for creating and manipulating quantum materials.
Dr. David Bacon, a former OIST researcher now at University College London, concludes, "We've opened the gates to a wide variety of bosons in Floquet engineering. This has strong potential for creating and directly manipulating quantum materials. We're excited to continue exploring this field and taking the first practical steps towards realizing these novel materials."
The future of quantum alchemy is here, and it's more exciting than ever!
