Jack Featherstone / OIST
A promising and powerful engineering breakthrough could soon allow researchers to alter the properties of materials by exciting electrons to higher-than-normal energy levels.
In physics, the Floquet engineering involves changes in the properties of a quantum material induced by a driving force, such as high power light.
The resulting effect causes a change on the behavior of the material, introducing new quantum states with properties that do not occur under normal conditions. Given its promising applications, Floquet engineering has been of interest to researchers for many years.
Now, a team of scientists from the Okinawa Institute of Science and Technology (OIST) and Stanford University claims to have developed a new method for achieving Floquet physics, more efficient than previous methods, which depend on light.
The method was detailed in a published on Monday in Nature Physics.
21st Century Alchemy
The team’s new approach leverages what is known for excitementswhich turned out to be much more powerful in coupling with quantum materials than existing methods “due to the strong Coulomb interaction, particularly in two-dimensional materials”, explains Keshav DaniOIST researcher, announcing the discovery.
For this reason, says Dani, excitons “can thus achieve strong Floquet effects avoiding the challenges posed by light“.
The team says this method offers an innovative means of exploring various applications, which include “future exotic quantum devices and the materials that Floquet’s engineering promises.”
Such unique phenomena could allow applications in materials science almost similar to alchemyto the extent that the concept of creating new materials simply by shining light on them sounds more like science fiction than the most advanced engineering of the 21st century.
Floquet Engineering
In the past, the effects of Floquet have remained elusive in the laboratoryalthough investigations over the years have demonstrated their potential, as long as they can be achieved in practical conditions.
However, a important limiting factor has been the reliance on intense light as the primary driving force, which can also lead to damage or even vaporization of materials, thus limiting useful results, explains .
Typically, Floquet engineering focuses on achieving such effects under quantum conditionswhich challenge our usual expectations of time and space. When researchers employ semiconductors or similar crystalline materials as a medium, the electrons behave accordingly. one of these dimensions, space, allows.
This is due to the distribution of atomswhich confines the movement of electrons and, consequently, limits their energy levels. Such conditions represent only a “periodic” condition to which electrons are subjected.
However, if a powerful light shines on the crystal at a certain frequency, represents a additional periodic forcealthough now in the time dimension. The resulting rhythmic interaction between light (i.e. photons) and electrons leads to additional changes in their energy.
By controlling the frequency and intensity of the light used as this secondary periodic force, electrons can be led to have unique behaviorswhich also cause changes in the material they inhabit during the time they remain excited.
From light to excitement
“Until now, Floquet’s engineering has been synonymous with driving forces of light,” he says Xing Zhucurrently a PhD student at OIST and co-author of the article.
However, how light couples poorly with matterresearchers have been limited in the past to achieving such effects mostly on the femtosecond scale.
“Such high energy levels tend to vaporize the material, and the effects are very ephemeral. In contrast, excitonic Floquet engineering requires much lower intensities,” says Zhu.
According to the Gianluca Stefanucciprofessor at the University of Rome Tor Vergata, and also co-author of the study, the exciters are an ideal alternative to photons because they carry self-oscillating energy that can affect surrounding material at frequencies that can be controlled through proper tuning.
“As excitons are created from the material’s own electrons, they couple much more strongly with the material than light,” explains Stefanucci.
“And, crucially, significantly less light is needed to create an exciton population dense enough to serve as an effective periodic force for hybridization — which is what we observe now”, he adds.
In the past, the OIST team has conducted exciton research using a specially designed assembly called TR-ARPESwhich stands for “time- and angle-resolved photoemission spectroscopy.”
During the experiments, the team excited a semiconductor material very thin, atomic thickness, with light, while recording the energy levels of the electrons inside, which allowed observing the manifestation of Floquet effects and, additionally, measure electronic signals at the femtosecond scale.
Significantly, this allowed researchers to evaluate the effects of Floquet associated with optical phenomena independently those related to excitonic behavior.
“It took dozens of hours of data acquisition to observe Floquet replicas in light,” he said. Vivek PareekPresidential Postdoctoral Investigator at the California Institute of Technology and first author of the study
Despite the amount of data required, the team managed to achieve excitonic Floquet effects, confirms Pareek, “and with a much stronger effect”.
The team says their results prove that Floquet’s effects can be achieved under such conditions and can be reliably generated using a more powerful means (excitations, in this case) than only light can provide.
This paves the way for the use of these capabilities in a range of applications that could aid the development of useful quantum materials and devices.
Second David Baconco-first author of the study, the discovery “opened the door to applied Floquet physics“, an achievement that is “very exciting, given its strong potential to directly create and manipulate quantum materials”.
“We don’t have the recipe for this yet.“, added Bacon, “but now we have the required spectral signature for the first practical steps”.
