An international team of researchers recently demonstrated for the first time the use of an ultrafast X-ray technique (A Novel X-ray Imaging Technique) without a lens to map time changes. This new technique described in Nature Physics makes it possible to observe the exact changes of quantum matter at the nanometric scale.
Using light to produce periodic changes in quantum materials is becoming a new way to produce new properties, such as the generation of superconductivity or nanoscale topological defects. However, visualizing the growth of a new system in a solid is not easy, in part because of the large amount of space and time involved in the process.
Although in the last two decades, scientists have described the changes in light caused by the so-called nanoscale dynamics, real planetary images have not been created, so no one has seen them. .
In a new study published in Nature Physics, ICFO researchers Allan S. Johnson and Daniel Pérez-Salinas, former ICFO Professor Simon Wall, in collaboration with colleagues from Aarhus University, Sogang University, Vanderbilt University, Max Born Institute, Diamond Light Source, from ALBA Synchrotron, Utrecht University and Pohang Laboratoire Accelerator, has. pioneered a new imaging technique that allows capturing time-lapse changes in vanadium oxide (VO2) light with high resolution and long durations.
The new technique implemented by the researchers is based on hyperspectral X-ray imaging at the level of the free electron laser, which allowed them to visualize and clearly understand, at the nanometric scale, the insulator-metal transition process and this of good. know quantum matter.
VO2 crystals are widely used to study light-induced time transitions. It is the first element of a solid transition that is followed by time-resolved X-ray diffraction and its electronic nature is investigated using ultrafast X-ray absorption techniques for the first time. However, if heat is applied to the material, it is possible to break the dimers of the vanadium ion pairs and make the transition from the coating to the metal level.
In their experiment, the authors prepared a thin VO2 sample with a gold coating to define the sample area. Next, the samples were transported to the X-ray free-electron laser facility at the Pohang Accelerator Laboratory, where the laser pulse caused a short period of time, before being analyzed by the laser-ray pulse. The camera captures the scattered X-rays and converts the diffraction pattern into an image using two different methods: Fourier Transform (FTH) Holography and Coherent Diffractive Imaging (CDI). Photographs are taken at several time-delayed X-ray wavelengths to create a film of the system with 150 femtosecond temporal resolution and 50 nm spatial resolution, but also with detailed hyperspectral information.
Amazing work of pressure
The new method has allowed researchers to better understand seasonal changes in VO2. They found that stress plays a bigger role in the seasonal changes caused by light than expected or previously.
“We found that time passing is not as dramatic as people think! Instead of a non-equilibrium process, what we saw was that we were misled by the fact that the ultrafast evolution is lead to great pressure in the sample.millions of times greater than air. This pressure changes the material’s properties and takes time to relax, giving the impression that there is a passing time,” explains Allan Johnson, a postdoctoral researcher at ICFO. “Using the method our images, we found that, at least in this case, there is no correlation between the picosecond dynamics we observed and any nanoscale changes or special processes. So it seems that some of these conclusions should be investigated.
To determine the role of pressure in the system, it is necessary to use hyperspectral images. “By combining imaging and spectroscopy into one unique image, we can retrieve a wealth of information that allows us to see the full spectrum of features and understand exactly where they came from,” Johnson continued. “It is important to look at each part of our crystal and find out whether it is a normal or an unusual process – and with this information, we can determine that during the transition period, all areas of our crystal is. the same, except for pressure”.
Research is difficult
One of the main challenges the researchers faced during the experiment was to ensure that the VO2 crystal sample returned to its initial state each time the laser was illuminated. To make sure this would happen, they did the first experiment at synchrotrons where they took several crystal samples and repeatedly shone laser light on them to test their ability to return to their original state.
The second challenge is to get the X-Ray Free Electron Laser, a large research facility where the window time to conduct those competitive experiments is very demanding because there are few of them in the world. “We had to spend two weeks in isolation in South Korea because of the COVID-19 restrictions before we got one drug and only had five days for the test to work, so it was a very difficult time,” Johnson recalls.
Although the researchers describe the current work as a basic discovery, the potential applications of this method may be different, since they can “find moving polarons in to make materials, try to create superconductivity itself, or even help us understand the new nanotechnology by looking at the drawings inside. “. nanoscale devices,” Johnson concluded.
