Using light to create phase transitions in quantum materials is a new method for engineering new properties such as superconductivity or nanoscale topological defects, but visualizing the growth of these phases in solids is challenging due to the wide variety of spatial and temporal scales involved in the process. . Scientists have described light-induced phase transitions in quantum materials using nanoscale dynamics, but creating real space images has proven difficult, so no one has seen them yet.

In a new study published in the journal Nature PhysicsICFO researchers Allan S. Johnson and Daniel Pérez-Salinas, led by former ICFO Professor Simon Wall, Aarhus University, Sogang University, Vanderbilt University, Max Born Institute, Diamond Light Source, ALBA Synchrotron, Utrecht University and Accelerator Laboratory Pohang in vanadium oxide (VO2) introduced a new imaging method that allows capturing the light-induced phase transition with high spatial and temporal resolution.

The new technique implemented by the researchers is based on coherent X-ray hyperspectral imaging using a free electron laser, which allows them to visualize and better understand the nanoscale insulator-metal phase transition in this well-known quantum material.

Nanoscale X-ray spectroscopy of transition phases. Ultra-fast video of a light-induced phase transition in VO2 at the nanoscale, where insulating fields of several hundred nanometers transition into a metallic phase when a powerful laser pulse excites them at t=0. Image credit: ICFO/Allan Johnson

The VO2 crystal has been widely used to study light-induced phase transitions. It was the first material whose solid-state transition was tracked by time-dependent X-ray diffraction and whose electronic nature was studied for the first time using ultrafast X-ray absorption techniques. At room temperature, VO2 is in the insulating phase. However, if light is given to the material, it is possible to break the dimers of the vanadium ion pairs and transition from the insulating phase to the metallic phase.

The study authors prepared fine VO samples in their experiments. ₂ with a gold mask to determine the field of view. The samples were then taken to a free-electron X-ray laser at the Pohang Accelerator Laboratory, where an optical laser pulse induced a phase transition followed by examination with an ultrafast X-ray laser pulse. The camera captured the scattering X-rays and the coherent scattering patterns were converted into images using two different approaches: Fourier transform holography (FTH) and coherent diffraction imaging (CDI). Images were taken at a range of time delay and X-ray wavelengths to produce a movie of the process with 150 femtosecond time resolution and 50 nm spatial resolution, as well as full hyperspectral information.

The surprising role of pressure

The new methodology allowed the researchers to better understand the dynamics of the phase transition in VO.₂. They found that pressure plays a much larger role in light-induced phase transitions than previously expected or predicted.

“We found that the transitional phases weren’t as exotic as people thought! Rather than a truly out-of-equilibrium phase, we saw what was wrong with the fact that the ultra-fast transition inherently causes massive internal pressures in the sample that are millions of times greater than atmospheric pressure. This pressure changes the properties of the material and takes time to relax, gives the impression that a temporary phase is taking place,” says ICFO PhD student Allan Johnson. “Using our imaging method, we found that, at least in this case, there was no connection between the picosecond dynamics we saw and any nanoscale changes or exotic phases. So it looks like some of these results will need to be revised.”

The use of hyperspectral imaging was crucial to determine the role of pressure in the process. “By combining imaging and spectroscopy into one great image, we can gain a lot more information that allows us to really see detailed features and decipher exactly where they come from,” Johnson continues. “It was important to look at each part of our crystal and determine whether it was a normal or exotic non-equilibrium phase, and with this information we were able to determine that all regions of our crystal were the same during phase transitions.

complex studies

One of the main problems the researchers faced during the experiment was making sure that the VO was the crystalline sample. ₂ returned to the first stage every time and after laser illumination. To ensure this happens, they conducted preliminary experiments in synchrotrons, where they took several crystal samples and repeatedly illuminated them with a laser to test their ability to restore themselves to their original state.

The second challenge was gaining access to the free electron X-ray laser, large research facilities where time windows for experiments are very competitive and in demand as there are only a few in the world. “Due to the restrictions from COVID-19, we had to stay in quarantine for two weeks in South Korea before we only had five days to start the trial, so it was a busy time,” Johnson recalls.

Although the researchers describe this work as fundamental research, the technique’s potential applications could be diverse as they “can look at polarons moving through catalytic materials, attempt to image superconductivity itself, and even help us understand new nanotechnology.” nanoscale devices.” “,” Johnson concludes.