The crystal VO2 has actually been commonly used in studying light-induced stage transitions. If light is applied to the product, it is possible to break the dimers of the vanadium ion pairs and drive the transition from an insulating to a metallic stage.
The samples were taken to the X-ray Free Electron Laser facility at the Pohang Accelerator Laboratory, where an optical laser pulse caused the transient stage, before being penetrated by an ultrafast X-ray laser pulse. Rather of a really non-equilibrium stage, what we saw was that we had actually been misinformed by the reality that the ultrafast transition inherently leads to giant internal pressures in the sample millions of times greater than climatic. “This was important to look at each part of our crystal and figure out whether it was a normal or an unique out-of-equilibrium phase-and with this information we were able to figure out that during the stage shifts all the regions of our crystal were the same, except for the pressure”.
The brand-new strategy implemented by the researchers is based on coherent X-ray hyperspectral imaging at a complimentary electron laser, which has enabled them to imagine and better comprehend, at the nanoscale, the insulator-to-metal stage transition in this really popular quantum material.
Nanoscale X-Ray spectroscopy of transient phases An ultrafast video of the photoinduced phase transition in VO2 at the nanoscale, in which insulating domains several hundred nanometers in size are switched to the metal phase when a strong laser pulse excites them at t= 0. Credit: ICFO/ Allan Johnson
The crystal VO2 has been commonly utilized in studying light-induced stage shifts. It was the very first material to have its solid-solid shift tracked by time-resolved X-ray diffraction and its electronic nature was studied by utilizing for the first time ultrafast X-ray absorption methods. At space temperature, VO2 is in the insulating phase. Nevertheless, if light is used to the material, it is possible to break the dimers of the vanadium ion pairs and drive the transition from an insulating to a metal phase.
The samples were taken to the X-ray Free Electron Laser center at the Pohang Accelerator Laboratory, where an optical laser pulse caused the short-term phase, prior to being probed by an ultrafast X-ray laser pulse. Images were taken at a range of time hold-ups and X-ray wavelengths to build up a film of the process with 150 femtosecond time resolution and 50 nm spatial resolution, but likewise with full hyperspectral details.
The surprising role of the pressure
The new method allowed the researchers to better comprehend the characteristics of the stage transition in VO2. They discovered that pressure plays a much larger role in light-induced phase transitions than previously expected or assumed.
Rather of a genuinely non-equilibrium phase, what we saw was that we had actually been deceived by the fact that the ultrafast shift fundamentally leads to huge internal pressures in the sample millions of times greater than atmospheric. “Using our imaging approach, we saw that, at least in this case, there was no link between the picosecond dynamics that we did see and any nanoscale changes or exotics phases.
To identify the role played by the pressure while doing so, it was essential to utilize the hyperspectral image. “By combining imaging and spectroscopy into one great image, we are able to recover much more details that permits us to in fact see comprehensive features and figure out precisely where they come from,” continues Johnson. “This was vital to look at each part of our crystal and figure out whether it was a typical or an exotic out-of-equilibrium phase-and with this details we had the ability to figure out that during the stage transitions all the regions of our crystal were the same, other than for the pressure”.
Challenging research
Among the main challenges the scientists dealt with during the experiment was to ensure that the crystal sample of VO2 returned to its original starting phase each time and after being illuminated by the laser. To guarantee that this would take place, they conducted preliminary experiments at synchrotrons where they took a number of crystal samples and consistently shone the laser on them to evaluate their capability to recuperate back to their initial state.
The second difficulty resided in having access to an X-Ray totally free electron laser, big research centers where the time windows to carry out the experiments are really competitive and in need because there are just a few worldwide. “We had to spend 2 weeks in quarantine in South Korea due to the COVID-19 limitations before we got our one shot of simply 5 days to make the experiment work, so that was an extreme time” Johnson recalls.
Although the scientists explain the present work as essential research, the prospective applications of this method could be diverse, given that they might “take a look at polarons moving inside catalytic materials, try imaging superconductivity itself, or perhaps help us understand unique nanotechnologies by viewing and imaging inside nanoscale devices” concludes Johnson.
Reference: “Ultrafast X-ray imaging of the light-induced stage transition in VO2” by Allan S. Johnson, Daniel Perez-Salinas, Khalid M. Siddiqui, Sungwon Kim, Sungwook Choi, Klara Volckaert, Paulina E. Majchrzak, Søren Ulstrup, Naman Agarwal, Kent Hallman, Richard F. Haglund Jr, Christian M. Günther, Bastian Pfau, Stefan Eisebitt, Dirk Backes, Francesco Maccherozzi, Ann Fitzpatrick, Sarnjeet S. Dhesi, Pierluigi Gargiani, Manuel Valvidares, Nongnuch Artrith, Frank de Groot, Hyeongi Choi, Dogeun Jang, Abhishek Katoch, Soonnam Kwon, Sang Han Park, Hyunjung Kim, and Simon E. Wall, 22 December 2022, Nature Physics.DOI: 10.1038/ s41567-022-01848-w.
A crystalline lattice melting, creatively represented here as a snowflake, is superimposed upon its coherent X-ray scattering pattern. Credit: ICFO/ Patricia Bondia
Utilizing light to produce transient stages in quantum materials is an unique method for engineering new residential or commercial properties like superconductivity or nanoscale topological defects, nevertheless, envisioning the growth of these phases in solids is challenging due to the vast array of spatial and time scales included in the procedure.
Researchers have actually described light-induced phase transitions in quantum materials through nanoscale dynamics, however producing real area images has shown hard, leading to no one having actually seen them.
In the brand-new study published in Nature Physics, ICFO scientists Allan S. Johnson and Daniel Pérez-Salinas, led by previous ICFO Prof. Simon Wall, in partnership with colleagues from Aarhus University, Sogang University, Vanderbilt University, the Max Born Institute, the Diamond Light Source, ALBA Synchrotron, Utrecht University, and the Pohang Accelerator Laboratory, have actually originated a new imaging approach that enables the capture of the light-induced stage transition in vanadium oxide (VO2) with high spatial and temporal resolution.