How does the insulator-to-metal transition propagate at sound speed in quantum materials?

The development of ultrafast lasers in the last decades, rewarded by the Physics Nobel Prizes in 2018 and 2023, has opened up entire new fields of exploration of matter. Recent advances in the use of ultrafast light-matter interaction offer the possibility of controlling the macroscopic properties and functionalities of matter, via pathways inaccessible to classical thermal mechanisms. This raises hope to use the switching between these out-of-equilibrium functional states in future ultrafast devices. However, a global physical picture of the mechanisms at play in such ultrafast, photo induced macroscopic phase transitions, is still lacking.

Now, an international team involving the France-Japan International Research Laboratory DYNACOM (Dynamical Control of Materials), CNRS Physics and the ESRF has taken a major step towards understanding a potentially universal, but until now underestimated, ultrafast photoinduced transition mechanism.

Their research shows that ultrafast photoexcitation of the canonical Mott insulators V2O3 generate internal stresses, which are relaxed by launching strain waves from free surfaces. They also discovered that that the complete photoinduced insulator-to-metal transition occurs only in the wake of the compressive strain wave.

Céline Mariette, scientist at ID09, with Aleksandra Chumakova on the beamline. Credits: S. Candé.

To achieve these results, the scientists came to the ESRF’s beamline ID09, where they carried out time-resolved analysis of the photoinduced structural changes at the picosecond timescale. Céline Mariette, a scientist at the ESRF and co-author of the publication, explains: “A first Breakthrough experiment was performed with the ID09 pink beam before EBS, however, the gain in beam intensity achieved with the EBS allowed us to use a monochromatic beam. This was a real game changer, since we were then able to characterise unambiguously the photo-induced metallic phase, and importantly to study the low laser excitation regimes, where the photo-induced structural changes are tiny”. The results at the ESRF were combined with additional measurements in ultrafast laser pump at Tohoku University in Japan and MAX IV in Sweden.

The findings of this work may influence other fields, in particular for quantum materials exhibiting phase transitions involving elastic deformations. It will also help assessing the ultimate performance of future innovative devices, such as hardware neural networks for artificial intelligence based on Mott insulators.

Reference:

T.Amano et al., Nature Physics (2024). doi : 10.1038/s41567-024-02628-4

Top image: Schematic representation of the mechanism of phase transition propagation at the speed of sound. From left to right: photo-excitation, precursors, and propagation on a macroscopic scale.