Discovery of the "cradle" for photoinduced phase transitions

The DYNACOM (Dynamical Control of Materials) research team, a CNRS International Research Laboratory (France), led by Professor Shin-ichi Ohkoshi at the Graduate School of Science of the University of Tokyo, Professor Eric Collet and Researcher Marco Cammarata at the University of Rennes, and Professor Hiroko Tokoro at the Faculty of Pure and Applied Sciences of the University of Tsukuba, collaborated with the SLAC National Accelerator Laboratory (US) and the European Synchrotron Radiation Facility to develop the world's first ultrafast simultaneous monitoring system for X-ray absorption and X-ray diffraction under light irradiation using an X-ray free-electron laser. The research team successfully conducted an experiment to monitor photoinduced phase transitions using this system.

The material used in the experiment is a type of Rubidium-Manganese-Iron Prussian Blue (RbMn[Fe(CN)₆]), a compound first reported by Ohkoshi et al. in 2002, in which a portion of Mn in the compound has been replaced with Co (Cobalt).

The photoinduced charge-transfer phase transition of this material is known to cause electrons to move between metal ions upon light irradiation at room temperature, leading to irreversible changes in magnetism and color.

In this study, they observed photoinduced phase transition using the measurement system they developed. They found that inverse Jahn-Teller distortion (flattened octahedral structure) of Mn³⁺ occurs in only 50 femtoseconds after light irradiation, followed by a charge transfer from Fe²⁺ to Mn³⁺ in 190 femtoseconds, and a charge-transfer polaron (a quasiparticle state in which the charge transfer state generated by electron transfer and the surrounding local lattice distortion behave together) in 2.1 picoseconds.

They revealed a series of processes, where the charge-transfer polaron acts as a "cradle," and exerts internal pressure within the crystal, causing a chain reaction of electron transfers that ultimately leads to a phase transition in the entire crystal. They elucidated for the first time a series of processes from the quantum mechanical process of photo-excitation to the phase transition along a time axis.

The research team expects that this will provide important insights into the fundamental principle of the control of physical properties by light and will contribute significantly to the establishment of material design guidelines for applications such as optical memories, writable optical switching devices, and photonic and quantum devices.

Light is a powerful external stimulus that can control the color, magnetism, electrical conductivity, and other physical properties of materials on an extremely short time scale of less than one trillionth of a second and is attracting attention as a fundamental technology for next-generation optical devices and quantum functional materials.

In particular, "photoinduced phase transitions," in which the state of an entire material switches upon light irradiation, is an important phenomenon that is expected to be applied to optical memories and optical switching devices.

However, the detailed mechanism of how light-induced quantum mechanical electronic excitations evolve into crystal structure changes and thermodynamic phase transitions from the femtosecond order has not been clarified until now.

The reason is that it was extremely difficult to simultaneously capture the multi-step progression of ultrafast phenomena at the electron-atomic level and concerted structural changes of the entire crystal in terms of both time and space using a single experimental technique.

The present result is noteworthy as a groundbreaking achievement that overcame such difficulties and succeeded in simultaneous observation for the first time.

The new concept that a charge-transfer polaron "cradle" generated by light functions as an internal pressure source to drive phase transition in the entire crystal of a material is a previously unexplained finding and is an important achievement in understanding the fundamental principle of the light-induced phase transition phenomenon. The study results were published online in Nature Materials on March 19.

Journal Information
Publication: Nature Materials
Title: Multiscale phase nucleation driven by photoinduced polarons in a volume-changing material
DOI: 10.1038/s41563-026-02521-w