Shinichi Okoshi (Professor, Department of Chemistry)

Hiroko Tokoro (Professor, Department of Pure and Applied Sciences, University of Tsukuba)

Key Points of the Presentation

• We have successfully observed that picosecond (ps)-scale strain wave propagation proceeds while inducing a phase transition in a solid.
• The spatio-temporal dynamics of the photo-induced metal-semiconductor transition in lambda-type-titanium pentoxide ( _λ-Ti3O5 ) at room temperature was revealed at the Swiss X-ray Free Electron Laser Facility (Swiss-FEL).
• This is the first pilot experimental data of time-resolved X-ray powder diffraction at the large facility Swiss-FEL, which is strongly promoted by the EU as a priority issue. The structural change was confirmed in less than 500 femtoseconds, which is the world's highest time resolution.

Announcement Summary

Professor Shinichi Okoshi of the Graduate School of Science, The University of Tokyo, and Professor Yuko Tokoro of the University of Tsukuba, in collaboration with Dr. Celine Mariette, Dr. Maciej Rolland, and Dr. Marco Camarata of the Department of Physics, University of Rennes, France, as part of an international collaboration at the IM-LED ^{(Note 1)}, CNRS International Joint Laboratory, France, We are promoting research on the photo-induced phase transition of lambda-type titanium pentoxide ( _λ-Ti3O5)^{(Note 2)}, which is the only metal oxide that exhibits a photo-induced phase transition at room temperature. Ultrafast X-ray powder diffraction experiments (time resolution: 500 fs) at ^the Swiss X-ray Free Electron Laser (Swiss-FEL) ^{(Note 3)} have revealed that the structure of _{Ti3O5 crystal} is deformed in 500 femtoseconds (fs) by light irradiation and that the phase transition is induced by strain waves propagating in the crystal in picosecond order _{from the Ti3O5 surface} where light is irradiated. The phase transition is induced by strain waves that propagate from the Ti3O5 surface to the crystal in picoseconds. Elastic body model analysis revealed that the phase transition from _beta-Ti3O5^{(Note 4)}_{to λ-Ti3O5} occurs within 16 ps (picoseconds) as the propagating "strain wave" progresses, an order of magnitude faster than the phase transition (~100 nanoseconds) induced by thermal diffusion This is an order of magnitude faster than the thermal diffusion phase transition (~100 nanoseconds). This is the first observation of such a phase transition phenomenon caused by strain wave propagation. The phase transition using strain waves as a mechanism is likely to be applicable to various other solid materials as well.

This study is the first valuable experimental data observed as the first time-resolved X-ray powder diffraction pilot experiment at the EU large facility Swiss-FEL, and demonstrates that the latest X-ray free electron laser (XFEL) light source can be used to investigate the propagation of atomic motion and lattice distortion in real time on the femtosecond scale. The data are valuable experimental data observed in the field.

Announcement

Research on optical phase transitions in solids has attracted academic and industrial attention in terms of optical memory and optical switching materials, etc. In 2010, Professor Okoshi and his colleagues discovered a new type of metal oxide, lambda-type-tri-titanium pentoxide ( _λ-Ti3O5 (λ-Ti3O5), a new metal oxide capable of photoswitching (write and erase) at room temperature. _λ-Ti3O5 is composed only of the commonplace metal ions titanium and oxygen ions. In addition to the photo-induced phase transition phenomenon, current-induced phase transition and long-term thermal storage performance have also been found, and it is expected to be developed for industrial applications (Figure 1).

Figure 1: Lambda-type titanium pentoxide, a metal oxide that can be switched (written and erased) by light irradiation at room temperature.

In this research project, the crystal structure change of _Ti3O5 immediately after light irradiation was measured by femtosecond time-resolved X-ray diffraction experiments at the Swiss X-ray Free Electron Laser Facility (Figure 2).

Figure 2: (a) Schematic of time-resolved X-ray powder diffraction at Swiss-FEL. (b) _{Ti3O5 sample} used for the measurements. (c) Observed Debye-Schuller rings. (d) Example of time variation of diffraction intensity. (e) Light-induced phase transition from _{β-Ti3O5to λ-Ti3O5}. (f) Light-induced change of Debye-Schuller rings.

Femtosecond laser irradiation causes an instantaneous change in the position of the titanium ions constituting the valence band, resulting in a partial volume change, and the observed changes in lattice volume and phase fraction over time, including inside the crystal not illuminated by light, show that the lattice volume of _{λ-Ti3O5andβ-Ti3O5} and microstrain rate and phase fraction both increase linearly from 0 to 16 ps (Figure 3 left) ^{(Note 5} ). This suggests that the increase in lattice volume of _λ-Ti3O5 and the transition from _β-Ti3O5 to _λ-Ti3O5 occur simultaneously, suggesting that strain propagation (strain waves) is involved in the phase transition. Analysis using an elastic body model showed that the results reproduced the lattice deformation well (Fig. 3, right). It takes 16 ps for the acoustic strain wavefront to reach 100 nm at the crystal interface. This coincides with the volume minimum of _β-Ti3O5, and is thought to be the result of compression by the area transformed to _λ-Ti3O5, which has a larger volume.

Figure 3: (a) Time dependence of lattice volume change (dark orange) and microstrain parameters (light orange) of _λ-Ti3O5. (b) Time dependence of lattice volume change of _β-Ti3O5 (blue) and phase fraction change of _λ-Ti3O5 (red). (c) Simulated time dependence of lattice volume change (dark orange) and microstrain parameter (light orange) of _λ-Ti3O5 using elastic body model. (d) Simulations using the elastic body model for the time dependence of the lattice volume change of β _-Ti3O5 (blue) and the phase fraction change of _λ-Ti3O5 (red).

These results suggest that the switching of _{Ti3O5 nanocrystals} occurs on the picosecond scale simultaneously with the propagating strain wavefront and is an order of magnitude faster than the phase transition by thermal diffusion (~100 ns). This result suggests that the optical phase transition in _Ti3O5 is a coherent propagation of the transition, in contrast to the random, thermally induced growth. The changes observed in this study are linear and markedly different from conventional models of nucleation and nucleation growth, in which the time dependence of the phase fraction of the new phase often varies in a sigmoidal curve. In addition, measurements at the European Synchrotron Radiation Facility (ESRF) ^{(Note 6)} show that the phase transition due to thermal diffusion occurs on a timescale of about 100 nanoseconds, much slower than the transition due to strain waves (Figure 4).

Figure 4: Time dependence of the phase fraction ( _ΔXλ) of _λ-Ti3O5 from subpicoseconds to milliseconds. The magenta squares show data measured at the European Synchrotron Radiation Facility (ESRF) at X-ray energy E = 11.5 keV and incident angle Ψ = 0.35°. The phase fraction changes are normalized by the maximum value so that they can be directly compared with the time variation (gray triangles) observed at Swiss-FEL ( E = 6.5 keV, incident angle Ψ = 0.55°). The slow time region is shown on a logarithmic time scale.

This work was performed by the IM-LED research team at the CNRS Institute as a prelude to the first time-resolved X-ray diffraction measurements at the large Swiss-FEL facility, demonstrating that with a modern XFEL light source it is possible to study interatomic motion and lattice distortions in real time on the femtosecond scale. The study demonstrated that the latest XFEL light source can be used to study interatomic motion and lattice distortion in real time on a femtosecond scale.

Journal

Journal name	Nature Communications
Title of paper	Strain Wave Pathway to Semiconductor-to-Metal Transition Revealed by Time Resolved X-ray Powder Diffraction
Author(s)	Céline Mariette, Maciej Lorenc, Hervé Cailleau, Eric Collet, Laurent Guérin, Alix Volte, Elzbieta Trzop, Roman Bertoni, Xu Dong, Bruno Lépine, Olivier Hernandez, Etienne Janod, Laurent Cario, Vinh Ta Phuoc, Shin-ichi Ohkoshi, Hiroko Tokoro, Luc Patthey, Andrej Babic, Ivan Usov, Dmitry Ozerov, Leonardo Sala, Simon Ebner, Pirmin Boehler, Andreas Keller, Roland Oggenfuss, Thierry Zamofing, Sophie Redford, Seraphin Vetter, Rolf Follath, Pavle Juranić, Akos Schreiber, Paul Beaud, Vincent Esposito, Yunpei Deng, Gerhard Ingold, Majed Chergui, Giulia Mancini, Roman Mankowsky, Cristian Svetina, Serhane Zerdane, Aldo Mozzanica, Alexei Bosak, Michael Wulff, Matteo Levantino, Henrik Lemke, Marco Cammarata
DOI Number	10.1038/s41467-021-21316-y
Publication URL	https://www.nature.com/articles/s41467-021-21316-y

Explanation of Terms

Note 1: IM-LED (Impacting materials with light and electric fields and watching real time dynamics), CNRS International Joint Institute, France.

IM-LED is jointly managed by Professor Shinichi Okoshi of The University of Tokyo and Professor Eric Collet of the University of Rennes as coordinators. This international joint Institute (a major international joint research project between France and Japan (LIA: Laboratoire International Associé) consisting of the University of Tokyo, Kyoto University, The University of Tokyo, Tokyo Institute of Technology, and Tohoku University from the Japanese side, and the University of Rennes, Nantes University, University of Versailles, and University of Maine from the French side. The LIA aims to develop research on materials that respond to light and electric fields, and the dynamics of their response, leading to the basis for next-generation ultrafast communications and optical computers.
[French Embassy website: "Establishment of the French-Japanese Joint Institute "IM-LED" in the field of materials" ()
(https://jp.ambafrance.org/article10924) ] up

Note 2: Lambda-type titanium pentoxide ( _λ-Ti3O5 )

Lambda-type titanium pentoxide is a titanium dioxide material with a new crystal structure discovered by Professor Shinichi Okoshi et al. in 2010 [Nature Chemistry, 2, 539 (2010)], and recently a new concept of heat storage ceramics has been proposed based on the properties of this material [NatureCommunications, 6, 7037 (2015); Science Advances, 6, 5264 (2020)]. _λ-Ti3O5 has metallic properties and also exhibits a variety of phase transition phenomena such as light-induced phase transition, pressure-induced phase transition, and current-induced phase transition with _β-Ti3O5. ↑up

Note 3: Swiss X-ray Free Electron Laser Facility (Swiss-FEL )

This is a new X-ray free electron laser facility at the Paul Scherrer Institute in Switzerland, which has just started operation in 2019. The first time-resolved measurements of the time dynamics of the optical phase transition between _{λ-Ti3O5andβ-Ti3O5} have now been performed.
[SwissFEL HP: "First time resolved Pilot Experiment by SwissFEL: Semiconductor to metal transition in _Ti3O5 nanocrystals" (https://www .psi.ch/en/bernina-group/scientific-highlights/first-time-resolved-pilot-experiment-by-swissfel )] ↑

Note 4 Beta-type - titanium pentoxide ( _beta-Ti3O5 )

A brown crystalline phase of the conventionally known titanium pentoxide, which exhibits semiconducting properties. ↑up

Note 5

_λ-Ti3O5 has three titanium sites (Ti1, Ti2, and Ti3), where the valence band is composed by Ti3-Ti3 pairs and the conduction band by Ti2 empty orbitals. Observation of the structural change immediately after laser irradiation shows that the bond angles and distances around the Ti3 site change at 500 femtoseconds, indicating the structural change dynamics when the electrons in the valence band are excited by the light. ↑up

Note 6 European Synchrotron Radiation Facility (ESRF)

The European Synchrotron Radiation Facility (ESRF) is a joint research facility located in Grenoble, France, and is jointly funded by 22 countries. ↑↑