Propagation of insulator-to-metal transition driven by photoinduced strain waves in a Mott material | Nature Physics

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Ultrafast photoexcitation can generate internal compressive stress in Mott insulators that lead to strain waves from free surfaces. These photoinduced elastic waves can trigger phase transitions in materials. However, a comprehensive physical picture of the phase transformation dynamics that includes acoustic-scale propagation has not yet been developed. Here we demonstrate that such a strain-wave mechanism drives the ultrafast insulator-to-metal phase transition in granular thin films of the Mott material V2O3. Our time-resolved optical reflectivity and X-ray diffraction measurements reveal that an inverse ferroelastic shear occurs before the insulator-to-metal transition, which propagates in the wake of a compressive strain wave. These dynamics are governed by the domain size and film thickness, respectively. Our results clarify the morphological conditions for the ultrafast phase transition that is favoured in granular thin films and hindered in single crystals. The resulting physical picture sheds light on the ultrafast phase transitions in quantum materials and future devices based on Mott insulators.

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Datasets generated during the current study are available from the corresponding authors on reasonable request. Source data are provided with this paper.

Codes generated during the current study are available from the corresponding authors on reasonable request.

A Correction to this paper has been published: https://doi.org/10.1038/s41567-024-02699-3

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M. Lorenc and E.J. acknowledge the Agence Nationale de la Recherche for financial support under grant nos. ANR-16-CE30-0018 (‘Elastica’), ANR-19-CE29-0018 (‘Multicross’) and ANR-23-CE30-0027 (‘FASTRAIN’). S.I. and H.K. are supported by JST CREST (JPMJCR1901). S.I. also acknowledges MEXT Q-LEAP (JPMXS0118067426) for their financial support. L.C. acknowledges the Région Pays de la Loire for its financial support through the Mott-IA project. J.L. acknowledges the support from VR (Grant No. 2023-05136). E.P. acknowledges support from the CNRS and the French Agence Nationale de la Recherche (ANR), under grant no. ANR-22-CPJ2-0053-01. This study is co-funded by the European Union (ERC, PhotoDefect, 101076203). Views and opinions expressed are, however, those of the author(s) only and do not necessarily reflect those of the European Union or the European Research Council. Neither the European Union nor the granting authority can be held responsible for them. We acknowledge MAX IV Laboratory for time on the FemtoMAX beamline under proposal nos. 20210400 and 20221240. Research conducted at MAX IV, a Swedish national user facility, is supported by the Swedish Research Council under contract no. 2018-07152, the Swedish Governmental Agency for Innovation Systems under contract no. 2018-04969 and Formas under contract no. 2019-02496. We acknowledge the ESRF for provision of the synchrotron radiation facilities and use of beamline ID09 under proposal nos. hc3747, ma5554 and ma5744, and thank R. Garlet, B. Richer and Y. Watier for their help in setting up the cryo-cooling system. We thank A. Merabet for preparing the FIB lamella extracted from one of the thin films studied in this paper. Some measurements were performed using the IMN’s characterization platform, PLASSMAT, Nantes, France. We thank financial support from the NEXT initiative through national funding by the French National Research Agency (ANR) under the Programme d’investissements d’Avenir (reference no. ANR-16-IDEX-0007).

Department of Physics, Tohoku University, Sendai, Japan

Tatsuya Amano, Hirotake Itoh, Yohei Kawakami & Shinichiro Iwai

Nantes Université, CNRS, Institut des Matériaux de Nantes Jean Rouxel, IMN, Nantes, France

Danylo Babich, Julio Guzman-Brambila, Julien Tranchant, Benoit Corraze, Laurent Cario, Mohammad Dolatabadi & Etienne Janod

Univ Rennes, CNRS, IPR (Institut de Physique de Rennes)—UMR 6251, Rennes, France

Ritwika Mandal, Julio Guzman-Brambila, Alix Volte, Elzbieta Trzop, Marina Servol, Ernest Pastor, Maryam Alashoor, Hervé Cailleau & Maciej Lorenc

ESRF—The European Synchrotron Radiation Facility, Grenoble, France

Julio Guzman-Brambila, Alix Volte, Céline Mariette, Matteo Levantino & Mikhail Kozhaev

MAX IV Laboratory, Lund University, Lund, Sweden

Alix Volte, Jörgen Larsson, Andrius Jurgilaitis, Van-Thai Pham, David Kroon, John Carl Ekström & Byungnam Ahn

CNRS, Univ Rennes, DYNACOM (Dynamical Control of Materials Laboratory)—IRL2015, The University of Tokyo, Tokyo, Japan

Elzbieta Trzop, Marina Servol, Ernest Pastor, Julien Tranchant, Benoit Corraze, Laurent Cario, Vinh Ta Phuoc, Hirotake Itoh, Yohei Kawakami, Hervé Cailleau, Maciej Lorenc, Shinichiro Iwai & Etienne Janod

Department of Physics, Lund University, Lund, Sweden

Jörgen Larsson

GREMAN UMR 7347 CNRS, Université de Tours, Tours, France

Vinh Ta Phuoc, Rodolphe Sopracase & Mathieu Guillon

Department of Applied Physics, Graduate School of Engineering, Nagoya University, Nagoya, Japan

Yuto Nakamura & Hideo Kishida

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H.C., M. Lorenc, S.I. and E.J. coordinated the project. T.A., H.I., Y.K. and S.I. performed and analysed the tr-reflectivity experiments. Y.N. and H.K. measured the steady-state reflectivity on thin films. V.T.P. and R.S. built the setup for high-pressure reflectivity, and measured the single crystal with help from M.G., B.C. and D.B. A.J., V.-T.P., D.K., J.C.E., B.A. and J.L. developed the FemtoMAX setup for tr-XRD. R.M., J.G.-B., A.V., E.T., M.S., E.P., M.A., E.J. and M. Lorenc proposed the experiment at FemtoMAX and performed it with the help of local contacts. C.M., M. Levantino and M.K. designed the ESRF tr-XRD experiment. J.G.-B., R.M., E.T., M.S., B.C. and E.J. performed the experiment at the ESRF. R.M., J.G.-B. and A.V. analysed the tr-XRD data. J.G.-B. established the correction function for sample displacement adapted to the geometry of the FemtoMAX beamline. D.B., J.T., B.C., E.J. and L.C. prepared and characterized the thin films and single crystals. M.D. performed the STEM study. H.C., M. Lorenc, S.I., E.J., B.C. and L.C. analysed and discussed all the results. H.C., M. Lorenc, S.I. and E.J. wrote the manuscript with important contributions from C.M. and E.P., critical reading from J.L., A.J. and V.-T.P., and help from all co-authors.

Correspondence to Hervé Cailleau, Maciej Lorenc, Shinichiro Iwai or Etienne Janod.

The authors declare no competing interests.

Nature Physics thanks Michael Sentef and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.

Publisher’s note Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

(a) Steady-state X-Ray diffraction (XRD) pattern measured in a Bragg-Brentano geometry on the 270 nm thick film used for time-resolved XRD. The average crystallite size at room temperature, estimated by the Scherrer formula, is 40 ± 5 nm. Compared to theoretical intensities (red bars), the enhancement of (hk0) Bragg peaks with respect to other (hkl) indicates a preferential orientation with the c-axis of the grains mostly oriented parallel to the substrate surface (see Extended Data Fig. 9). (b): Scanning Electron Microscopy (SEM) image of a similar film prepared together with the previous one, but deposited on a SiO2/Si substrate, and allowing a reliable estimate of the thickness. (c) TEM image of the film shown in (b) measured on the S/TEM Themis Z G3 electron microscope, (Thermo Fisher Scientific). This TEM sample was prepared by Focus Ion Beam (FIB) in a ZEISS Crossbeam 550 L scanning electron microscope. The white circle indicates the typical beam size used for collect electron diffraction images shown in (d). (d) Representative electron diffraction image collected close to the free surface. In (c), the free surface was covered by Platinum during FIB lamella preparation process. The electron diffraction images show exclusively Bragg peaks diffracted by multiple grains hit by the electron beam and absolutely no trace of amorphous phase. This rules out the possibility that the 9 ps delay for volume change shown in Fig. 4b is related to an thick amorphous phase at the surface of the V2O3 films. (e) XRD pattern of the 106 nm thick film used for tr-XRD both at ID09 and FemtoMAX beamlines. Zoom on the (024)H Bragg peak in PM phase, (022)M peak below the PM to AFI transition temperature, measured under steady state at ID09. A clear broadening of this Bragg peak appears in the monoclinic phase. Simulation of the (022)M Bragg peak leads to an estimate of the ferroelastic monoclinic domains size of 20 ± 5 nm (→ average radius = 10 ± 2.5 nm). This diffracting domain is half that at 300 K (see simulation of Bragg peak with a domain size of 40 nm). (f) SEM images (side and top views) of a 116 nm film prepared together with the one used for tr-reflectivity, but deposited on a SiO2/Si substrate. (g) Temperature dependence of resistivity (single crystal) and resistance (thin films) of the samples used in this study. The large amplitude of the resistivity jump at the transition and the high transition temperature (160 K upon heating) attest the quality and stoichiometry of the single crystal. Crystals from the same batch are used both for steady state and time-resolved reflectivity. The films show a jump of similar amplitude at the transition, with a slightly reduced transition temperature (147 K upon heating on the 116 nm thick film) as usually observed in V2O3 films. The absence of hysteresis below 120 K (tr-reflectivity thin film) and 107 K (tr-XRD) ensures the absence of phase coexistence in the different time-resolved studies performed in this work.

Source data

In Fig. 2 of the main article, we unveil a weaker photoinduced response of the single crystal with respect to thin film. In order to check if it results from an anisotropic response of V2O3, we have performed polarization dependent tr-reflectivity after a proper orientation of a V2O3 single crystal by XRD and a polishing to reveal a surface suitable for tr-reflectivitty. (a) optical image of the V2O3 crystal used for tr-reflectivity, with a surface defined by the the directions ${\bf{c}}$ and $2{\bf{a}}+{\bf{b}}$. (b) time-resolved reflectivity performed with polarization directions of the pump and the probe aligned along the ${\bf{c}}$ and $2{\bf{a}}+{\bf{b}}$ directions after photoexcitation at 4 mJ.cm−2 at 120 K. Overall, the polarization effect is very weak and is hence not the cause of the weaker photoinduced response of the crystal shown in Fig. 2 of the main text.

Source data

The dashed lines correspond to XRD patterns without (blue) and with (red) photoexcitation of 10.5 mJ/cm2 (delay 400 ps) converting 100 % of the volume into a metallic hexagonal phase. These XRD patterns were refined with Rietveld method (blue and red lines), allowing to extract the monoclinic cell parameters (space group 15, I2/a). We found for the unexcited initial phase: aM = 7.321(3) Å, bM = 4.988(1) Å, cM = 5.5325(28) Å, β = 96.96(2)°, VM = 200.54(15) Å3 and aM = 7.364(7) Å, bM = 4.953(2) Å, cM = 5.464(7) Å, β = 97.63(4)°, VM = 197.54(15) Å3 for the photo-excited phase 100 % converted into the hexagonal phase (delay = 400 ps, fluence = 10.5 mJ/cm2). As the photo-conversion towards the hexagonal phase is complete, the photo-induced XRD pattern can equally be refined with a hexagonal unit cell, yielding aH = 4.9505(6) Å, cH = 13.966(6) Å and hence 2/3*VH = 197.6 (2) Å3. Overall, the photoinduced volume change ΔV/V determined with this refinement procedure amounts to −1.50 ± 0.12 %. However this refinement procedure is tedious and requires advanced corrections of sample to detector distance drifts. Therefore we used another strategy to estimate the volume change at any time delay and fluence, based on the simple Q shift of the (110)H Bragg peak. This is justified since: (1) the photoinduced change of the pseudo-hexagonal cH parameter (determined as ${c}_{H}=2\sqrt{{c}_{M}^{2}+\frac{1}{4}{a}_{M}^{2}-{a}_{M}{c}_{M}\,\cos \beta }$) is as low as ∆cH/cH = +0.16 %, demonstrating that most of the volume change result from the large decrease of the pseudo-hexagonal aH parameter, (2) the refinement demonstrates that the two monoclinic Bragg peaks deriving from the (110)H, that is the (020)M and (211)M, are almost superimposed in the monoclinic phase (${Q}_{{(211)}_{M}-}{Q}_{{(020)}_{M}} < 0.004 \, {\text{\AA }}^{-1}$). As a consequence, the photoinduced shift ΔQ of the (110)H Bragg peak directly gives the relative change of the pseudo-hexagonal aH parameter ΔQ/Q = -ΔaH/aH and the relative volume change ΔVH/VH = 2 ΔaH/aH + ΔcH/cH ≈ 2 ΔaH/aH. In the example shown in Extended Data Fig. 3, this method yields ΔVH/VH = −1.48 ± 0.10 %, in perfect agreement with the refinement method. All the estimate of the volume changes presented in Figs. 3 and 4 are obtained using this strategy.

Source data

(a) Evolution of transient reflectivity with fluence measured on the same 116 nm film as in Fig. 3a, the same conditions (pump 0.89 eV, probe at 0.25 eV), but at 100 K instead of 10 K. (b) Fluence dependency of the volume change measured at 87 K on the 106 nm film (see Methods), as deduced from the integration of the differential XRD pattern around the (110)H Bragg peak for a pump-probe delay of 200 ps (c) Same as (b) for the symmetry change, deduced from the integration of differential XRD around the (116)H peak. These three plots unveil a clear threshold fluence around 1 mJ/cm2 above which the electronic part (a) and the structural components (b and c) of the insulator-to-metal transition are initiated. The threshold is reduced from 2 mJ/cm2 at 10 K (see Fig. 3) to 1 mJ/cm2 around 85–100 K.

Source data

(Left) Evolution of the (110)H Bragg peak for pump-probe delays shorter (top) and longer (bottom) than 9 ps. (Right) Same for the (116)H Bragg peak, split into three peaks (013)M, (−222)M and (−411)M in the initial monoclinic state. Vertical bars indicate the Bragg peak positions from the refinement of XRD pattern of the 116 nm film shown in Extended Data Fig. 3, in black monoclinic, in red hexagonal phase. These panels evidence that the monoclinic to hexagonal transition is completed in less than 4 ps (see top right part), whereas volume change occurs slower after a onset shift of roughly 9 ps (left part). The fitting procedure of the individual peaks is described in the Methods section.The key results shown in Fig. 4, that is the temporal evolution of the symmetry and volume changes, are extracted from the fitted FWHM of the (116)H and shift of the (110)H peaks, respectively (see complementary discussion in Extended Data Fig. 3).

Source data

(a) Temporal evolution of the unit cell volume change evaluated by the Q-shift of the (110)H Bragg peak, as used in Fig. 4 of the article, and by the whole powder pattern refinement. Both methods lead to the same conclusion, that is that the average volume is unchanged during the first 9 ps, before decreasing during 16 ps (time required for a strain wave propagating at the speed of sound to travel though the film from the surface to the substrate). (b) Time dependence of the αH and γH angles of the pseudo-hexagonal unit cell (see description below). The evolution of αH and γH is similar with the time dependence of (116)H’s FWHM displayed in Fig. 4a, with a very fast evolution during the first ps and a plateau reached after less than 4 ps. Moreover, the values of αH and γH at the plateau are extremely close to those expected for a $R\bar{3}c$ symmetry, that is 90° and 120° respectively. Inset: temporal evolution of Rwp showing that the quality of the refinement is essentially delay-independent.

Source data

Structural evidence for a long plateau of time-independent structural change between 100 ps and 3 ns after initial photoexcitation, followed by a complete relaxation in the 3–100 ns range in the thin film (106 nm) measured at ID09 at 87 K and for a fluence (10.5 mJ/cm2) inducing a full AFI-to-PM transformation. (a) and (b) Temporal evolution of γH and αH, angles of the pseudo-hexagonal unit cell (see definition in Extended Data Fig. 6) evidencing the photoinduced symmetry change to the hexagonal symmetry (γH ≈ 120° and αH ≈ 90°) lasting up to 3 ns, and its relaxation towards the monoclinic symmetry (γH ≈ 119.65° ≠ 120° and αH ≈ 91.1° ≠ 90°). (c) Temporal evolution of the relative volume change, showing a plateau at −1.5 % up to 3 ns, and then relaxing to its initial value between 3 and 100 ns.

Source data

Time-resolved reflectivity measured at low pump laser fluence (0.1 mJ/cm2, photon energy = 0.89 eV), showing electronic recombination time in the nanosecond range.

Source data

Typical 2D detector images measured a) at ID09 on the 2D Rayonix CCD detector and d) at Femtomax on the 2D pilatus photon counting detector. b) The unfolded ID09 image shows the available azimuthal range. c), e) and f) Intensity along the (110)H Bragg peak for the thin film measured at ID09 (106 nm), and for the thick (270 nm) and thin (106 nm) film measured at Femtomax, respectively. The intensity is integrated over 1 deg and 1.5 deg for Femtomax and ID09 data respectively. These profiles were refined using a pseudo Voigt function with linear background: $I=(1-\Gamma ).A.exp(\frac{{(x-{x}_{0})}^{2}}{2{\sigma }^{2}})+\Gamma .A.\frac{{\sigma }^{2}}{{(x-{x}_{0})}^{2}+{\sigma }^{2}}+{y}_{0}+{y}_{1}.x$ σwas found equal to 16 degrees and 18 degrees for the thin films (for ID09 c) and Femtomax f) profiles respectively) and to 30 degrees for the thick film (Femtomax profile e)).

Source data

Steady-state and photoinduced reflectivity data on V2O3 thin film and single crystal. Steady-state and photoexcited XRD patterns on V2O3 thin films.

Steady-state and photoexcited XRD patterns on V2O3 thin films: effect of laser fluence.

Photoinduced reflectivity and XRD: time scans.

Source data for Extended Data Fig. 1.

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Source data for Extended Data Fig. 3.

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Source data for Extended Data Fig. 9.

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Amano, T., Babich, D., Mandal, R. et al. Propagation of insulator-to-metal transition driven by photoinduced strain waves in a Mott material. Nat. Phys. (2024). https://doi.org/10.1038/s41567-024-02628-4

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Received: 06 August 2023

Accepted: 05 August 2024

Published: 17 September 2024

DOI: https://doi.org/10.1038/s41567-024-02628-4

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