Propagating insulator-to-metal transition in the wake of photoinduced strain waves in a Mott material

The study asks how fast and by what mechanism photoexcitation can drive a material across a phase boundary, specifically whether long-wavelength acoustic (strain) waves can coherently drive macroscopic phase transformations in correlated materials. For transitions involving volume change and/or ferroelastic shear, the ultimate time scale should be set by the propagation of strain waves (acoustic phonons) rather than by optical phonon periods. While coherent optical-phonon-driven pathways are well established, the ability of strain waves to transform a macroscopic state has been little explored. Many quantum materials, including V₂O₃, exhibit coupled isosymmetric Mott insulator-to-metal transitions (IMT) with volume change and symmetry-breaking ferroelastic transitions with shear. The central question is how volume and shear strain dynamics contribute to triggering an ultrafast IMT and how these propagate across relevant length scales. Focusing on V₂O₃, whose phase diagram contains isosymmetric PI–PM and symmetry-breaking AFI–PM transitions with significant volume change and ferroelastic domain formation, the authors aim to construct a comprehensive dynamical picture that includes acoustic-scale propagation. Using time-resolved reflectivity (electronic response) and ultrafast X-ray diffraction (structural response) on granular thin films and single crystals under conditions minimizing laser heating (0.89 eV pump, base T well below coexistence regime), they explore whether the IMT can propagate at the sound velocity in the wake of a photoinduced compressive strain wave, and how ferroelastic shear recovery precedes and couples to the IMT. They also assess the role of sample morphology (granular films vs single crystals) in enabling or hindering this pathway.

The work situates itself within several strands of prior research: (1) Ultrafast control of quantum materials along nonthermal pathways and coherent dynamics governed by optical phonons have been widely studied. (2) Picosecond acoustics established generation and propagation of strain waves, typically via laser heating of metallic transducers creating tensile waves. (3) Prior ultrafast studies on V₂O₃ using near-IR pumps on single crystals and epitaxial films reported transient states involving excitation into a₁g orbitals stabilized by A_g coherent phonons and interpretations invoking laser heating and nucleation-growth kinetics near coexistence, with complexity due to ferroelastic nanotexturing. Strong THz fields can induce ultrafast electronic switching by tunneling but revert to slower thermal evolution near the AFI–PM transition temperature. (4) Recent works highlighted elastically driven photoinduced transitions and volume expansion/contraction effects in materials with first-order isosymmetric transitions. However, a comprehensive understanding of how strain waves couple to electronic IMT in Mott systems, and how shear and volume components interplay and propagate at acoustic velocities, remained incomplete.

Samples: Single crystals of V₂O₃ were grown by vapor transport using sulfur as transport agent; thin films (100–270 nm) were deposited by reactive sputtering on (0001) sapphire and annealed in reducing atmosphere, yielding granular, polycrystalline, single-phase films. Domain/grain sizes were characterized by XRD (Scherrer analysis) and SEM: ~40 nm grains at 300 K (R3c), and ~20 ± 5 nm ferroelastic domains at 87 K (I2/a). Transport showed large resistivity jumps with transition temperatures typical of stoichiometric V₂O₃ (films ~147 K on heating, single crystals ~160 K). Pump/probe optical penetration depth at 0.89 eV was estimated to be ~200–400 nm. Experimental conditions minimized heating by using near-gap pumping (0.89 eV) and base temperatures well below the AFI–PM coexistence region. Transient reflectivity: A Ti:sapphire regenerative amplifier (800 nm, ~100 fs, 1 kHz) pumped OPAs. Pump: 1400 nm (0.89 eV). Probe: 0.1–1.2 eV spectral range. Cross-polarized pump/probe, spot sizes ~400–500 µm (pump) and 100–200 µm (probe). Samples in He cryostats (down to ~10 K). Detection used monochromator and InGaAs or HgCdTe detectors with shot-by-shot normalization. Time resolution 150–200 fs. Polarization and orientation dependence on single crystals was tested and found weak. Time-resolved X-ray diffraction (tr-XRD): Two setups were used. - MAX IV FemtoMAX (sub-ps): 9 keV X-ray pulses (~100 fs, 10 Hz) in UHV GIXS endstation, sample tilt 6°, spot ~60×570 µm². Scattering collected with a Pilatus detector at Q up to 4.5 Å⁻¹. Pump: 1300 nm (OPA), incident at 23°, s- or p-polarized; footprint 300 µm × 2.8 mm; peak fluence up to 60 mJ/cm². Synchronization by RF phase locking with a delay monitor (jitter ~210 fs FWHM). Fluence rescaling accounted for reflectivity differences to compare with optical data. - ESRF ID09 (high Q-resolution, ~100 ps): Single-bunch mode; 15 keV X-rays, Si(111) monochromator (ΔE/E ~10⁻⁴), reflection geometry, 6° incidence, footprint ~50×250 µm²; Rayonix MX170-HS detector at 150 mm. Pump: 1300 nm, 1.5 ps, 1 kHz stroboscopic mode with interleaved negative delays as reference. N₂-flow cryostat. Data analysis: tr-reflectivity spectra compared to static AFI vs PM reflectivity to identify metallization. tr-XRD azimuthally integrated patterns analyzed to track Bragg peak shifts and narrowing. Volume contraction estimated primarily from Q-shifts of the (110)_H peak (ΔV/V ≈ 2 Δa_H/a_H, given negligible Δc_H/c_H), validated by Rietveld refinements on fully transformed states. Symmetry change (monoclinic→hexagonal) tracked via narrowing and intensity redistribution of (116)_H-derived peaks. Fluence-threshold and saturation behaviors were extracted from fluence series at fixed delays. Thickness dependence was probed on 106 nm and 270 nm films; single-crystal reference data were acquired for comparison. Additional steady-state high-pressure reflectivity provided context for pressure-induced metallization similarities.

- In granular V₂O₃ thin films (116–270 nm), strong photoexcitation (0.89 eV pump) induces a complete AFI→PM insulator-to-metal transition (IMT) on sub-100-ps timescales; the transient reflectivity at ~300 ps matches the steady-state AFI→PM spectral difference without rescaling. - Ultrafast tr-XRD (FemtoMAX and ESRF) confirms full structural transformation: disappearance of monoclinic splitting and recovery of hexagonal R3c symmetry (notably in the (116)_H family), and volume contraction ΔV/V ≈ −1.5% consistent with the thermal AFI→PM transition. - A clear fluence threshold for macroscopic IMT is observed in thin films: ~2 mJ/cm² at 10 K, reduced to ~1 mJ/cm² at ~85–100 K. Above threshold, both electronic (ΔR/R) and structural signatures (volume contraction, symmetry recovery) increase nonlinearly and saturate around 8–10 mJ/cm². - At intermediate fluence (e.g., 3.9 mJ/cm²), tr-XRD shows distinct AFI and PM Bragg peaks, evidencing macroscopic phase coexistence; at higher fluence, single-phase PM peaks indicate complete conversion. - Dynamics separate into two regimes: • Symmetry change (inverse ferroelastic shear): completes in <3 ps (domain-scale), evidenced by rapid narrowing/merging of (116)_H-derived peaks. • Volume contraction (isosymmetric component): exhibits an onset after ~9 ps, then a linear evolution over a thickness-dependent acoustic timescale—~16 ps across a 106 nm film and ~40 ps across a 270 nm film—consistent with longitudinal sound velocities of ~7–7.3 nm/ps. - The ferroelastic domain size at low temperature (~20 ± 5 nm) and estimated transverse sound velocity (~4 nm/ps) rationalize the sub-3-ps shear recovery within domains. - Transient reflectivity closely tracks the structural evolution: a fast initial increase (consistent with a high-volume paraelastic PI-like state) followed by a linear rise concurrent with volume propagation, culminating in a long-lived metallic state (hundreds of ps). - Single crystals show no fluence threshold and no complete macroscopic IMT under comparable conditions; responses suggest only short-length-scale coexistence, consistent with lateral clamping suppressing volumetric deformation in continuous media. - The total duration of dynamics above threshold is largely fluence- and temperature-independent (10–100 K), consistent with coherent strain-wave propagation setting the timescale. - No pronounced strain oscillations due to reflections are observed, distinguishing the mechanism from conventional ps acoustics and supporting a self-launched internal compressive strain wave pathway from free surfaces. - Mechanistic picture: long-lived photoexcited electronic states generate internal shear and compressive stresses that launch shear strain waves within ferroelastic domains and compressive longitudinal waves through the film. The IMT propagates at the sound velocity in the wake of the compressive strain wave. Cooperative feedback via compressive (positive chemical) pressure and electronic Mott physics underpins the observed fluence threshold and macroscopic transformation.

The findings demonstrate that in V₂O₃ granular thin films the ultrafast IMT is not initiated by nucleation-and-growth governed by thermal kinetics, but rather is driven by propagating strain waves initiated by photoexcited electronic precursors. This addresses the core question by showing that the macroscopic phase transformation proceeds at an acoustic speed limit. The symmetry-breaking (ferroelastic) and isosymmetric (volume) components decouple in time and scale with different length scales: shear recovery is domain-limited and completes within a few picoseconds, while the volume contraction propagates across the full film thickness over tens of picoseconds. The tight temporal correlation between reflectivity (electronic) and XRD (structural) changes indicates that electronic metallization follows the structural evolution, especially the volume contraction, emphasizing the central role of the isosymmetric order parameter in the IMT. The observed fluence threshold and its reduction at elevated base temperatures signify a cooperative mechanism requiring a critical electronic excitation density. As compressive strain builds, it acts like positive chemical pressure that further stabilizes the metallic state, providing a positive feedback loop that amplifies the transformation once initiated. Symmetry considerations suggest a linear–quadratic coupling between volume strain and ferroelastic order (ΔV/V · φ²), consistent with the rapid recovery of the paraelastic state preceding independent volume evolution. The lack of complete transformation in single crystals underscores the importance of morphology: granular films are free to deform volumetrically in three dimensions, whereas single crystals experience lateral clamping that inhibits the necessary longitudinal volume contraction for a macroscopic IMT. The near temperature-independence of the dynamics away from coexistence further supports a coherent, nonthermal strain-wave pathway. Overall, the results extend ultrafast control paradigms by incorporating acoustic-scale propagation and highlight how elastic degrees of freedom, domain structure, and morphology govern nonequilibrium phase transitions in Mott materials.

This work establishes a strain-wave-driven pathway for the ultrafast insulator-to-metal transition in V₂O₃ granular thin films. The transformation propagates at sound velocity behind a compressive strain wave, with inverse ferroelastic shear completing within ~3 ps and isosymmetric volume contraction unfolding over thickness-limited acoustic times. A clear fluence threshold and saturation are observed, with macroscopic phase coexistence at intermediate fluence and complete conversion at higher fluence. The mechanism is favored in granular thin films and hindered in single crystals due to mechanical clamping. These insights clarify the active, central role of volume contraction and the interplay between symmetry-breaking and isosymmetric components in driving IMT, offering a physical framework applicable to a range of quantum materials with elastic-coupled phase transitions. Future work should develop time-dependent Ginzburg–Landau models including propagating strain fields and their coupling to electronic order, systematically compare to systems like VO₂ where ferroelasticity dominates, and explore device-level implications where switching speeds are bounded by acoustic velocities.

- The initial ~9 ps delay before the onset of volume contraction is observed but not fully explained; possible contributions include granular microstructure effects and coupling between shear and volume strains. - A comprehensive microscopic theory (e.g., time-dependent Ginzburg–Landau including strain propagation and fluence/temperature dependence) is not yet provided. - Penetration depth at 0.89 eV has appreciable uncertainty, which can affect excitation profiles; however, estimates indicate sufficient bulk excitation of the films. - Epitaxial films and interfaces, where substrate clamping and anisotropic strain may play strong roles, were not the focus here; single-crystal transformations were incomplete under studied conditions. - Magnetic dynamics, while likely coupled to ferroelasticity, were not directly probed in time-resolved measurements.