The ability to manipulate materials' phases along coherent transformation pathways has profound implications for materials science and technology. Understanding the dynamic limits of photoinduced phase transitions is crucial for realizing this potential. For transitions involving volume and/or shear deformations, the propagation of strain waves becomes a critical factor. While coherent dynamics along established pathways are well-studied, the role of strain waves in macroscopic state transformations remains under-explored. Previous research has touched upon volume expansion dynamics in photoinduced processes in materials undergoing first-order isostructural transitions, but a comprehensive understanding is still lacking. Many quantum materials, such as Mott insulators, exhibit phase transitions coupling isosymmetric Mott IMTs (involving only volume change) and symmetry-breaking ferroelastic transitions (involving changes in crystal system due to shear strain). The dynamics of both volume and shear strain can influence the ultrafast IMT. This study focuses on V₂O₃, a prototypical Mott material, to investigate the underlying physics of the dynamical strain wave pathway. Understanding this mechanism in V₂O₃ is particularly important given its established role as a model system in Mott physics. The generation and propagation of photoinduced strain waves have been studied in the context of picosecond acoustics. However, coupling these strain waves with electronic phase transitions, particularly IMTs in Mott materials, presents a significant challenge. Conventional picosecond acoustics typically uses laser heating of external transducers to generate longitudinal tensile strain waves, while in the case of insulators, internal stresses are generated through ultrafast electronic photoexcitation, initiating self-supported waves from the surface. The morphology of the material strongly influences this dynamic strain response, affecting the efficiency of the ultrafast IMT. The V₂O₃ phase diagram at thermal equilibrium exhibits both isosymmetric and symmetry-breaking IMTs, making it an ideal system for studying the combined effects of volume and shear strain on ultrafast IMT dynamics.

Previous studies on ultrafast photoinduced dynamics related to IMT in V₂O₃ insulating phases have used near-IR pumping on single crystals and epitaxial films. One study attributed the transient state to electron excitation in bonding a₁g orbitals, stabilized by atomic displacements associated with coherent optical phonons. Other studies have discussed photoinduced AFI-to-PM dynamics using ultrafast laser heating and nucleation-growth models, but these models may not be fully applicable to non-equilibrium ultrafast dynamics. The role of ferroelastic nanotexturing has also been observed to influence the dynamics. Furthermore, intense THz pumping can induce ultrafast electronic switching via quantum tunneling, but slower thermal evolution is observed near the AFI-PM transition temperature. These prior investigations highlight the complexity of the phenomenon and emphasize the need for a comprehensive approach that considers both electronic and structural dynamics simultaneously.

This research combines time-resolved (tr) transient reflectivity to probe IMT electronic changes and X-ray diffraction (XRD) to probe structural dynamics, starting from the AFI phase. Experimental conditions were chosen to minimize heating effects by using a pump photon energy close to the optical gap (0.89 eV) and a base temperature below the AFI-PM coexistence regime to avoid heterogeneous nucleation. The study employed V₂O₃ thin films (116 nm thick, with approximately 40 nm grain size) and single crystals. Transient reflectivity measurements covered a broad spectral range to detect the metallic state. Ultrafast X-ray diffraction, performed at FemtoMAX and ESRF, provided sub-picosecond and high Q-resolution data respectively, to observe structural changes. Data analysis included azimuthal integration of 2D XRD images and Rietveld refinement of XRD patterns to determine structural parameters (volume contraction and symmetry breaking). The fluence dependence of the transient reflectivity was investigated at various time delays to determine the threshold for macroscopic IMT. The thickness dependence of the volume contraction and shear change were studied by comparing results from films of 106nm and 270nm thickness.

The study reveals a complete photoinduced IMT in the V₂O₃ thin film, evidenced by the spectral response of transient reflectivity matching the difference in static reflectivity between AFI and PM phases. In contrast, the single crystal exhibited a significantly weaker response, indicating the absence of a macroscopic IMT. Time-resolved XRD confirmed the structural transformation from the monoclinic AFI phase to the rhombohedral PM phase. A shift of Bragg peaks towards larger Q indicated a photoinduced compressed state, while intensity redistribution revealed the disappearance of monoclinic distortion, implying a complete reverse symmetry breaking. The volume contraction (ΔV/V = -1.5%) was consistent with thermal equilibrium values. A clear fluence threshold for macroscopic IMT was observed in the thin films (~2 mJ/cm² at 10 K), whereas no threshold was observed in the single crystal. At intermediate fluences, phase separation between AFI and PM phases was observed. The temporal signature of strain wave dynamics was evident in the propagation of the phase transformation on the acoustic time scale. The monoclinic-to-hexagonal transition was completed in less than 3 ps, while the volume contraction required longer times (16 and 40 ps for 106 nm and 270 nm films respectively), scaling with film thickness. These dynamics support a strain wave scenario: initial stresses (shear and volumic) are relaxed by wave propagation from interfaces. The transient reflectivity closely followed the structural dynamics, with a fast initial increase followed by a slower, linear increase coinciding with volume change propagation. The overall duration of the dynamics remained constant above the fluence threshold. The faster shear dynamics, associated with symmetry-breaking, scaled with domain size, while the volume dynamics scaled with film thickness. The absence of complete transformation in the single crystal is consistent with a strain-wave picture due to the lateral clamping effect. The granular nature of the thin film facilitates independent deformation of grains, promoting macroscopic IMT.

The results highlight the crucial role of volume contraction in driving the macroscopic IMT. For a given volume, the I-to-M transition involves primarily electronic reorganization, immediately following volume change propagation. This contrasts with systems where slower thermal activation governs local dynamics. The fluence threshold suggests a critical electronic excitation level triggering self-amplification of the IMT at the macroscopic scale. The decreased fluence threshold at higher temperatures aligns with the reduced distance to the phase transition line. Long-range elastic interactions, creating effective chemical pressure, reinforce cooperativity, linking electronic Mott physics and volume contraction. The study’s findings establish the necessity of dynamic volume contraction for macroscopic IMT, clarifying the roles of shear and volume strain waves. The differences from conventional picosecond acoustics and the importance of sample morphology are also highlighted. The presented physical picture likely extends beyond V₂O₃ to other quantum materials involving elastic deformations during phase transitions.

This work demonstrates the pivotal role of photoinduced strain waves in driving ultrafast insulator-to-metal transitions in V₂O₃. The study highlights the importance of volume contraction and the distinct dynamics of shear and volume strain waves. The findings reveal differences from conventional picosecond acoustics and underscore the crucial impact of sample morphology. The proposed physical model, based on strain wave propagation, offers a framework for understanding ultrafast phase transitions in a broader range of quantum materials and has implications for the development of ultrafast devices based on Mott insulators.

The study primarily focuses on granular thin films and single crystals of V₂O₃. The generalizability of these findings to other morphologies or materials requires further investigation. The precise mechanisms behind the observed fluence threshold and the delayed onset of volume dynamics warrant further exploration through theoretical modeling and additional experimental studies. The limitations on the evaluation of the pump laser penetration depth might introduce some uncertainty into the quantitative aspects of the excitation. Advanced techniques could improve precision.