One of the main challenges in ultrafast material science is to trigger phase transitions with short pulses of light. Here we show how strain waves, launched by electronic and structural precursor phenomena, determine a coherent macroscopic transformation pathway for the semiconducting-to-metal transition in bistable Ti3O5 nanocrystals. Employing femtosecond powder X-ray diffraction, we measure the lattice deformation in the phase transition as a function of time. We monitor the early intra-cell distortion around the light absorbing metal dimer and the long range deformations governed by acoustic waves propagating from the laser-exposed Ti3O5 surface. We developed a simplified elastic model demonstrating that picosecond switching in nanocrystals happens concomitantly with the propagating acoustic wavefront, several decades faster than thermal processes governed by heat diffusion.

Upon cooling, the aperiodic inclusion compound n-nonadecane/urea presents a hexagonal-to-orthorhombic group-subgroup phase transition at T cl that increases the structure's superspace dimensionality from four to five. This paper reports on pretransitional phenomena in such a high-dimensional space, generalizing the critical results previously reported at a lower dimensionality. Very high-resolution diffraction data reveal anomalously large correlation lengths along the aperiodic direction, with all correlation lengths diverging at T cl . This could be explained by low-frequency phason excitations that soften at T cl at the critical wave vector, in accordance with an increase in the critical diffuse scattering intensity. The physics of phase transitions in crystalline materials was studied extensively during the last decades of the 20th century. With respect to group-subgroup structural instabilities, the order parameters, as measures of symmetry breaking, as well as related critical phenomena, were derived and measured.