Strain wave pathway to semiconductor-to-metal transition revealed by time-resolved X-ray powder diffraction

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. Ultrafast control of materials draws interest. Here, the authors extend X-ray powder diffraction to the femtosecond timescale to follow the photo-induced semiconductor to metal transition in titanium pentaoxide, observing a phase front that moves at the speed of sound and proposing a little explored mechanism.

Automated summary

Researchers have used femtosecond powder X-ray diffraction to study the transition from semiconductor to metal in Ti3O5 nanocrystals, finding that strain waves and electronic precursor phenomena determine the transformation pathway. The study demonstrates that in nanocrystals, picosecond switching occurs simultaneously with the propagating acoustic wavefront.