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.

Aperiodic spin state ordering of bistable molecules and its photoinduced erasing.

We describe a novel type of ordering phenomenon associated with the incommensurate occupational modulation of bistable molecular magnetic state in a spin-crossover material. This unusual type of aperiodicity resulting from the ordering of multistable electronic states opens new possibilities for addressing such materials by light. Here we show that light can switch the crystal from four- to three-dimensional periodic structure. Mixing aperiodicity, multistability, and photoinduced phenomenà opens new perspectives for directing complex order and function in material science.

Successive dynamical steps of photoinduced switching of a molecular Fe(III) spin-crossover material by time-resolved x-ray diffraction.

We investigate the out-of-equilibrium switching dynamics of a molecular Fe(III) spin-crossover solid triggered by a femtosecond laser flash. The time-resolved x-ray diffraction and optical results show that the dynamics span from subpicosecond local photoswitching followed by volume expansion (nanosecond) and thermal switching (microsecond). We present a physical picture of the consecutive steps in the photoswitching of molecular materials.

Down to the quantum limit at the neutral-to-ionic phase transition of (BEDT-TTF)-(ClMe-TCNQ): symmetry analysis and phase diagram.

We report on the neutral-to-ionic (N-I) phase transition in the one-dimensional organic complex (BEDT-TTF)-(ClMeTCNQ). The X-ray studies at room temperature show that the neutral phase of (BEDT-TTF)-(ClMeTCNQ) is already characterized by a polar long-range ordering, at variance with other charge-transfer compounds comprising noncentrosymmetric molecules. From a detailed neutron diffraction study of this complex under high pressure, we present the phase diagram of the N-I transition down to the quantum limit. We discuss the symmetry breaking associated with the transition and the evolution of its first-order character under pressure.

One-dimensional fluctuating nanodomains in the charge-transfer molecular system TTF-CA and their first-order crystallization.

We report on the direct observation by x-ray diffuse scattering measurements of thermally induced one-dimensional nanoscale ordered fluctuations in the mixed-stack charge-transfer molecular system tetrathiafulvalene-p chloranil (TTF-CA), the prototype for the neutral-ionic phase transition. The unusual physical properties of this compound are considered to be driven by such one-dimensional excitations. The results are discussed in relation to previous experimental and theoretical experiments both at thermal equilibrium and under light irradiation.