Multiscale out-of-equilibrium dynamics driven by pulsed laser excitation in spin-crossover materials: A combined thermoelastic and mechanoelastic study

In this paper, we present an elastic model coupled with a heat propagation process in order to reproduce the out-of-equilibrium dynamics of spin crossover materials driven by femtosecond laser excitation: a multiscale out-of-equilibrium dynamics driven by pulsed laser excitation in spin-crossover materials (thermoelastic step), the thermal switching (thermal step), and the subsequent relaxation to the initial state on cooling. The simulations were performed for open boundaries two and three-dimensional samples, composed of individual molecules linked by springs, which stand for elastic interactions. This building-up of the samples allows the propagation of elastic waves, which leads to accumulation of high spin molecules towards edges at the maximum of the thermoelastic step. We first show that a simple model with a single “temperature” reproduces the thermoelastic, the thermal step and the relaxation to the original equilibrium state. However, the too slow thermalization of the lattice obtained in this model does not correspond to the experimental data. Therefore, to overcome this drawback, we consider either an inhomogeneous photoexcitation or different “temperatures” for the lattice and the spin states. The effect of the sample size, which prevents the existence of a thermal step in the case of nanoparticles is also discussed, as well as the three-dimensional model.