Effect of intermolecular interactions and elastic frustration on the dynamical properties of the isothermal relaxation of 1D spin crossover chains.

We consider an open one-dimensional spin-crossover chain, in which each site can be in either a low spin (LS) or a high spin (HS) state. The sites interact elastically through nearest neighbor (nn) and next-nearest neighbor (nnn) springs with local equilibrium distances depending on the spin states. The system's Hamiltonian is solved numerically using the Monte Carlo method, applied on both spin states and atomic displacements. This study focuses on the investigations of the isothermal relaxation of a photoinduced HS metastable chain, by analyzing the interplay between the electronic and structural properties along this process. The obtained results indicate that the nucleation and growth mechanisms of LS domains during relaxation are significantly influenced by the amplitude of the intermolecular interactions. Thus, increasing the latter reduces the number of HS/LS clusters due to the high cost of stored elastic energy at HS/LS interfaces. In the second part, we inject an elastic frustration between the equilibrium nn and nnn bond lengths, resulting in the emergence of two distinct relaxation regimes, which depend on the frustration rate, ξ. A detailed analysis of the effect of ξ on the isothermal HS to LS relaxation reveals the stabilization of rich intermediate self-organized electronic structures with long lifetime along this process. Thus, these results clearly demonstrate that the shape of the relaxation curves transforms from a continuous to a two-step behavior, which is reminiscent of the thermal dependence of the order parameters of such models in equilibrium thermodynamics.