Monte Carlo Simulation for the Macroscopic Phase Separation in Photo-induced Low-spin to High-spin State Transitions

Abstract

Photo-induced cooperative low-spin (LS) to high-spin (HS) conversion phenomena are studied on the basis of a model which includes the long range interaction between complexes originated from the coupling with the acoustic phonons. A short range interaction which mimics the coupling with the optical phonons is also included. The Monte Carlo simulation method is applied in the two- and the three-dimensional cases. It is pointed out that the macroscopic phase separation of HS and LS domains during the conversion process is caused by the volume striction effect through the coupling with acoustic modes. A quantity which describes the coexistence condition of two phases in the photo-induced conversion process is presented. Other characteristic features observed in experiments; the existence of the threshold intensity and the incubation time are also reproduced. The abrupt step-like conversion observed in experiments is ascribed to the de-pinning effect of the volume striction.