The time of a photoinduced spin-Peierls phase transition

The time τ of the spin-Peierls phase transition is analyzed theoretically as a function of the duration τ_p of the exciting light pulse and the average number x₀ of absorbed photons per magnetic ion after the transmission of the pulse. It is shown that the phase transition occurs at x₀ > x_c. The critical value x_c is determined as a function of the duration τ_p of the light pulse. A photoinduced variation in the optical reflection coefficient R is calculated as a function of time t. The results of calculation are compared with experimental data on ultrafast photoinduced melting of the low-temperature spin-Peierls phase into potassium tetracyanoquinodimethan (K-TCNQ)