Dynamics of phonons in Sr₃Ir₄Sn₁₃: an experimental study by ultrafast spectroscopy measurements

We report a study of ultrafast dynamics of photoexcited electrons and phonons in Sr₃Ir₄Sn₁₃ using dual-color transient reflectivity change (ΔR/R) measurements. Time resolved optical spectroscopy of collective excitations reveal the marked features near its structural phase transition temperature K. Two distinctive oscillatory timescales in have been clearly resolved. The rapid THz-range oscillations are attributed to the dynamics of the optical phonons which strongly correlate to the structural phase transition. The slow GHz-range oscillatory phenomenon which only occurs below about 150 K is associated with the dynamic response of the longitudinal-acoustic phonons. These low-energy phonons show a softening feature on approaching the transition temperature, also indicating a strong relevance to the structural phase transition. The information that we demonstrated would provide a deeper understanding of the structural phase transition in Sr₃Ir₄Sn₁₃.