Ultrafast dynamics of soft phonon modes in perovskite dielectrics observed by coherent phonon spectroscopy

Coherent phonons were observed in perovskite dielectrics LaAlO₃ and KMnF₃ using ultrafast polarization spectroscopy. Ultrafast dynamics and softening of phonon modes, which contribute to structural phase transitions, were studied. The temperature dependences of the phonon frequency and relaxation rate were obtained from the observed damped oscillation of coherent phonons. In LaAlO₃, typical softening of the phonon frequency was observed toward the phase-transition temperature Tc ($~750$ K). The observed relaxation for the soft-mode phonons is explained well by a population decay due to anharmonic phonon-phonon coupling. In KMnF₃, two structural phase-transition temperatures, Tc1 $(=187$ K) and Tcm ($~81$ K), were found through birefringence measurement. At temperatures below Tcm, three modes of coherent phonons were observed and the phonon modes were found to undergo softening toward Tcm. The observed dependences of the coherent phonon signal on the pumping polarization and the temperature suggest an orthorhombic structure, instead of a monoclinic structure, at temperatures below Tcm. The sudden decrease in the phonon frequency at Tcm indicates the first-order character of the phase transition at Tcm. Notable changes in birefringence and phonon frequency were observed at around 90 K, but no distinct anomaly is found, which suggests that the structure undergoes continuous change. Above 100 K, the Eg mode in the tetragonal symmetry is found to undergo softening toward Tc1. A coherent phonon signal of relaxation type appears at around Tc1 and survives even at near room temperature, which suggests the existence of structural disorder even in the cubic symmetry above Tc1.