Vibrational-Anharmonicity-Assisted Phase Transitions in Perovskite Oxides Under Terahertz Irradiation

Despite extensive research interests in perovskite oxides, low-energy consumption, nondestructive, and manoeuverable methods for phase transition in perovskite oxides are still under exploration, and the underlying mechanisms remain ambiguous. Here, optical susceptibility, including electron and anharmonic phonon contributions, is used to evaluate Gibbs free-energy variations of ${\mathrm{Pb}\mathrm{Ti}\mathrm{O}}_{3}$ and ${\mathrm{Ba}\mathrm{Ti}\mathrm{O}}_{3}$ under terahertz irradiation. This corresponds to an off-resonant light-controlled phase transition, rather than the resonant approaches that excites hot carriers over electronic band or infrared-active vibrations in the phonon band. We show that intermediate terahertz light can trigger polarization change between ferroelectric orientation variants of ${\mathrm{Pb}\mathrm{Ti}\mathrm{O}}_{3}$ at room temperature. Similarly, the phase transformation from low-symmetric ferroelectric phase to high-symmetric paraelectric structure in ${\mathrm{Pb}\mathrm{Ti}\mathrm{O}}_{3}$ can be driven by changing the polarization and intensity of the incident light. Analogous results are obtained in ${\mathrm{Ba}\mathrm{Ti}\mathrm{O}}_{3}$. In detail, the phonon spectrum and optical susceptibility exhibit strong temperature dependence, in which we show significant effects of anharmonic vibration. In order to explicitly show its nonlinear optical nature, we perform an alternating electric field dressed ab initio molecular dynamics simulation, which maps onto the Raman-active phonon excitation under off-resonant terahertz irradiation.