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.

Figure 1

Schematic illustration of geometric phase transition in ${\mathrm{Pb}\mathrm{Ti}\mathrm{O}}_3$. (a) Crystal structures of the cubic paraelectric and tetragonal ferroelectric unit cells. The polar displacement is along the z axis. (b) The atomic structures of ${\mathrm{Pb}\mathrm{Ti}\mathrm{O}}_3$ in FE₁, PE, and FE₂ states. (c) Schematic plot of the lattice potential energy surface (PES) with respect to polarizations P_x and P_z.

Figure 2

DFT-calculated fundamental properties of electrons and phonons of ${\mathrm{Pb}\mathrm{Ti}\mathrm{O}}_3$. (a) Electronic band dispersion along the high-symmetry k path, where blue and red curves denote the FE₁ and $\mathrm{PE}$ phase, respectively. The temperature-dependent phonon spectra of the (b) ${\mathrm{FE}}_1$ phase and (c) PE phase. The LO-TO splitting effect is included. The gray-shaded areas are the unstable imaginary phonon frequency regime, which is denoted as negative values. The dashed curves represent the phonon dispersion according to the standard second-harmonic approximation, while the solid curves are the finite-temperature phonon structures obtained from the SCPH method.

Figure 3

Real (solid curve) and imaginary (dashed curve) parts of the electronic susceptibility components of PE and FE₁ phase in the ${\mathrm{Pb}\mathrm{Ti}\mathrm{O}}_3$.

Figure 4

(a) Real and (b) imaginary parts of the ionic susceptibility components of ${\mathrm{Pb}\mathrm{Ti}\mathrm{O}}_3$. The electric dipole for FE₁ is along z. The dashed vertical lines in (b) mark the frequency of the first peak in the imaginary part of susceptibility at 300 K. (c) Schematic plot of the ionic displacement modes of the first dominant peak along each direction at room temperature.

Figure 5

(a) Relative GFE of ${\mathrm{Pb}\mathrm{Ti}\mathrm{O}}_3$ as a function of LPTL electric field magnitude at room temperature. (b) Nonresonant phonon response under alternating electric field of FE ${\mathrm{Pb}\mathrm{Ti}\mathrm{O}}_3$. Only a Raman-active mode appears here.

Figure 6

Geometric structures of ${\mathrm{Ba}\mathrm{Ti}\mathrm{O}}_3$ in (a) cubic (C), (b) tetragonal (T), (c) orthorhombic (A), and (d) rhombohedral (R) phases. The black arrow represents the direction of polarization of the ion ($\mathrm{Ti}$, $\mathrm{O}$) referenced by the C phase.

Figure 7

Temperature-dependent phonon spectra of ${\mathrm{Ba}\mathrm{Ti}\mathrm{O}}_3$ in the (a) C, (b) T₁, (c) A, and (d) R phase. The dashed curves represent the phonon dispersion according to the second-harmonic approximation, while the solid curves are from the SCPH scheme.

Figure 8

Relative GFE of ${\mathrm{Ba}\mathrm{Ti}\mathrm{O}}_3$ as a function of the LPTL electric field magnitude under room temperature. The symbols T₁ (T₂), C, R, and A denote tetrahedral, cubic, rhombohedral, and orthorhombic phases, respectively. Here T₁ and T₂ refer to that the electric polar direction is parallel and perpendicular to the light electric field direction, respectively.