Multiferroic materials possess more than one type of ferroic order in a single phase, which means that they exhibit two or more of the primary ferroic properties, namely, ferroelectricity, ferromagnetism, and ferroelasticity, which are coupled to some degree. As a prototypical ferroelectric and ferroelastic perovskite, BaTiO3 (BTO) has gained a lot of attention, due to the intriguing properties that arise from the coupling between its ferroelectric and ferroelastic orders and its wide range of current and potential applications, which include applications to energy storage, photovoltaics, and memory devices.
In this thesis, ab-initio electronic structure methods and atomistic simulations are used to study the effects of ultrashort (<1 ps) above-band-gap optical laser pulses on BTO’s structure, ferroelectricity, and lattice dynamics, in its ferroelectric rhombohedral (R3m) phase. To calculate properties of the photoexcited state, a constrained form of density functional theory (DFT) [1–3] is used, which keeps the density of electrons occupying the conduction band fixed. This is to study phonon dynamics on time scales shorter than typical electron-hole recombination times. We find that photoexcitation could selectively excite and soften a coherent A1 ferroelectric mode (FM) phonon. The FM eigenvector can be viewed as the shape of the distortion that relates the ferroelectric (FE) crystal structure to its higher-symmetry paraelectric (PE) structure (Pm ̄3m). Photoexcitation reduces both the magnitude of this distortion, and hence the magnitude of the polarisation field |P|, and the energy barrier for reversal of the direction of P. It does so by reducing the degree to which BTO is ionic: photoexcitation returns electrons from O to Ti ions, thereby reducing the magnitudes of their charges, and increases their polarisabilities. This weakens the Coulomb interactions (e.g., Ti-O attraction) that stabilise ferroelectricity. Our results indicate that a transient photo-induced lowering of both the coercive field (Ec) and the FE-PE transition temperature (TC) is possible. We also suggest that pump-probe spectroscopy could be used to induce a purely-displacive transition to the higher-symmetry Pm ̄3m phase at low temperature.
Using a polarisable-ion model (PIM) for atomistic simulations, parameterised for both the electronic ground state and photoexcited states, we simulate the effects of photoexcitation on larger time and length scales. Our results show a reduction of both TC and |P| with increasing photoexcited carrier density (x), and a reduction of |P| with increasing temperature, T. The calculated infrared (IR) spectra show a remarkable softening of the FM with photoexcitation, and demonstrate the carrier-dependence and temperature-dependence of the highly-anharmonic low-frequency central mode (CM). As the temperature approaches TC, the CM is strongly coupled with the FM in the low-frequency region of the IR spectrum.
The results on photoexcited BTO are also used to interpret experimental investigations of the effects of electron doping on structure and ferroelectricity in BTO, including a recently-reported polar metallic phase [4–6]. It is suggested that, like photoexcitation, doping reduces the magnitudes of Coulomb interactions, particularly the Ti-O attraction, which reduces TC, the magnitude of the ferroelectric distortion, and the FM frequency. It is suggested that a polar metallic phase can exist if enough delocalised and itinerant conduction band electrons (CBEs) for metallic conductivity can exist before the CBEs localised around Ti and O ions critically weaken the Coulomb interactions that stabilise the ferroelectric distortion.
These results may help to guide the design and optimisation of BTO-based optoelectronic devices.