Photoinduced Structural Phenomena in Ferroelectric Oxide Electronic Materials Investigated by Synchrotron X-ray Diffraction

Abstract

My thesis research investigates photoinduced phenomena in ferroelectric electronic oxide materials, with a focus on changes that occur on sub-nanosecond timescales and length scales ranging from single unit cells to the mesoscopic ferroelectric polarization pattern. The experiments employ time-resolved synchrotron X-ray diffraction and microscopy. A series of X-ray diffraction experiments and theoretical calculations were employed in order to understand photoinduced phenomena from a structural perspective. The dynamics of the photoinduced phenomena were analyzed to obtain insight into the mechanisms of lightinduced structural effects in ferroelectric materials. Taken together, the results show that bound charges due to the polarization discontinuity at domain boundaries or interfaces with non-polar materials have an important role in photoinduced structural phenomena in ferroelectrics with nanoscale domain patterns. The photoexcitation phenomena reported in this thesis are excited by ultrashort optical pulse. Optical absorption is followed by a rapid deformation of the equilibrium structure via a series of pathways that are reported in detail. This thesis reports studies of photoinduced phenomena in three ferroelectric materials system. The first results involve the dynamics of ferroelectric nanodomain patterns within PbTiO3/SrTiO3 superlattices. Previous studies have found that the steady-state domain pattern can be modified by changing mechanical and electrostatic boundary conditions. In a series of in-situ X-ray diffraction experiments, we have discovered a photoinduced transformation to uniform polarization state. Thermodynamic calculations reveal that the uniform polarization state is energetically stabilized by the screening of bound charges. An analysis of the relaxation dynamics indicates that trapped charge carriers have an important role in setting the concentration of mobile charge carriers. The results are reported in Ahn et al., Phys. Rev. Lett. 119, 057601 (2017). A second study involves low-strain BaTiO3 thin films. Photoexcitation leads to a reorientation of the domain walls in this system on a sub-nanosecond timescale. The reorientation is observed only in the room-temperature regime in which two phases of the domain pattern coexist. Electrostatic calculations show that the domain wall reorientation results from the screening of the bound charges at domain walls. An alternative model based on an elastic response to optically induced expansion is not consistent with the experimental results. The bound charges can arise due to roughness and disorder of the domain walls and from weak in-plane polarization components, both of which are reduced in the hightemperature single-domain phase. Finally, we report an optically induced transformation between structural phases of a compressively strained BiFeO3 film. These structural phases have distinct electronic and magnetic properties coupled to the crystal structures and thus the system has a potential to enable properties to be manipulated significantly via this phase transformation. Timeresolved synchrotron X-ray diffraction microscopy showed the photoinduced phase transformation on nanosecond timescale. Thermodynamic free energy calculations provide insight into the phase transformation [Ahn et al., Phys. Rev. Lett. 123, 045703 (2019)].