dc.description.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)]. | en_US |