Center for Correlated Electron Systems


Ultrafast Phenomena in Ferroic Complex Oxide Heterostructures

January 29, 2021l Hit 301
Date : February 4, 2021 14:00 ~ 15:00
Speaker : Dr. Hyeon Jun Lee (University of Wisconsin Madison)
Location : Bldg. 19, Rm. 210 / Zoom(

Epitaxial ferroelectric oxide heterostructures are highly engineered nanomaterials exhibiting complex configurations of their electrical polarization, ranging from nanoscale striped domains to chiral vortices. The key aspect of the physical mechanisms to modulate the complex domains/phases in ferroelectrics is straightforward: tailoring energy landscape determined by the competition between elastic and electrostatic energy. External stimuli such as electric field and optical excitation perturbs ferroelectric order parameter, polarization, resulting in the microscopic dimensional and electrical modification. The fundamental time scales of the transitions can range from timescale corresponding to electron-electron scattering to a few microseconds due to slow dynamics such as domain wall motion. Probing the time-dependent dynamics is crucial to understand mechanisms of the transitions and discover new physical phenomena that can exist at ultrafast time scale.  In this presentation, We will discuss the electric field and laser-driven ultrafast transitions in ferroelectric epitaxial thin films at various timescales.

First, we discuss both the electric-field-driven transient and permanent phase transformations in highly strained BiFeO3 thin films and their transformation dynamics at the nanoscale. We found that the permanent phase transformation was triggered below and above a coercive voltage of the polymorphic phase, indicating that ferroelectric polarization switching could promote permanent phase transformations. We also found that the transient transformations evolve via complex phase boundary motions between the coexisting phases, whereas permanent transformations occurred via nucleation of the other phases.

Second, we discuss nanosecond dynamics of electric field driven ferroelastic switching and non-volatile 180° domain switching in 90° ferroelastic domain wall. It is found that the application of electric fields induces spatially heterogeneous domain switching processes via intermediate domain structures with rotated polarization vectors. In addition, the domain switching time is shown to be inversely proportional to the magnitude of the applied electric field, and electric fields higher than 480 kV/cm are found to complete the ferroelastic switching within nanoseconds.

Third, we report that complex strain pulses produced by the photoexcitation drive a time-dependent distortion of nanodomains in PbTiO3/SrTiO3 superlattice film on SrRuO3/SrTiO3 substrate. The picosecond dynamics of structural and domain distortion following photoexcitation were probed using femtosecond x-ray free electron laser (XFEL) diffraction. Acoustic pulses resulting from the absorption of the incident optical beam lead to changes in the x-ray diffraction pattern of the SL over the picosecond time scales. The picosecond scale variation of the nanodomain diffuse scattering intensity is consistent with a larger polarization change than would be expected due to the polarization-tetragonality coupling of uniformly polarized ferroelectrics. The strong coupling is consistent with polarization rotation facilitated by the reorientation of the in-plane component of the polarization at the domain boundaries of the striped polarization structure.



Host : Prof. Tae Won Noh