Opportunities for ultrahigh speed and density information processing open up with the discovery of nanoscale atomic whirls induced by electric field stimuli. Scientists from the Institute of Physics contributed to a study uncovering how polarization whirls react to THz pulses on the nanometre scale, which was published in the April 15th issue of Nature.
The study pays attention to the dynamical aspects of ferroelectric superlattices. Using terahertz excitation and femtosecond X-ray diffraction measurements, the researchers observed ultrafast collective polarization whirl dynamics.
The experiment and calculations were undertaken by a broad collaboration of the Czech team with Argonne National Laboratory, The Pennsylvania State University, University of California, Berkeley and other American institutions.
Polarization dynamics in the ‘vortexon’ mode of the PbTiO3/SrTiO3 superlattice. Each arrow represents the polarization of one perovskite unit cell (the distance between neighbouring arrows is ~4 Å). 8 unit-cell thick layers of non-polar SrTiO3 are on the top and the bottom, 16 unit cells of vortex-hosting PbTiO3 layer are in the middle. The animation is the result of all-atom lattice dynamics calculations with first-principles-based interatomic potentials.
“The atomistic picture of the observed excitations could be unveiled primarily due to the computational method, developed by Dr. Marek Pasciak, said the PI of the project at the Institute of Physics, Dr. Jiří Hlinka. “In particular, his method allowed to identify transverse oscillations of polar vortices. They act as the most coherent and electric-field susceptible excitations at the sub-THz frequencies," added Dr. Christelle Kadlec, a team member charged by experimental exploration of the interaction of the superlattice with the THZ radiation.
The experiment and the calculations demonstrate that the frequency of this oscillation is tunable by temperature or by the selection of a suitable substrate and that the THs pulses can excite the polarization whirls on the nanometre scale, which opens up opportunities for electric-field-driven data processing in topological structures with ultrahigh speed and density.
It is all about atomic shifts
Electric polarization characterizes the state of mater in which centres of masses of positively and negatively charged particles are shifted with respect to each other. This relative shift can be represented as a vector. In many crystalline materials, such a polarization vector can be defined on the nanometre scale of the basic unit cell which is then periodically repeated in the bulk. In such a case, the same vector characterizes micro and macro scales. This is true for ferroelectric materials where the polarization reacts to the electric field, which can enhance or reorient the displacements of ions in the crystal. Since it is all about atomic shifts, mechanical strain in the material is also directly coupled to the dielectric polarization, leading to several applications such as piezoelectric actuators or transducers.
In some situations, micro and macro scales are not identical. For instance, in antiferroelectric materials, where the size and the direction of the polarization in neighbouring areas is opposite, the polarization averages out to zero on the macroscopic scale. More complicated polarization topologies have been predicted theoretically and recently realised in synthetic materials. In so-called superlattices, in which layers of a ferroelectric material PbTiO3 are separated by layers of a normal dielectric SrTiO3, a special structure of nanosized polarization whirls has been successfully created.
DOI: doi.org/10.1038/s41586-021-03342-4