Newswise — Researchers at the University of Arkansas investigating critical properties of ferroelectric thin films – which have applications in sensors, actuators and memory devices – have shed light on an overlooked phase of the material.

 Since its discovery about four decades ago, the Berezinskii-Kosterlitz-Thouless (BKT) theory has provided a new understanding of phase transitions in two dimensions, propelling discoveries in high-energy physics, atomic physics, nuclear physics, statistical physics and nonlinear systems. The BKT phase is confined to two dimensions and embodies an unusual state of matter (e.g., superconductivity, superfluidity) that departs from traditional three-dimensional physics (length, width and height). Work on the BKT theory led to the Nobel Prize in physics in 2016.

 The BKT phase was thought to be precluded in ferroelectrics, however. In a paper published Sept. 12 in the journal Physical Review Letters, research assistant professor Yousra Nahas and BeIPD-COFUND fellow Sergei Prokhorenko, along with Distinguished Professor of physics Laurent Bellaiche and in collaboration with professor Igor Kornev from Ecole Centrale Paris, found an intermediate phase with BKT-defining features, the most salient of which is the condensation of tightly bound pairs of dipolar topological defects. They discovered a subtle interplay between interactions and the confining two-dimensional geometry of thin-film of barium titanate, a prototypical ferroelectric. In addition to highlighting the fundamental richness of low-dimensional ferroelectrics and widening the realm of BKT physics, the research opens the way to prospective uses of topological features within ferroelectric-thin film.

 The research was supported by Army Research Office (ARO), the Defense Advanced Research Projects Agency (DARPA), the University of Liege in Belgium, and the European Union in the context of the FP7-PEOPLE-COFUND-BeIPD project. Simulations were done at the Arkansas High Performance Computing Center.