Engineering Phase Transitions and Dielectric Properties of Nano-Ferroelectrics

My group is interested in the behavior of ferroelectric materials that present non-trivial polar orders (vortexes [1,2], skyrmions [3]) and properties (chirality [4], negative capacitance [5,6]) in situations of reduced dimensionality (ultra-thin layers or films) and/or subject to suitable electrostatic boundary conditions. In this talk I will review our most recent theoretical results for one of the model systems in the field, the PbTiO₃/SrTiO₃ ferroelectric/dielectric superlattices where many of the above effects were first demonstated. In particular, I will show how layer thickness and epitaxial strain can be tuned to induce phase transitions that yield a giant negative-capacitance response. I will also discuss our ongoing efforts to study the dynamical behavior of these nano-ferroelectrics, which have recently been shown to display interesting subterahertz resonances [7].<br/><br/>Work done in collaboration with M. Graf and H. Aramberri, postdocs working at the Luxembourg Institute of Science and Technolgoy. Funded by the Luxembourg National Research Fund through Grants INTER/RCUK/18/12601980 and C18/MS/12705883/REFOX.<br/><br/>[1] A.K. Yadav et al., "Observation of polar vortices in oxide superlattices", Nature 530, 198 (2016).<br/>[2] A.R. Damodaran et al., " Phase coexistence and electric-field control of toroidal order in oxide superlattices", Nature Materials 16, 1003 (2017).<br/>[3] S. Das et al., "Observation of room-temperature polar skyrmions", Nature 568, 368 (2019).<br/>[4] P. Shafer et al., " Emergent chirality in the electric polarization texture of titanate superlattices", PNAS 115, 915 (2018).<br/>[5] P. Zubko et al., "Negative capacitance in multidomain ferroelectric superlattices", Nature 534, 524 (2016).<br/>[6] S. Das et al., "Local negative permittivity and topological phase transition in polar skyrmions", Nature Materials 20, 194 (2021).<br/>[7] Q. Li et al., "Subterahertz collective dynamics of polar vortices", Nature 592, 376 (2021).