STUDY OF COMPLEX FERROIC OXIDES BY LARGE-SCALE FIRST-PRINCIPLES SIMULATIONS

Doctoral thesis (2020)

The main goal of this thesis was to explore the possibility that ferroelectric materials, characterized by a spontaneous and switchable electric polarization, may present topologically non-trivial ... [more ▼]

The main goal of this thesis was to explore the possibility that ferroelectric materials, characterized by a spontaneous and switchable electric polarization, may present topologically non-trivial structures akin to the skyrmions that occur in their ferromagnetic counterparts. The main tool used in the investigation was atomistic simulation based of first-principles effective models (“second-principles methods”), applied to two model systems: ferroelectric PbTiO3 and ferroelectric/paraelectric superlattices made of PbTiO3 and SrTiO3. More precisely, the simulations were used to analyze multidomain configurations in these compounds, motivated by previous reports that they may present non-trivial structural features. The main finding of the thesis is that, indeed, a simple multidomain configuration in PbTiO3 – namely, a columnar nanodomain with polarization opposed to that of its surrounding matrix – is sufficient to generate a dipole texture – associated to the rotation of the polarization at the domain wall between nanodomain and matrix – with the topology of a skyrmion. This constitutes the first prediction of an electric skyrmion in a simple ferroelectric material. Further, it is shown that the properties and topology of this skyrmion can be tuned by external electric and elastic fields, as well as by temperature, obtaining novel effects such as topological and iso-topological phase transitions. Finally, the investigation of the PbTiO3/SrTiO3 superlattices reveals that the skyrmion structures can be obtained as the ground state solution for such systems. This latter study was developed in the context of a collaboration with experimental groups at UC Berkeley and elsewhere, which led to the first experimental confirmation of electric skyrmions. Hence, in conclusion, the theoretical work in this thesis has been an integral part of the discovery of electric skyrmions in ferroelectric materials. [less ▲]