| Abstract [eng] |
Strontium titanate (SrTiO3) is an incipient ferroelectric perovskite whose structural and polar properties are highly sensitive to lattice distortions and reduced dimensionality. In twisted bilayered SrTiO3 the moiré geometry introduces spatially varying local atomic environments, providing an additional degree of freedom to modify the structural be- haviour and polarization through the twist angle. This thesis investigates how these properties evolve with twist angle using a combination of first-principles calculations and machine-learning-based atomistic simulations. Density functional theory (DFT) calculations were used to generate the reference dataset for both bulk and layered SrTiO3 systems. The datasets were used to train sep- arate Allegro/NequIP-based machine-learning interatomic potentials which were sub- sequently employed with LAMMPS to relax bulk and twisted bilayered structures. This approach enabled the investigation of large moiré supercells which would be computa- tionally demanding to treat directly with DFT. The relaxed bulk SrTiO3 system was analyzed over a range of lattice parameters to identify characteristic structural regions, including I4/mcm-, I4cm-, Ima2-, and R3c- like configurations, which were used as references for interpreting the local structural environments of the twisted bilayers. The relaxed twisted bilayered structures develop strongly nonuniform polarization patterns that vary with twist angle. The out-of-plane polarization (P z) exhibits spa- tially distributed regions of positive and negative polarization across the moiré super- cell. The relative in-plane and out-of-plane polarization contributions evolve approxi- mately symmetrically around 45.041◦. At small twist angles, the in-plane polarization forms vortex-like textures, whereas at intermediate angles, strong out-of-plane polariza- tion components emerge. Local structural analysis reveals a corresponding evolution of the identifiable bulk-reference-like environments, from Ima2-like character at 10.389◦, through I4cm-like character near 45.041◦, to stronger I4/mcm-like character at 83.267◦, although mixed and low-symmetry environments remain dominant throughout. Overall, these results establish a connection between twist angle, local structural reconstruction and spatial organization of the polarization field, highlighting moiré engineering as a route for generating complex polar states in twisted SrTiO3 bilayers. |
