BaTiO 3 ultrathin films (thickness ≈ 1.6 nm) with in-and out-of-plane polarization are studied by first-principles calculations. Out-of-plane polarization is simulated using the method proposed by Shimada et al. [Phys. Rev. B 81, 144116 (2010)], which consists in building a supercell containing small domains with alternating up and down polarization. This allows one to investigate the properties of defect free BaTiO 3 ultrathin films with polarization perpendicular to the surface, as a function of in-plane lattice constant, i.e., epitaxial strain. The configurations with polarization perpendicular to the surface (c phase) are found stable under compressive strain, while under tensile strain, the polarization tends to lie in-plane (aa phase), along [110]. In the c phase, the most stable domain width is predicted to be 1 to 2 lattice constants, and the magnitude of the surface rumpling varies according to the direction of the polarization (upwards versus downwards), though its sign is unchanged, the oxygen anions pointing in all cases outwards. Finally, all the surfaces studied are found to be insulating. Analysis of the atom-projected electronic density of states gives insight into the surface contributions to the electronic structure. An important reduction of the Kohn-Sham band gap is predicted at TiO 2 terminations in the c phase (≈ 1 eV with respect to the aa phase). The Madelung potential at the surface plays the dominant role in modifications of the surface electronic structure.
We present a spatial and wave-vector resolved study of the electronic structure of micron sized ferroelectric domains at the surface of a BaTiO(3)(001) single crystal. The n-type doping of the BaTiO(3) is controlled by in situ vacuum and oxygen annealing, providing experimental evidence of a surface paraelectric-ferroelectric transition below a critical doping level. Real space imaging of photoemission threshold, core level and valence band spectra show contrast due to domain polarization. Reciprocal space imaging of the electronic structure using linearly polarized light provides unambiguous evidence for the presence of both in- and out-of-plane polarization with two- and fourfold symmetry, respectively. The results agree well with first principles calculations.
We have used low energy electron microscopy (LEEM) to study the evolution of the surface structure of BaTiO3(001) during the ferroelectric-paraelectric phase transition (120 °C, P4mm-Pm3m). Transient surface structures appear under heating at temperatures slightly above TC. Intersections between polar domain walls and the sample surface persist in the surface layer at 126.0 °C while the bulk has already transformed into the cubic phase. The wall signals are criss-crossed by a second set of stripe patterns with roughly perpendicular orientation at 126.3 °C. These surface patterns coarsen under further heating to 126.9 °C. The LEEM image is dominated in the ferroelectric state by intersections of ferroelastic/ferroelectric 90° walls and the surface. The intersection lines are charged.
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