Electronic Structures, Stabilities, and Magnetism of SrTiO 3 Monolayer and Ultrathin Nanotubes

Based on density functional theory, we predict the electronic structures, stabilities, and magnetism of ultrathin SrTiO3 nanotubes (u-STONTs) with different terminals and curvatures, which are constructed by rolling up the rectangular (001) STO monolayer. By comparing the total energies and cohesive energies of STO monolayer and u-STONTs, it is found that the STO monolayer is more stable than TiO2-out terminal NTs, but not as stable as SrO-out terminal NTs, where (18, 0) SrO-out NT is the most stable among various tubular structures. In STO monolayer, the stable ferromagnetic coupling is closely related to Ti vacancies instead of O vacancies, although Ti vacancy is more difficult to generate in any conditions. On the contrary, Ti vacancies are more likely to emerge in SrO-out NTs under O-rich condition, and it can induce a large magnetic moment and ferromagnetic coupling. Under O-poor condition, O vacancies are much easier to form in NTs, and the magnetic coupling induced by O vacancies depends upon the kinds of vacancies. And the absence of twofold-coordinated O atoms in the axial direction can lead to a long-range ferromagnetic coupling in SrO-out NT.