Structural and electronic properties of monodomain ultrathin PbTiO_3/SrTiO_3/PbTiO_3/SrRuO_3 heterostructures: A first-principles approach

摘要

First-principles calculations within the local density approximation were carried out to explain the ground state and electronic properties of a vacuum/PbTiO_3/SrTiO_3/PbTiO_3/SrRuO_3 multilayer in a monodomain phase. Open-circuit boundary conditions were assumed, considering the electric displacement field, D, as the fundamental electrical variable. The direction and the magnitude of D can be monitored by proper treatment of the PbO surface layer, introducing external fractional charges Q in the surface atomic layers by means of virtual crystal approximation. Different excess or deficit surface charges (from Q = ± 0.05 to Q = ±0.15) were considered, corresponding to small values of the polarization (up to ±0.16 C/m~2) in both directions. The layer-by-layer electric polarization, tetragonality, and the profile of the electrostatic potential were computed, as well as the projected density of states, as a function of electric displacement field. The magnitude of D is preserved across the dielectric layers, which translates into a polarization of the SrTiO_3 spacer layer. The tetragonality of the two PbTiO_3 layers is different, in good agreement with experimental x-ray diffraction techniques, with the layer closer to the free surface exhibiting a smaller value. This is attributed to the interplay with surface effects that tend to contract the material in order to make the remaining bonds stronger. Our calculations show how the final structure in this complex oxide heterostructure comes from a delicate balance between electrical, mechanical, and chemical boundary conditions.