Controlling properties of PbTiO₃/SrTiO₃ superlattices by photo-excited carriers

In the last decade, studies of PbTiO₃/SrTiO₃ superlattices (PTO/STO Sls) showed that they host exotic phenomena such as negative capacitance. These phenomena often accompany real space topological solitons such as flux closure domain polar skyrmions and vortices. Interestingly, recent studies also showed that light is yet another way to induce new properties and even metastable out-of-equilibrium phases in these Sls. One of such phases is the so-called super crystal phase induced by optical excitation. In this work, using a constrained-Density Functional Theory scheme, we report on the impact of the photo-excited carriers on the electrical polarization, internal electrostatic field, and electronic charge density of monodomain PTO/STO Sls. From energetics and electrostatic analysis, we found that the polarization in the direction close to open-circuit conditions (that is the out-of-plane direction) grows under illumination. On the contrary, the polarization in directions close to short-circuit conditions (that are the in-plane directions) is reduced by illumination, as reminiscent of results in bulk ferroelectrics. These photo-induced features therefore result in the rotation of the polarization towards the out-of-plane direction in the superlattices, explaining some recent experimental observations, and yielding a light-induced structural transition. Using standard semiconducting modelling, we further demonstrate that the photo-excited electrons and holes migrate at opposite PTO/STO interfaces to screen bound polarization charges and reduce the electrostatic depolarizing field. This is the main driving force that favors the out-of-plane direction of the polarization under illumination.