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

In the last decade, studies of PbTiO₃/SrTiO₃ (PTO/STO) superlattices showed that they host exotic phenomena such as negative capacitance [1]. These phenomena often accompany real space topological solitons such as flux closure domain polar skyrmions and vortices [1]. Interestingly, recent studies also showed that light is yet another way to induce new properties and even metastable out-of-equilibrium phases in these superlattices. One of such phases is the so-called super crystal phase induced by optical excitation [2]. In this work, using a constrained-Density Functional Theory scheme [3], we study the impact of the photo-excited carriers on the electrical polarization, internal electrostatic field, and electronic charge density of monodomain PTO/STO superlattices. From energetics and electrostatic analysis, we found that the polarization in the direction close to open-circuit conditions grows under illumination. On the contrary, the polarization in directions close to short-circuit conditions is reduced by illumination, as reminiscent of results in bulk ferroelectrics [4]. 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 [5-6], 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.

References
[1] S. Das et al., APL Mat. 2020, 8, 120902

[2] V. A. Stoica et al., Nat. Mater. 2019, 18, 377

[3] C. Paillard et al., Phys. Rev. Lett. 2016, 116, 247401

[4] C. Paillard et al., Phys. Rev. Lett. 2019, 123, 087601

[5] Y. Ahn et al., Phys. Rev. Lett. 2017, 119, 057601

[6] H. J. Lee et al., Phys. Rev. 2021, X 11, 031031