Changing soft mode dynamics of the polar metal EuTiO₃ with photodoping

EuTiO₃ (ETO) is a paraelectric (PE), antiferromagnetic (AFM) insulator under ambient conditions, but can be driven to a ferromagnetic (FM), ferroelectric (FE) state under sufficient tensile strain—achievable by using DyScO₃ (DSO) as a substrate. First principles simulations of ETO on DSO indicate that the FE order is disrupted under photodoping. The free electrons generated by photodoping occupy Ti d-states, perturbing the Ti-O bond that drives the FE phase transition. In experiment, an initial reduction in polar order for low photodoping concentrations is observed, but this reduction saturates under increasing photodoping at a non-zero polar amplitude. Thus, at high doping levels, strained EuTiO₃ behaves like an intrinsic polar metal, exhibiting a polar distortion which is insensitive to changes at the Fermi level. This transition from doped ferroelectric to polar metallic behavior implies a never-before-seen change in distortion character under doping. We show first principles evidence of changes in the TiO₆ displacement patterns under doping and discuss several hypotheses which may explain this new phenomenon.