Rise and fall of insulating state in oxide perovskites

Paramagnetic (PM) ABO3 perovskites appear either as metals or as insulators or as systems able to interconvert between such phases. Here, we demonstrate the role of structural and magnetic symmetry breaking in PM phases identifying: (i) False PM metals, predicted incorrectly to be metals because of ignoring symmetry breaking energy lowering whereas in reality they are PM insulators (e.g., PM-SrMnO3); (ii) Real PM insulators due to effective symmetry breaking leading to stabilization and gapping (e.g., JT distorted PM-LaMnO3); (iii) Extrinsic metals formed by application of external knobs such as temperature or pressure on true insulators, suppressing symmetry breaking thus restoring (i) as an actual (not false) metal (e.g., PM-LaTiO3). (iv) Intrinsic true metal not benefiting from symmetry breaking, i.e., when symmetry breaking does not open the band gap (e.g., PM-SrVO3). We will show that the rise of the insulating state emerges when (i) is replaced by (ii), whereas the fall of the insulating state and emergence of narrow-gap state emerges when (ii) transforms to (iii) under the application of external knobs, or when or the would-be Mott insulator (ii) lacks the ability to symmetry break and is this naturally a metal (iv).