Prediction of ultra-flat bands in twisted moiré oxides

The emergence of flat bands in moiré superlattices of twisted bilayer structures has recently gathered significant attention. Flat bands feature new phases and electronic behavior due to strong electronic correlations. In this work, driven by the recent experimental demonstrations of free-standing oxide membranes [1,2], we explore electronic properties of twisted oxide membranes. These membranes are expected to provide a much stronger coupling between the layers than typical two-dimensional van der Waals structures and thus may lead to a broader spectrum of exotic properties. We consider a prototypical oxide, SrTiO₃, and design SrTiO₃ bilayers with a relative twist between the layers. Using calculations based on density functional theory, we predict the emergence of ultra-flat bands in these bilayers at relatively high twist angles. These bands are formed due to the strong localization of electronic states exhibiting moiré periodicity. Using a tight-binding approach, we create simple toy models which qualitatively explain our results. With proper doping, the flat bands at the Fermi energy may give rise to superconductivity, Mott insulating phases, and other strongly correlated properties.

1. D. X. Ji et al., Nature 570, 87 (2019).

2. H. S. Kum et al., Nature 578, 75 (2020).