Strong interfacial electron-phonon coupling at the BaPbO$_3$/BaBiO$_3$ bilayer

The recent discovery of 2D superconductivity at the interface of BaPbO$_3$ (BPO) and BaBiO$_3$ (BBO) has motivated us to study in depth the electronic and structural properties and the relation between them in this particular heterostructure, by means of firs-principles calculations. From the structural relaxations of the simulated bilayer, our results indicate that the breathing distortions, the charge ordering and the semiconducting behaviour that characterize the parent compound BBO in its bulk form, are preserved at the innermost layers of the BBO side of the BPO/BBO bilayer. On the other hand, at the interface, there is a partial breaking of the breathing distortions with a concomitant charge transfer between the interfacial Bi ions and the on top BPO layer. We show that two types of carriers coexist at the interface, the delocalized 3D like $sp$ states coming from Pb ions and the quasi 2D $s$ states from the Bi ones. Remarkably, we obtain a substantial coupling between the 2D Bi states with the interfacial stretching phonon mode and suggest that this coupling may play an important role in the physical mechanism behind the experimentally observed 2D superconductivity. We hope these findings will motivate future research to explore different interfaces with charge ordered semiconductors as BBO in order to trigger this fascinating 2D behavior.