Two-dimensional superconductivity driven by interfacial electron-phonon coupling in a ${\mathrm{BaPbO}}_3/{\mathrm{BaBiO}}_3$ bilayer

The recent discovery of two-dimensional (2D) superconductivity at the interface of $\mathrm{Ba}\mathrm{Pb}{\mathrm{O}}_3$ (BPO) and $\mathrm{Ba}\mathrm{Bi}{\mathrm{O}}_3$ (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 first-principles calculations. Our results indicate that the breathing distortions, the charge ordering, and the semiconducting behavior 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 three-dimensional-like $sp$ states coming from Pb ions and the quasi-2D $s$ states from the Bi ones. We obtain a substantial electron-phonon coupling between the 2D Bi states with the interfacial stretching phonon mode and a large density of states that can explain the critical temperature experimentally observed below 3.5 K. We hope these findings will motivate future research to explore different interfaces with charge-ordered semiconductors such as BBO in order to trigger this fascinating 2D behavior.

Figure 1

(a) Schematic representation of the $\mathrm{Ba}\mathrm{Bi}{\mathrm{O}}_3/\mathrm{Ba}\mathrm{Pb}{\mathrm{O}}_3$ interface. We show half of the structure used in our simulations. Numbers 1–3 represent the plane number. [(b), (c)] Octahedral environment of Bi atoms belonging to the bulk and interface regions, respectively. (Green, Ba; magenta, Bi; red, O; and gray, Pb atoms). (d) Bi ($s$) and Pb ($sp$) partial density of states (PDOS) projected onto the planes labeled in panel (a).

Figure 2

(Top) HSE band structure of the BPO2/BBO6 heterostructure. The $s$ character of interfacial ${\mathrm{Bi}}^{5+}$ (red circles) and ${\mathrm{Bi}}^{3+}$ (blue circles) and the $sp$ character of Pb (dark green circles) are shown. (Bottom) Charge density obtained from the states crossing the Fermi level from below (left) and from above (right) corresponding to charge density mainly centered at the interfacial ${\mathrm{Bi}}^{3+}$ and Pb (and ${\mathrm{Bi}}^{5+}$) atoms, respectively.

Figure 3

HSE bandsstructures of the BPO2/BBO4 heterostrusture in the undistorted (black solid line) and ST (dashed red lines) distortions. In the right insets, the ST frozen phonon displacements are shown. The oxygen atoms's displacements considered are 0.044 Å.

Figure 4

Schematic evolution of $T_c$ for the BBO family of superconductors according to the McMillan modified equation.