Topological Phases in (111)-oriented BaBiO3 heterostructures

Abstract

Graphene--a representative two dimensional (2D) system whose low-energy electrons are described by the massless Dirac fermion--has been drawn great attention, and it triggered the outburst of 2D materials research. Among various capable applications, graphene was suggested as a platform for topological electronics; following the quantum spin Hall (QSH) phase pioneered by Kane and Mele, diverse topological phases have been introduced in graphene via generating the mass-gap at the Dirac point. For example, graphene can host the QSH phase through the gap-opening induced by the spin-orbit coupling (SOC), the quantum valley Hall (QVH) phase by the inversion symmetry breaking, and the quantum anomalous Hall (QAH) phase by the time-reversal symmetry breaking, respectively. To be more optimal for topological electronics applications, however, the large SOC and the bandgap tunability are necessary, which are hardly accessible in graphene.

In an attempt to find practical candidates, we suggest that the (111)-oriented BaBiO$_3$-bilayer (BBL) sandwiched by large gap perovskite oxides can provide an ideal platform for topological electronics. The low energy electronic structure of the (111) BaBiO$_3$-bilayer heterostructure is simply governed by the half-filled Bi 6$s$-orbital forming a buckled honeycomb lattice, which results in the $s$-orbital Dirac fermions. The strong hybridization between the neighboring Bi-6$s$ and Bi-6$p$ orbitals enables the large SOC in the $s$-orbital band system. Finally, the abundant order parameters of oxide perovskite materials could break the symmetry in this oxide heterostructure, and open a gap at the Dirac point, which possibly leads to the engineering of the multiple topological phases.

The $s$-orbital Dirac fermion and its various topological phases in the (111) BBL heterostructure emerge from the confluence of three research areas in condensed matter physics: Dirac materials, oxide heterostructures, and topological electronics. It brings the graphene-like electronic structure in the oxide system, modifies and controls the bandgap at the Dirac point, and realizes the versatile topological phases. By taking account of the charge, spin, valley and pseudospin degrees of freedom of the Dirac fermion and the various quantum states of the oxide perovskites, we may find a zoo of topological quantum matters in this combined research area.

Our main results are summarized as follows: We present, based on first-principles calculations combined with the tight-binding analysis, that the perovskite heterostructure of BBL grown along the (111) direction can host QSH and QVH phases with appropriate choices of neighboring layers. When the same materials in the top and bottom layers sandwich the BBL, the $s$-orbital Dirac cone is emerging within the bandgap of sandwich layers. Due to the large spin-orbit coupling of the Bi atom and $s$-$p$ hybridization, the Dirac cone that mainly consists of Bi $s$-orbital could opens a sizable non-trivial gap which turns the system into the QSH phase. For an asymmetric configuration, where the top and bottom layers are different, QVH phase with spin-valley coupling arises as a result of the inversion symmetry breaking. In addition, we suggest a ferroelectric control of topological phases in BaTiO$_3$/BBL/BaTiO$_3$ heterostructure where the QSH and QVH phases can be selected via switching the polarization directions of BaTiO$_3$ layer. The (111) BBL heterostructure is proposed to be a feasible platform for spintronics and valleytronics as well as for topological engineering of the two-dimensional electron system.