Large-gap quantum anomalous Hall states induced by functionalizing buckled Bi-III monolayer/Al${}_2$O${}_3$

Abstract

Chiral edge modes inherent to the topological quantum anomalous Hall (QAH) effect are a pivotal topic of contemporary condensed matter research aiming at future quantum technology and application in spintronics. A large topological gap is vital to protecting against thermal fluctuations and thus enabling a higher operating temperature. From first-principle calculations, we propose Al${}_2$O${}_3$ as an ideal substrate for atomic monolayers consisting of Bi and group-III elements, in which a large-gap quantum spin Hall effect can be realized. Additional half-passivation with nitrogen then suggests a topological phase transition to a large-gap QAH insulator. By effective tight-binding modelling, we demonstrate that Bi-III monolayer/Al${}_2$O${}_3$ is dominated by $p_x,p_y$ orbitals, with subdominant $p_z$ orbital contributions. The topological phase transition into the QAH is induced by Zeeman splitting, where the off-diagonal spin exchange does not play a significant role. The effective model analysis promises utility far beyond Bi-III monolayer/Al${}_2$O${}_3$, as it should generically apply to systems dominated by $p_x,p_y$ orbitals with a band inversion at $\Gamma$.

9 pages with 4 figures

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