Strain-Induced Nontrivial Topology and a Negative Poisson’s Ratio in the Sn₂BN Monolayer

Two-dimensional (2D) quantum spin Hall (QSH) insulators have attracted a great deal of attention since they enable the dissipationless transport of electrons, which serve as ideal candidates for designing miniaturized electronic devices. Here, through first-principles calculations, the Sn 2 BN monolayer is proved as a 2D material with excellent thermodynamic stability that can realize the QSH effect under the action of biaxial strain. Such a nontrivial topology is characterized through the band inversion related to Sn-p x, y with B-p z orbitals at the Γ point induced by spin–orbit coupling (SOC) and confirmed by the Z 2 invariant and gapless helical edge states. More interestingly, by employing specific tensile strain, the Sn 2 BN monolayer can host the characteristic of a negative Poisson’s ratio. In addition to biaxial strain, the external electric field is also an effective technique to tune the global band gap of the Sn 2 BN monolayer. Our findings reveal the strain-induced outstanding topological and mechanical properties in the Sn 2 BN monolayer, which provide a platform for designing multifunctional quantum devices.