In this work, we report that ${\mathrm{Bi}}_2{\mathrm{Ta}}_3{\mathrm{S}}_6$-family superconductors exhibit nontrivial band topology. They possess a natural quantum-well structure consisting of alternating stacks of ${\mathrm{TaS}}_2$ and honeycomb Bi layers, which contribute superconducting and topological properties, respectively. Symmetry-based indicators $({\mathbb{Z}}_4;{\mathbb{Z}}_2{\mathbb{Z}}_2{\mathbb{Z}}_2)=(2;000)$ reveal that the topological nature arises entirely from the Bi layers, which belong to a quantum spin Hall phase characterized by a $p_x\text{−}{p}_y$ model on a honeycomb lattice. The topological zigzag (ZZ) and armchair (AC) edge states are obtained. Using vasp2kp, the in-plane $g$ factors of these topological edge states are computed from the ab initio calculations: $g_{x/y}^{\mathrm{ZZ}}=2.07/1.60$ and $g_{x/y}^{\mathrm{AC}}=0.50/0.06$. The strong anisotropy of the edge-state $g$ factors allows us to explore Majorana zero modes in the Bi monolayer on a superconductor, which can be obtained by exfoliation or molecular beam epitaxy. Furthermore, using the experimental superconducting gap $\mathrm{\Delta }$ and the computed $g$ factors, we obtain the phase diagram, which shows that the in-plane field $B_y>2.62\mathrm{T}$ can generate corner Majorana zero modes in the Bi monolayer of the superconductor ${\mathrm{Bi}}_2{\mathrm{Ta}}_3{\mathrm{S}}_6$. A similar paradigm also applies to the ${\mathrm{Bi}}_2{\mathrm{Ta}}_3{\mathrm{S}}_6$ bulk with the emergence of Majorana hinge states. These natural quantum-well superconductors therefore offer ideal platforms for exploring topological superconductivity and Majorana zero modes.

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• Majorana zero modes
• Topological edge modes
• Topological superconductors
• Honeycomb lattice
• Topological materials
• Bogoliubov-de Gennes equations
• Density functional calculations