Higher-Order Topology and Nodal Topological Superconductivity in Fe(Se,Te) Heterostructures

We show, theoretically, that a heterostructure of monolayer ${\mathrm{FeTe}}_{1-x}{\mathrm{Se}}_x$—a superconducting quantum spin Hall material—with a monolayer of FeTe—a bicollinear antiferromagnet—realizes a higher order topological superconductor phase characterized by emergent Majorana zero modes pinned to the sample corners. We provide a minimal effective model for this system, analyze the origin of higher order topology, and fully characterize the topological phase diagram. Despite the conventional $s$-wave pairing, we find a rather surprising emergence of a novel topological nodal superconductor in the phase diagram. Featured by edge-dependent Majorana flat bands, the topological nodal phase is protected by an antiferromagnetic chiral symmetry. We also discuss the experimental feasibility, the estimation of realistic model parameters, and the robustness of the Majorana corner modes against magnetic and potential disorder. Our work provides a new experimentally feasible high-temperature platform for both higher order topology and non-Abelian Majorana physics.

Figure 1

(a) Schematic plot of bicollinear antiferromagnetic order in FeTe. The circle and its arrow represent the Fe atom and its magnetic moment. (b) Schematic plot of the FTS/FeTe heterostructure with corner-localized Majorana modes.

Figure 2

With $(A,B,m,M,\mathrm{\Delta })=(1,1,2,0.2,0)$, dispersions of $\tilde{x}$ edge (AFM) and $\tilde{y}$ edge (FM) are plotted in (a) and (b), respectively. (c) Topological phase diagram for a fixed $\mu =0.2$. The white dashed line shows the analytical results of Eq. (4). (d) Energy spectrum of $H_{\mathrm{FTS}}$ with open boundary conditions in both $\tilde{x}$ and $\tilde{y}$ directions, which clearly reveals four Majorana zero modes. (e) Spatial profile of the Majorana zero modes in (d). (f) Topological phase diagram with respect to $M$ and $\mu$ at a fixed $\mathrm{\Delta }$.

Figure 3

(a) Position of SC nodes in the Brillouin zone for the nodal SC phase. (b) Fermi surface of the corresponding normal band structure and the zero-pairing contours (white dashed lines). (c) and (d) show the dispersions of the AFM edge and the FM edge for the nodal SC phase, respectively. The AFM edge hosts symmetry protected Majorana flat bands.