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High-$T_c$ Superconductor Fe(Se,Te) Monolayer: an Intrinsic, Scalable and Electrically-tunable Majorana Platform

A monolayer of the high-$T_c$ superconductor FeTe$_{1-x}$Se$_x$ has been predicted to realize a topologically non-trivial state with helical edge modes at its boundary, providing a novel intrinsic system to search for topological superconductivity and Majorana zero modes. Evidence in favor of a topological phase transition and helical edge modes has been identified in recent experiments \cite{Peng2019}. We propose to create Majorana zero modes by applying an in-plane magnetic field to the FeTe$_{1-x}$Se$_x$ monolayer and by tuning the local chemical potential via electric gating. Owing to the anisotropic magnetic couplings on edges from a topological band inversion, an in-plane magnetic field drives the system into an intrinsic high-order topological superconductor phase with Majorana corner modes, without fabricating heterostructures. Furthermore, we demonstrate that Majorana zero modes can occur at other different locations, including the domain wall of chemical potentials at one edge and certain type of tri-junction in the 2D bulk. Our study not only demonstrates FeTe$_{1-x}$Se$_x$ monolayer as a promising Majorana platform with scalability and electrical tunability and within reach of contemporary experimental capability, but also provides a general principle to search for realistic realization of high-order topological superconductivity.