Topological and superconducting properties of monolayered CoN and CoP: A first-principles comparative study

Abstract

Two-dimensional systems that simultaneously harbor superconductivity and nontrivial band topology may serve as appealing platforms for realizing topological superconductivity with promising applications in fault-tolerant quantum computing. Here, based on first-principles calculations, we show that monolayered CoN and CoP with the isovalent FeSe-like structure are stable in freestanding form, even though their known bulk phases have no resemblance to layering. The two systems are further revealed to display intrinsic band inversions due to crystal field splitting, and such orderings are preserved with the inclusion of spin-orbit coupling (SOC), which otherwise is able to open a curved band gap, yielding a non-zero Z₂ topological invariant in each case. Such a mechanism of topologicalization is distinctly contrasted with that identified recently for the closely related monolayers of CoX (X = As, Sb, Bi), where the SOC plays an indispensable role in causing a nontrivial band inversion. Next, we demonstrate that, by applying equi-biaxial tensile strain, the electron-phonon coupling strength in monolayered CoN can be significantly enhanced, yielding a superconducting transition temperature (T_c) up to 7–12 K for the Coulomb pseudopotential of μ* = 0.2–0.1, while the CoP monolayer shows very low T_c even under pronounced strain. Their different superconducting behaviors can be attributed to different variations in lattice softening and electronic density of states around the Fermi level upon pressuring. Our central findings enrich the understanding of different mechanisms of band inversions and topologicalization and offer platforms for achieving the coexistence of superconductivity and nontrivial band topology based on two-dimensional systems.