Competition between superconductivity and ferromagnetism in 2D Janus MXH (M = Ti, Zr, Hf, X = S, Se, Te) monolayer

Abstract

Two-dimensional Janus transition-metal chalcogenide hydrides (2D-JTMCs) have generated a lot of interest as potential 2D superconductors, with numerous studies showing a transition from a metallic to a superconducting state at low temperature. They feature a three-layered structure consisting of a central transition metal layer, with chalcogen atoms below and hydrogen atoms above. The opening of a superconducting gap can stabilize a material when a metal has a high density of states at the Fermi level. Magnetic states also commonly result from a high density of states at the Fermi level in the unpolarized material (Stoner, 1938 [1]). In this study, we systematically investigate the instability of the normal state to superconductivity and ferromagnetism in two-dimensional hexagonal 2H and 1T Janus transition-metal chalcogenide hydrides (MXH), where $M=\text{Ti, Zr, Hf}$ and $X=\text{S, Se, Te}$. In the normal metallic phase, Fermi surface instability leads to the formation of Cooper pairs, driving a superconducting phase transition characterized by a finite superconducting gap energy. We estimate the critical temperature ($T_c$) using the Allen–Dynes formula and compute the superconducting gap by solving the anisotropic Migdal–Eliashberg equations, predicting $T_c$ values in the range of 10–35 K. However, most JTMCs favor ferromagnetism over the normal metallic phase, with energy differences of 10–30 meV . Even after accounting for the superconducting gap energy, ferromagnetism remains energetically preferred. Using DFT with an on-site Coulomb interaction $U$, we find that some JTMCs exhibit intrinsic half-metallic ferromagnetism. Our results suggest that a high electronic density of states in the normal metallic phase can drive the emergence of intrinsic half-metallic ferromagnetism, which is essential for spintronic and valleytronic applications.