Superconductivity and topological aspects of two-dimensional transition-metal monohalides

Two-dimensional(2D)superconducting states have attracted much recent interest,especially when they coexist with nontrivial band topology which affords a promising approach towards Majorana fermions.Using first-principles calculations,we predict van der Waals monolayered transition-metal monohalides MX(M=Zr,Mo;X=F,Cl)as a class of 2D superconductors with remarkable transition temperature(5.9–12.4 K).Anisotropic Migdal-Eliashberg theory reveals that ZrCl have a single superconducting gap∆~2.14 meV,while MoCl is a two-gap superconductor with∆~1.96 and 1.37 meV.The Z_(2)band topology of 2D MX is further demonstrated that MoF and MoCl are candidates for realizing topological superconductivity.Moreover,the Dirac phonons of ZrCl and MoCl contribute w-shape phononic edge states,which are potential for an edge-enhanced electron-phonon coupling.These findings demonstrate that 2D MX offers an attractive platform for exploring the interplay between superconductivity,nontrivial electronic and phononic topology.