2D Mg-Cu Intermetallic Compounds with Nontrivial Band Topology and Dirac Nodal Lines*

Synthesis of vertical heterostructures that include atomically thin layers of materials with topologically nontrivial energy bands is desirable for exploring exotic quantum states. Here, the authors report on atomic-layer-by-layer deposition of magnesium on copper(111) surface by molecular beam epitaxy, monitored in situ by low-energy electron microscopy and diffraction, and modeled by ab initio theory. It is found that a 2D MgCu₂ intermetallic compound forms during initial Mg deposition and persists till a full monolayer of Mg is formed. Deposition of additional Mg triggers a phase transition from the commensurate MgCu₂ to an incommensurate Mg₂Cu layer and enables growth of the second layer of Mg. Ab initio calculations indicate non-trivial topology of the electron bands in the interfacial Mg₂Cu layer and the existence of Dirac nodal lines near the Fermi level. The new 2D Mg₂Cu material emerges as a promising platform to study new topological states of matter.