Layer dependent magnetism and topology in monolayer and bilayers ReX₃(X=Br, I)

The realization of robust intrinsic ferromagnetism in two-dimensional materials with the possibility to support topologically non-trivial states has provided the fertile ground for novel physics and next-generation spintronics and quantum computing applications. In this contribution, we investigated the formation of topological states and magnetism in monolayer and bilayer systems of ReX₃(X= Br, I), with PBE, ACBN0 (self-consistent Hubbard-U), excluding/including van der Waals (vdW) corrections and/or spin-orbit coupling. Bulk ReX₃(X= Br, I) is predicted to crystallize in space groupR3¯(#148), similar to CrI₃, with monolayer exfoliation energies that are comparable or less than that of graphite. The topological character of the monolayer and bilayer systems of ReX₃(X= Br, I) is derived from anomalous Hall conductivity computations. Topologically non-trivial states in ReX₃(X= Br, I) are absent in the Hubbard-Ucomputations if vdW interactions are included, a prediction that is attributed to the large Hubbard-Udifference between the chemical constituents, ΔU∼ 1.5-1.6 eV, and a significant ∼2.0%-3.6% compressive in-plane strain introduced by vdW interactions. In contrast to the fragile and likely absent topological states in ReX₃(X= Br, I), magnetic properties are robust and independent of the level of theory: ferromagnetic monolayers are coupled antiferromagnetically to bilayers, with an energy separation between ferromagnetic and antiferromagnetic bilayer spin configurations that could be as low as 0.02 meV/Re (f= 4.8 GHz), well within the microwave range. This suggests that layer dependent magnetism in ReX₃(X= Br, I) may support a microwave controllable magnetic qubit, consisting of a superposition of antiferromagnetic and ferromagnetic bilayer states.