Citation: Hussain, G.; Fakhredine, A.; Islam, R.; Sattigeri, R.M.; Autieri, C.; Cuono, G. Correlation-Driven Topological Transition in Janus Two-Dimensional Vanadates. Materials 2023, 16, 1649. https:// doi.org/10.3390/ma16041649 Academic Editor: Rüdiger Schmidt-Grund Received: 10 January 2023 Revised: 9 February 2023 Accepted: 13 February 2023 Published: 16 February 2023 Copyright: © 2023 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). materials Article Correlation-Driven Topological Transition in Janus Two-Dimensional Vanadates Ghulam Hussain 1 , Amar Fakhredine 2 , Rajibul Islam 1 , Raghottam M. Sattigeri 1 , Carmine Autieri 1, * and Giuseppe Cuono 1 1 International Research Centre MagTop, Institute of Physics, Polish Academy of Sciences, Aleja Lotników 32/46, 02668 Warsaw, Poland 2 Institute of Physics, Polish Academy of Sciences, Aleja Lotników 32/46, 02668 Warsaw, Poland * Correspondence: autieri@magtop.ifpan.edu.pl Abstract: The appearance of intrinsic ferromagnetism in 2D materials opens the possibility of inves- tigating the interplay between magnetism and topology. The magnetic anisotropy energy (MAE) describing the easy axis for magnetization in a particular direction is an important yardstick for nanoscale applications. Here, the first-principles approach is used to investigate the electronic band structures, the strain dependence of MAE in pristine VSi 2 Z 4 (Z = P, As) and its Janus phase VSiGeP 2 As 2 and the evolution of the topology as a function of the Coulomb interaction. In the Janus phase the compound presents a breaking of the mirror symmetry, which is equivalent to having an electric field, and the system can be piezoelectric. It is revealed that all three monolayers exhibit ferromagnetic ground state ordering, which is robust even under biaxial strains. A large value of coupling J is obtained, and this, together with the magnetocrystalline anisotropy, will produce a large critical temperature. We found an out-of-plane (in-plane) magnetization for VSi 2 P 4 (VSi 2 As 4 ), and an in-plane magnetization for VSiGeP 2 As 2 . Furthermore, we observed a correlation-driven topological transition in the Janus VSiGeP 2 As 2 . Our analysis of these emerging pristine and Janus-phased mag- netic semiconductors opens prospects for studying the interplay between magnetism and topology in two-dimensional materials. Keywords: correlation-driven topological transition; vanadates; density functional theory; 2D ferromagnetism 1. Introduction Since the observation of intrinsic ferromagnetism in two-dimensional layered materials (2D) such as CrGeTe 3 [1] and CrI 3 [2], the fields of magnetism and spintronics have received tremendous research attention in the 2D limit [3–14]. The atomically thin 2D magnetic materials are considered ideal systems, where the magnetic and spin-related features can effectively be controlled and modulated via proximity effects, electric field, magnetic field, strain, defects and optical doping [15–22]. Unlike bulk materials, where magnetic ordering is possible without magnetic anisotropy, long-range magnetic ordering in layered 2D materials is not conceivable in systems deprived of magnetic anisotropy, which is necessary to balance out thermal fluctuations [23]. Due to the fact that magnetic anisotropy is primarily caused by spin-orbit coupling (SOC) effects [24], SOC becomes a crucial characteristic. Furthermore, spintronic devices such as magnetic tunnel junctions and spin valves show enhanced performance based on 2D magnetic structures with substantial magnetic anisotropy [25–27]. It has been demonstrated that strain engineering is an effective method of tuning the magnetic, electronic and optical characteristics of materials [28–33]. The recently discovered new family of 2D layered materials MA 2 Z 4 , where M, A and Z represent the transition metal atoms (Mo, W, Hf, Cr, V), IV-elements (Si, Ge) and V-elements (N, As, P), respectively [34], has sparked intense interest in different studies [35–44]. These Materials 2023, 16, 1649. https://doi.org/10.3390/ma16041649 https://www.mdpi.com/journal/materials