Odd-even alternating exchange coupling and optical properties of 2D 2H-VSe2 film

Recently, extensive efforts have been focused on the magnetic properties of the 2H VSe2 system. However, the layer-to-layer magnetic exchange coupling in the 2H VSe2 system is still a controversial issue. Thus, we investigated thickness dependent exchange coupling by changing the film thickness from monolayer to four-layer and optical properties. The monolayer structure had a ferromagnetic (FM) ground state. However, with increasing thickness, we found layer-to-layer antiferromagnetic exchange coupling like in the well-known CrI3 system. So, the net magnetic moment of 1 mu B was found in the odd number of layers system, whereas no net magnetic moment was found in the even number of layers structure. In the monolayer, we obtained a direct band gap of 1.19 eV. Except for this monolayer, all other systems had an indirect band gap, and the band gap was decreased with increasing thickness. We also obtained an in-plane magnetic anisotropy at least up to four-layer thickness. The main contribution to the in-plane anisotropy originated from the Se atom because the in-plane spin-orbit coupling strength from the Se atom was almost twice larger than that of the V atom. In the trilayer structure, we obtained a very large refractive index of 2.7 and optical transparency of 75% in the visible frequencies.