New SubmissionElectronic excitations in 2D bilayers: Quantum Monte Carlo study of phosphorene, MoS₂, and h-BN

Two-dimensional (2D) materials are perhaps the currently most studied materials due to their unique electronic, optical, thermal, spin, and magnetic properties. New ways are explored to artificially stack 2D layers interacting via van der Waals (vdW) interactions in the so-called 2.5D materials with unique physical properties. Previously the effect of layering in 2D vdW materials was studied at the QMC-level for various bilayers but only in ground state. Here, using QMC methods, we study quasiparticle band gaps of homo vdW bilayers of phophorene, MoS₂, and h-BN, and describe the complexities brought about by adding the excited states. We found that electronic structure of 2D bilayers is affected by a number of factors, such as the nature of the interlayer interaction (vdW vs. partially covalent), degeneracy of the bands at a given k-point, supercell size, and time step error of the diffusion Monte Carlo. In pure vdW bilayers, such as h-BN, at small supercell sizes artificial excitons localized on one layer only are formed. Such excitons do not form at larger supercell sizes which in turn are more prone to time step errors, making such calculations computationally very demanding. After taking into account all the mentioned factors we have been able to highly accurately model the Γ → Γ excitation in phosphorene, Γ → K and Γ → K/2 excitations in MoS₂, and K → K and K → M excitations in h-BN.