Spin effects at mirror twin grain boundaries in monolayered dichalcogenides*

First-principles calculations are used to study the origin of the charge density wave observed in 1D inversion domain grain boundaries in monolayered MoSe₂. A small structural (Peierls) distortion with the same periodicity observed in the STM experiments can be reproduced. However, an opening of the gap does not require this distortion, which can be obtained with a suitable spin configuration for the edge electronic states. Spin-orbit effects on these peculiar interfacial states are also discussed. Remarkably, similar spin configurations can also be obtained for edge states in zigzag-edged domains.