Despite considerable interest in layered transition metal dichalcogenides (TMDs), such as MX 2 with M = (Mo, W) and X = (S, Se, Te), the physical origin of their topological nature is still poorly understood. In the conventional view of topological phase transition (TPT), the non-trivial topology of electron bands in TMDs is caused by the band inversion between metal d and chalcogen p orbital bands, where the former is pulled down below the latter. Here, we show that, in TMDs, the TPT is entirely different from the conventional speculation. In particular, MS 2 and MSe 2 exhibits the opposite behavior of TPT, such that the chalcogen p orbital band moves down below the metal d orbital band. More interestingly, in MTe 2 , the band inversion occurs between the metal d orbital bands. Our findings cast doubts on the common view of TPT and provide clear guidelines for understanding the topological nature in new topological materials to be discovered.2

Diamond Si is a semiconductor with an indirect band gap that is the basis of modern semiconductor technology. Although many metastable forms of Si were observed using diamond anvil cells for compression and chemical precursors for synthesis, no metallic phase at ambient conditions has been reported thus far. Here we report the prediction of pure metallic Si allotropes with open channels at ambient pressure, unlike a cubic diamond structure in covalent bonding networks. The metallic phase termed P6/m-Si_{6} can be obtained by removing Na after pressure release from a novel Na-Si clathrate called P6/m-NaSi_{6}, which is predicted through first-principles study at high pressure. We identify that both P6/m-NaSi_{6} and P6/m-Si_{6} are stable and superconducting with the critical temperatures of about 13 and 12 K at ambient pressure, respectively. The prediction of new Na-Si and Si clathrate structures presents the possibility of exploring new exotic allotropes useful for Si-based devices.

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The ability to create graphene nanoribbons with atomically sharp edges is important for various graphene applications because these edges significantly influence the overall electronic properties and support unique magnetic edge states. The discovery of graphene self‐folding induced by traveling wave excitation through atomic force microscope scanning under a normal force of less than 15 nN is reported. Most remarkably, the crystallographic direction of self‐folding may be either along a chosen direction defined by the scan line or along the zigzag or armchair direction in the presence of a pre‐existing crack in the vicinity. The crystalline direction of the atomically sharp edge is confirmed via careful lateral force microscopy measurements. Multilayer nanoribbons with lateral dimensions of a few tens of nanometers are realized on the same graphene sheet with different folding types (e.g., z‐type or double parallel). Molecular dynamics simulations reveal the folding dynamics and suggest a monotonic increase of the folded area with the applied normal force. This method may be extended to other 2D van der Waals materials and lead to nanostructures that exhibit novel edge properties without the chemical instability that typically hinders applications of etched or patterned graphene nanostructures.