Jun Jung1,Yong-Hyun Kim1
Korea Advanced Institute of Science and Technology1
Jun Jung1,Yong-Hyun Kim1
Korea Advanced Institute of Science and Technology1
A Kagome lattice is formed by corner-sharing triangles is known to have many emergent quantum phenomena. At the 1/3 electron filling, the Kagome lattice is semi-metallic with graphene-like Dirac cones. A breathing bond strength anisotropy between upper and lower triangles in this lattice opens the bandgap. This lattice can become a higher-order topological insulator under breathing anisotropy and stronger inter-site hopping (HOTI). Unlike a conventional topological insulator, there is a topologically protected state on a corner of a HOTI that is 2 dimensions lower than the bulk. There have been reports on experimental realizations of breathing Kagome lattices, but none on simple natural materials with an electronic breathing Kagome lattice. Here we theoretically prove that a monolayer hexagonal transition metal dichalcogenide (h-TMD) is an electronic breathing Kagome lattice material, and that it is a HOTI in particular for group 6 transition metal h-TMD. Due to the trigonal prismatic crystal field and <i>C<sub>3</sub></i> rotational symmetry, <i>d</i> orbitals form <i>sp<sup>2</sup></i>-like hybrid <i>d</i> orbitals. Under this hybrid basis, a single type of nearest-neighbor hopping dominates the electronic structure. Considering only this dominant inter-site hopping and an on-site crystal field, the h-TMD becomes a breathing Kagome lattice under this hybrid <i>d</i> orbital basis. As inter-site hopping is found to be stronger than on-site hopping, they are HOTI. These hybrid orbitals are hidden in a bulk wave function. Bulk bonds should be broken in order to directly detect the hybrid orbitals. On an edge of h-TMD nanoribbons, 1D system or hybrid orbitals exist. We also demonstrate that topologically protected corner states exist in h-TMD triangular nanoflakes. For group 6 h-TMDs, these corner states reside inside the bulk bandgap. Because h-TMDs are easily synthesizable and stable at ambient conditions, our findings provide an easily accessible platform for quantum physics on condensed matter systems.