Rhombohedral Sb$_{\mathrm{2}}$Se$_{\mathrm{3}}$ as an intrinsic topological insulator due to strong van der Waals inter-layer coupling

Topological insulators are a new class of quantum materials, which have insulating energy gaps in bulk form, but exhibit robust gapless surface states. It was theoretically predicted and experimentally confirmed that the binary tetradymites Bi$_{\mathrm{2}}$Te$_{\mathrm{3}}$, Bi$_{\mathrm{2}}$Se$_{\mathrm{3}}$, and Sb$_{\mathrm{2}}$Te$_{\mathrm{3}}$ are three-dimensional topological insulators. In this talk, we demonstrate by first-principles approach that the prevailingly believed trivial system of Sb$_{\mathrm{2}}$Se$_{\mathrm{3}}$ with relatively weaker spin-orbital coupling, is actually also a topological insulator, as characterized by its topologically protected surface states and the $Z_{\mathrm{2}}$ invariant. The underlying reason is the ubiquitous van der Waals interaction between quintuple layers. We also generalize our considerations to several related systems in an attempt to draw generic guiding principles.