Topological materials are a class of materials that exhibit unique electronic properties at their boundary (surface in 3D materials; edge in 2D materials) that are robust against imperfections or disturbances and are markedly different from their bulk properties. In other words, these materials could be insulators (i.e., resisting the flow of electrons or heat), and yet be conducting at their boundary (i.e., allowing electrons or heat to easily flow through them).