A pedagogic review on designing model topological insulators

Abstract

Following the centuries old concept of the quantization of flux through a Gaussian curvature (Euler characteristic) and its successive dispersal into various condensed matter properties such as quantum Hall effect, and topological invariants, we can establish a simple and fairly universal understanding of various modern topological insulators (TIs). Formation of a periodic lattice (which is a non-trivial Gaussian curvature) of 'cyclotron orbits' with applied magnetic field, or 'chiral orbits' with fictitious 'momentum space magnetic field' (Berry curvature) guarantees its flux quantization, and thus integer quantum Hall (IQH), and quantum spin-Hall (QSH) insulators, respectively, occur. The bulk-boundary correspondence associated with all classes of TIs dictates that some sort of pumping or polarization of a 'quantity' at the boundary must be associated with the flux quantization or topological invariant in the bulk. Unlike charge or spin polarizations at the edge for IQH and QSH states, the time-reversal (TR) invariant Z2 TIs pump a mathematical quantity called 'TR polarization' to the surface. This requires that the valence electron's wavefunction (say, ψ_{\uparrow} (k)) switches to its TR conjugate (ψ_{\downarrow}^* (-k)) odd number of times in half of the Brillouin zone. These two universal features can be considered as 'targets' to design and predict various TIs. For example, we demonstrate that when two adjacent atomic chains or layers are assembled with opposite spin-orbit coupling (SOC), serving as the TR partner to each other, the system naturally becomes a Z2 TI. This review delivers a holistic overview on various concepts, computational schemes, and engineering principles of TIs.