2020 Volume 75 Issue 5 Pages 289-294
We review a recent progress on higher-order topological systems. We start with a brief history of higher-order topological insulators (HOTIs). Then, we discuss two types of HOTIs. One is the Wannier-type HOTIs, which are extensions of the Su–Schrieffer–Heeger model, and the others are mass-type HOTIs, which are constructed based on topological insulators by adding extra terms. We also explain how these systems are realized in electric circuits. Finally, we extend the notion to non-Hermitian topological systems, where non-Hermitian terms are introduced by dissipation or nonreciprocity.