Observation of corner states in second-order topological electric circuits

Recently, the topological states in electric circuits have attracted a great deal of attention, and they have formed the basis of the field of so-called topoelectrical circuits. We realize a second-order topological electric circuit with capacitors and inductors, and we observe the corner states that exist in the bulk gap, the continuum of bulk modes, and the dependence on the corner shapes by measuring the resonance peaks of the corner impedance. More importantly, we broaden the second-order topological circuit to the non-Hermitian region by adding resistors. Interestingly, the non-Hermiticity tunes the amplitude of the corner impedance; however, it has no effect on the topological properties. Our system provides a highly tunable platform in which to explore the intriguing topological properties of higher-order topological insulators.