Non-Hermitian boundary and interface states in nonreciprocal higher-order topological metals and electrical circuits

Non-Hermitian skin-edge states emerge only at one edge in one-dimensional nonreciprocal chains, where all states are localized at the edge irrespective of eigenvalues. The bulk topological number is the winding number associated with the complex energy spectrum, which is well defined for metals. We study non-Hermitian nonreciprocal systems in higher dimensions, and propose to realize them with the use of electric diode circuits. We first investigate one-dimensional interface states between two domains carrying different topological numbers, where all states are localized at the interface. They are a generalization of the skin-edge states. Then we generalize them into higher dimensions. We show that there emerge a rich variety of boundary states and interface states including surface, line, and point states in three-dimensional systems. They emerge at the boundaries of several domains carrying different topological numbers. The resulting systems are the first-order, second-order, and third-order topological metals. Such states may well be observed by measuring the two-point impedance in diode circuits.