Nonlinear topological edge states, topological gap solitons, and self-induced topological edge states in nonlinear Su-Schrieffer-Heeger circuit lattices

Topological edge states typically arise at the boundaries of topologically nontrivial structures or at interfaces between regions with different topological invariants. When topological systems are extended into the nonlinear regime, linear topological edge states bifurcate into nonlinear counterparts, and topological gap solitons emerge in the bulk of the structures. Extensive studies of nonlinear topological edge states and topological gap solitons have been carried out. Following recent experimental observations in photonic systems, we leverage the strong and tunable nonlinearity of electric circuits and systematically investigate the localized states in nonlinear Su-Schrieffer-Heeger (SSH) circuit lattices. Besides revisiting the nonlinear topological edge states and topological gap solitons, we uncover a new type of self-induced topological edge states which exhibit the hallmark features of linear topological edge states, including sublattice polarization, phase jumps, and decaying tails that approach zero. A distinctive feature of these states is the boundary-induced power threshold for existence. Our work unveils new opportunities for exploring novel nonlinear topological states, and paves the way for the development of nonlinear topological circuits.