Nonlinearity-induced topological phase transition characterized by the nonlinear Chern number

TL;DR

This paper introduces a nonlinear extension of the Chern number to characterize topological phases in nonlinear systems, revealing amplitude-dependent topological transitions and boundary modes, thus expanding topological classification into the nonlinear regime.

Contribution

It proposes a nonlinear Chern number based on nonlinear eigenvalue problems, demonstrating bulk-boundary correspondence and topological phase transitions in nonlinear systems.

Findings

Nonlinear Chern number depends on oscillation amplitude.

Existence of amplitude-dependent topological edge modes.

Confirmation of nonlinear bulk-boundary correspondence.

Abstract

As first demonstrated by the characterization of the quantum Hall effect by the Chern number, topology provides a guiding principle to realize robust properties of condensed matter systems immune to the existence of disorder. The bulk-boundary correspondence guarantees the emergence of gapless boundary modes in a topological system whose bulk exhibits nonzero topological invariants. Although some recent studies have suggested a possible extension of the notion of topology to nonlinear systems such as photonics and electrical circuits, the nonlinear counterpart of topological invariant has not yet been understood. Here, we propose the nonlinear extension of the Chern number based on the nonlinear eigenvalue problems in two-dimensional systems and reveal the bulk-boundary correspondence beyond the weakly nonlinear regime. Specifically, we find the nonlinearity-induced topological phase transitions, where the existence of topological edge modes depends on the amplitude of oscillatory modes. We propose and analyze a minimal model of a nonlinear Chern insulator whose exact bulk solutions are analytically obtained and indicate the amplitude dependence of the nonlinear Chern number, for which we confirm the nonlinear counterpart of the bulk-boundary correspondence in the continuum limit. Thus, our result reveals the existence of genuinely nonlinear topological phases that are adiabatically disconnected from the linear regime, showing the promise for expanding the scope of topological classification of matter towards the nonlinear regime.