Spontaneous symmetry breaking in nonlinear binary periodic systems

Spontaneous symmetry breaking (SSB) occurs when modes of asymmetric profile appear in a symmetric, double-well potential, due to the nonlinearity of the potential exceeding a critical value. In this study, we examine SSB in a periodic potential where the unit cell itself is a symmetric double well, in both one-dimensional (1D) and two-dimensional (2D) periodic systems. Using the tight-binding model, we derive the analytical form that predicts the critical power at which SSB occurs for both 1D and 2D systems. The results show that the critical power depends significantly on the quasimomentum of the Bloch mode, and as the modulus of momentum increases, the SSB threshold decreases rapidly, potentially dropping to zero. These analytical findings are supported by numerical nonlinear eigenmode analysis and direct propagation simulations of Bloch modes.