Figure 2. Susceptibility near commensurate-incommensurate transition in t − t′-model.

For (a) t′ = −0.345t and (b) t′ = −0.267t. (c) A simple ‘Mexican hat’ model, , for , with A₄ > 0 and varying A₂: A₂ > 0 (red line), χ = 0 (blue line), and <0 (green line).