Spin correlations and <i>B_2g</i> nematicity in extremely hole doped iron pnictides

There is growing experimental evidence for B_2g nematic correlations in the extremely hole doped iron pnictide compounds AFe₂As₂ (A=K,Rb,Cs). In this work, we construct a general Ginzburg-Landau theory for the nematic transitions driven by incommensurate spin correlations. We find various nematic orders depending on the nature of the incommensurate spin correlations, and demonstrate a unified description of the nematicity in both the parent and doped iron pnictides. For the extremely hole doped iron pnictides, our analysis provides a mechanism for a B_2g nematic order developing from the pertinent incommensurate spin fluctuations. We further study an extended bilinear-biquadratic Heisenberg model, which provides additional evidence for the proposed mechanism.