XXZ Chain — Bethe Ansatz

H = -J Σ[SˣSˣ+SʸSʸ+ΔSᶻSᶻ]
Bethe eqs: e^(ik_j N) = Π_{l≠j} S(k_j,k_l)
S(k,q) = (e^i(k+q)+1-2Δe^ik) / (e^i(k+q)+1-2Δe^iq)
Δ=1: Heisenberg; Δ=0: XX; |Δ|>1: gapped