The spin-1/2 XXZ chain H = J Σ(SxSx + SySy + ΔSzSz) − h Σ Sz shows magnetization plateaus when the anisotropy Δ > 1 (Ising regime). The plateau at M=0 reflects a gap protected by symmetry.
Anisotropy Δ1.50
Field h / J1.00
Temperature kT / J0.30
H = J Σ(S⁺ᵢS⁻ᵢ₊₁ + h.c.)/2 + ΔSzᵢSzᵢ₊₁ − h·Szᵢ
Ising gap: Δ_gap ≈ J(Δ−1)
Plateau at M=0: exists for Δ > 1
Saturation field: h_s ≈ J(Δ+1)
Simulated via thermal Monte Carlo on classical Sz = ±1/2 spins