= Linear spin-wave approximation
The <linear spin-wave approximation> expands a <Holstein–Primakoff transformation> about a classical ordered state and retains the quadratic oscillator <Hamiltonian>. It gives the leading order-$S$ excitation energies above the order-$S^2$ classical energy. In a <Heisenberg antiferromagnet>, first make a <bipartite spin rotation>. The approximation also predicts a zero-point reduction of the order parameter; infrared-divergent <quantum depletion of Néel order> signals that the assumed ordered state is not a valid thermodynamic starting point.
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