One-dimensional planar spin-wave depletion 2026-10-25
The Bogoliubov vacuum has original-boson occupation , reducing the ordered spin by its average over momenta. For the zero-field planar ferromagnetic chain, . Even after excluding the exact uniform zero mode, the sum grows as . It invalidates a thermodynamic fixed-orientation expansion with finite dilute depletion; it does not represent a negative physical magnetization.