= Global spin-rotation symmetry about one axis
{title2=$[H,S^z_{\mathrm{tot}}]=0$}
If the <Hamiltonian> commutes with total $S^z$, simultaneous spin rotations $e^{-i\phi S^z_{\mathrm{tot}}}$ form a <circle group> <symmetry>. In a state of definite total $S^z$, the transverse one-point averages vanish because $S^x\pm iS^y$ change that eigenvalue. A chosen classical orientation or symmetry-breaking limit requires separate care when constructing a <linear spin-wave approximation>.
Back to article page
