Type-B Goldstone boson
= Type-B Goldstone boson
{title2=$\langle[Q_1,Q_2]\rangle\ne0$}
A gapless mode associated with a canonically paired pair of broken symmetry generators. In an isotropic ferromagnet, the nonzero expectation of $[S^x,S^y]=iS^z$ makes two transverse coordinates one dynamical pair, giving one quadratic magnon branch. Broken-generator counting without this pairing would incorrectly predict two independent modes. This contrasts with the linear phase mode of a <superfluid>.