title: Redundancies in Nambu-Goldstone bosons.
abstract: We propose a simple criterion to identify when Nambu-Goldstone bosons for different symmetries are redundant. It solves an old mystery why crystals have phonons for spontaneously broken translations but no gapless excitations for equally spontaneously broken rotations. Similarly for a superfluid, the Nambu-Goldstone boson for spontaneously broken Galilean symmetry is redundant with phonons. The most nontrivial example is Tkachenko mode for a vortex lattice in a superfluid, where phonons are redundant to the Tkachenko mode which is identified as the Boboliubov mode.
authors.affiliation: Department of Physics, University of California, Berkeley, California 94720, USA. hwatanabe@berkeley.edu
authors.forename: Haruki
authors.initials: H
authors.lastname: Watanabe
authors.forename: Hitoshi
authors.initials: H
authors.lastname: Murayama