Broken U(1) gauge symmetry and excitations in Bose liquids
We derive the low-energy effective field theory at finite temperature of Bose condensed liquids (e.g., superfluid 4He) directly from the breakdown of the global gauge symmetry U(1). The theory can immediately yield the Landau quantum hydrodynamics. The Bogoliubov type spectrum of low-energy collective excitations is given in the zeroth order of this theory and the momentum and temperature dependences of the damping of excitations are obtained by simple power counting. The well-known Hugenholtz-Pines theorem is understood in terms of the nonrelativistic Goldstone theorem. PACS: 67.40.Db, 11.15.Ex, 05.30.Jp