title: Criterion for stability of Goldstone modes and Fermi liquid behavior in a metal with broken symmetry.
abstract: There are few general physical principles that protect the low-energy excitations of a quantum phase. Of these, Goldstone's theorem and Landau-Fermi liquid theory are the most relevant to solids. We investigate the stability of the resulting gapless excitations--Nambu-Goldstone bosons (NGBs) and Landau quasiparticles--when coupled to one another, which is of direct relevance to metals with a broken continuous symmetry. Typically, the coupling between NGBs and Landau quasiparticles vanishes at low energies, leaving the gapless modes unaffected. If, however, the low-energy coupling is nonvanishing, non-Fermi liquid behavior and overdamped bosons are expected. Here we prove a general criterion that specifies when the coupling is nonvanishing. It is satisfied by the case of a nematic Fermi fluid, consistent with earlier microscopic calculations. In addition, the criterion identifies a new kind of symmetry breaking--of magnetic translations--where nonvanishing couplings should arise, opening a previously unidentified route to realizing non-Fermi liquid phases.
authors.affiliation: Department of Physics, University of California, Berkeley, CA 94720; and hwatanabe@berkeley.edu ashvinv@socrates.berkeley.edu.
authors.forename: Haruki
authors.initials: H
authors.lastname: Watanabe
authors.affiliation: Department of Physics, University of California, Berkeley, CA 94720; and Materials Science Division, Lawrence Berkeley National Laboratories, Berkeley, CA 94720 hwatanabe@berkeley.edu ashvinv@socrates.berkeley.edu.
authors.forename: Ashvin
authors.initials: A
authors.lastname: Vishwanath
keywords: Goldstone modes; non-Fermi liquids; spontaneous symmetry breaking; strong magnetic fields