Emergent $U(1)$ Symmetries and $τ$-$σ$ Duality in Gapless Superfluids or Superconductors

Abstract: A superfluid spontaneously breaks the usual $U(1)$ symmetry because of condensation. In this article, we illustrate six classes of emergent $U(1)$ symmetries naturally appear in infrared limits in a broad class of gapless topological superfluids (that either belong to a stable phase or are quantum critical). In gapless states we have considered, emergent $U(1)$ symmetry groups are embedded in an $Spin(4)=SU(2) \otimes SU(2)$ group that are algebraically isomorphic to an $SO(4)$ group. All $U(1)$ charges associated with symmetries are further invariant under an $SU(2)$ spin group or an equivalent of it but always break pre-existing higher space-time Lorentz symmetry of $SO(3,1)$ group. Emergent $U(1)$ symmetries can be further spontaneously broken only if interactions are strong enough and resultant strong coupling states become fully gapped. However if states remain gapless, emergent $U(1)$ symmetries are always present, despite that these states may exhibit much lower space-time symmetries compared to their weakly interacting gapless Lorentz symmetric counter parts. In the limit of our interests, we have identified all possible gapless real fermions with or without Lorentz symmetries and find that they all display emergent $U(1)$ symmetries in the infrared limit.We argue emergent $U(1)$ symmetries in infrared are intrinsic in a broad class of interacting gapless superfluid or superconducting states and are typically well defined in high dimensions where there are infrared stable fixed points dictating emergent properties.