Hydrodynamics of Borromean Counterfluids

Counterflow superfluidity in a system with $N\ge 3$ components is distinctively different from the $N=2$ case. The key feature is the difference between the number ($N$) of elementary vortex excitations and the number ($N-1$) of independent branches of phonon modes, that is, the number of superfluid modes is larger than the number of ordered phase variables. We formulate a hydrodynamic theory of this state. We show how all the dynamical and statistical aspects of this (``Borromean") type of ordering are naturally described by effective $N$-component theory featuring compact-gauge invariance. We also discuss how off-diagonal intercomponent couplings convert the Borromean supercounterfluid into a Borromean insulator, with an emphasis on the properties of a non-trivial state with broken time-reversal symmetry.