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Super-Solid phase in a U(2) symmetric S = 1 Magnet on the Triangular Lattice

A spin supersolid is characterized by the simultaneous breaking of lattice translation and continuous spin rotation symmetries. In this work, we study a spin-1 model with $U(2)\cong SU(2)\times U(1)/Z_2$ symmetry on the triangular lattice, and the phase diagram is figured out using a variational $\mathbb CP^2$ approach. We identify a novel $SU(2)$-supersolid phase which contains a 3-sublattice solid order and a spin-superfluid order. Unlike usual supersolid phases having noncollinear magnetic order and only one Goldstone mode, the $SU(2)$-supersolid phase has collinear Neel order and two Goldstone modes. Another important feature of this supersolid is that the magnon excitation spectrum has symmetry protected double degeneracy in the whole Brillouin zone. As by-products, several other ordered phases are obtained, including the ferromagnetic and the antiferromagnetic states breaking the $SU(2)$ symmetry, as well as genuine phases that completely break the $U(2)$ symmetry. Furthermore, the instabilities of $SU(3)$-flavor linear spin-wave theory are consistent with the phase boundaries between different ordered phases. %dispersions confirm the stability of the classically ordered phases and provides insights into their excitation spectra.