Supersolid states in a spin system: Phase diagram and collective excitations

Abstract

Phases analogous to supersolids can be realized in spin systems. Here we have obtained, using a bond-operator representation, a mean-field theory and a generalized spin-wave theory, the phase diagram of a frustrated dimer-spin-$\frac{1}{2}$ system on a square lattice and study the collective excitation spectra, focusing on the supersolid (SS) state. In the phase diagram on a parameter space of the exchange interaction and magnetic field, we have identified not only the SS phase, but a phase that has no counterpart in the Bose-Hubbard model and this state becomes dominant at the region where the enhancement of SS occurs in the Bose-Hubbard model. We then investigate the excitation spectrum and spin-spin correlation, detectable by neutron-scattering experiments. We obtain an analytic expression for the spin-wave velocity, which agrees with hydrodynamic relations. The intensity of excitation modes in the spin-spin correlation function is calculated and their change in the supersolid and superfluid states is discussed.