We study theoretically the collective excitations in a spin-1 $XY$ model with a quadratic Zeeman term and a long-range interaction that decays algebraically with the distance. Using the quantum-field theory based on the finite-temperature Green's function formalism, we analyze properties of the Nambu–Goldstone (NG) and Higgs modes in order to analytically evaluate the damping rate of the Higgs mode in the $XY$-ferromagnetic ordered phase near the quantum phase transition to the disordered phase. When the power of the algebraic decay is 3, as in the case of dipole-dipole interactions in Rydberg-atom systems, we show that in two dimensions the excitation energy of the Higgs mode exhibits a linear dispersion, whereas the dispersion of the NG mode becomes proportional to the square root of the momentum. We find that the damping of the Higgs mode is significantly suppressed by the long-range interaction. We also propose how to excite and probe the Higgs mode in Rydberg-atom experiments.

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• Cold atoms & matter waves
• Long-range interactions
• Rydberg gases
• Rydberg atoms & molecules
• Perturbative methods