Wannier states and spin supersolid physics in a triangular-lattice XXZ antiferromagnet near Ising limit

The antiferromagnetic Ising model on a triangular lattice is a paradigmatic example of frustrated magnetism. Its ground state lacks long-range order but exhibits macroscopic degeneracy and a finite zero-point entropy, as first shown by Wannier. Introducing a small transverse exchange interaction lifts the classical degeneracy and stabilizes a novel spin supersolid state. In this talk, we present thermodynamic and high-resolution neutron scattering studies on the pseudospin-1/2 triangular lattice antiferromagnet K₂Co(SeO₃)₂, a rare material realization of an easy-axis XXZ Hamiltonian close to the Ising limit [1]. BKT transitions signaling the onset of Ising and supersolid orders are clearly identified, and the Wannier entropy is experimentally recovered. In the supersolid phase, the magnetic excitation spectrum features a broad continuum, with anomalous roton-like minima at its lower boundary. Alongside gapless Goldstone excitations, a pseudo-Goldstone mode with a tiny gap of 0.06 meV is observed. A second continuum is seen at higher energy, replacing the single-spin-flip excitation in the Ising model. Under applied magnetic fields, the Goldstone and pseudo-Goldstone sectors evolve distinctively, and the continuum gradually transforms into coherent spin waves above a field-induced quantum critical point, where the system enters a 1/3-magnetization pletaeu state. These experimental observations show excellent quantitative agreement with quantum Monte Carlo simulations [2].