Wannier states and spin supersolid physics in the triangular antiferromagnet K$_2$Co(SeO$_3$)$_2$

TL;DR

This study combines neutron scattering and simulations to explore spin excitations and thermodynamics in a triangular antiferromagnet, revealing BKT transitions, Wannier entropy, and complex excitation spectra in the spin-supersolid phase.

Contribution

It provides the first experimental observation of Wannier entropy and detailed excitation spectra in the spin-supersolid phase of a triangular lattice antiferromagnet.

Findings

Identification of BKT transitions for Ising and supersolid order

Experimental recovery of Wannier entropy above the supersolid phase

Observation of a pseudo-Goldstone mode with a 0.06 meV gap

Abstract

We combine ultra-high-resolution inelastic neutron scattering and quantum Monte Carlo simulations to study thermodynamics and spin excitations in the spin-supersolid phase of the triangular lattice XXZ antiferromagnet K$_2$Co(SeO$_3$)$_2$ under zero and non-zero magnetic field. BKT transitions signaling the onset of Ising and supersolid order are clearly identified, and the Wannier entropy is experimentally recovered just above the supersolid phase. At low temperatures, with an experimental resolution of about 23 $\mu$eV, no discrete coherent magnon modes are resolved within a broad scattering continuum. Alongside gapless excitations, a pseudo-Goldstone mode with a 0.06 meV gap is observed. A second, higher-energy continuum replaces single-spin-flip excitations of the Ising model. Under applied fields, the continuum evolves into coherent spin waves, with Goldstone and pseudo-Goldstone sectors responding differently. The experiments and simulations show excellent quantitative agreement.