Spectroscopic Evidence of Supersolids in Quantum Ising Magnet K2Co(SeO3)2*

We report the neutron scattering results for the spin- 1/2 triangular-lattice antiferromagnet, K2Co(SeO3)2, which features nearest-neighbor Ising-like interactions with Jz = 2.96(2) meV and Jxy = 0.21(3) meV. For temperatures below 15 K, neutron spectroscopy reveals the gradual development of solidity, indicated by a quasi-two-dimensional √3 × √3 magnetic order associated with Z3 symmetry breaking. Below 0.3 K, the emergence of a Goldstone mode from low-energy continuum scattering provides evidence of superfluidity associated with breaking spin rotational U(1) symmetry. Notably, the presence of flat bands of excitations at quantized energies (ℏω = nJz, n = 0, 1, 2, 3) is a hallmark of the spatially modulated <Sz> on a triangular lattice, distinguishing K2Co(SeO3)2 from existing candidates. Our work demonstrates that K2Co(SeO3)2 is a spectacular example of a spin- 1/2 triangular-lattice quantum Ising magnet and provides spectroscopic evidence of zero-field supersolidity on a triangular lattice.