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Magnetic, thermodynamic, neutron diffraction and inelastic neutron scattering are used to study spin correlations in the easy-axis XXZ triangular lattice magnet K2Co(SeO3)2. Despite the presence of quasi-2D "supersolid" magnetic order, the low-energy excitation spectrum contains no sharp modes and is instead a broad and structured multi-particle continuum. Applying a weak magnetic field drives the system into an m = 1/3 fractional magnetization plateau phase and restores sharp spin wave modes. To some extent, the behavior at zero field can be understood in terms of spin wave decay. However, the presence of clear excitation minima at the M-points of the Brillouin zone suggest that the spinon language may provide a more adequate description, and signals a possible proximity to a Dirac spin liquid state.

Magnetic structures and spin excitations are studied across the phase diagram of the geometrically frustrated S = 3/2 quantum antiferromagnet Cs2CoBr4 in magnetic fields applied along the magnetic easy axis, using neutron diffraction, inelastic neutron scattering and THz absorption spectroscopy. The data are analyzed, where appropriate, using extended SU (4) linear spin wave theory. A minimal magnetic Hamiltonian is proposed based on measurements in the high field polarized state. It deviates considerably from the previously considered models. Additional dilatometry experiments highlight the importance of magnetoelastic coupling in this system., Comment: 14 pages, 10 figures, Accepted to Phys. Rev. B

High-resolution neutron and THz spectroscopies are used to study the magnetic excitation spectrum of Cs$_2$CoBr$_4$, a distorted-triangular-lattice antiferromagnet with nearly XY-type anisotropy. What was previously thought of as a broad excitation continuum [Phys. Rev. Lett. 129, 087201 (2022)] is shown to be a series of dispersive bound states reminiscent of "Zeeman ladders" in quasi-one-dimensional Ising systems. At wave vectors where inter-chain interactions cancel at the Mean Field level, they can indeed be interpreted as bound finite-width kinks in individual chains. Elsewhere in the Brillouin zone their true two-dimensional structure and propagation are revealed., Comment: 11 pages, 8 figures

We report an excellent realization of the highly non-classical incommensurate spin-density wave (SDW) state in the quantum frustrated antiferromagnetic insulator Cs$_2$CoBr$_4$. In contrast to the well-known Ising spin chain case, here the SDW is stabilized by virtue of competing planar in-chain anisotropies and frustrated interchain exchange. Adjacent to the SDW phase is a broad $m = 1/3$ magnetization plateau that can be seen as a commensurate locking of the SDW state into the up-up-down (UUD) spin structure. This represents the first example of long-sought SDW-UUD transition in triangular-type quantum magnets., Comment: 13 pages, 13 figures