Spin-density waves and fractional magnetization plateau in triangular Cs₂CoBr₄

Despite being essentially two-dimensional (2D), spin insulators on distorted triangular lattice can retain exotic features of one-dimensional (1D) physics. Quantum fluctuations in applied magnetic field lift the classical ground-state degeneracy, leading to rich phase diagrams hosting emergent states with collective excitations. Examples are frustration-induced magnetization plateaux, a typical feature of 2D triangular geometries, and incommensurate spin-density waves (SDW), which in 1D result from the perfect nesting of the underlying fermionic Fermi surface. This talk focuses on the triangular antiferromagnetic insulator Cs₂CoBr₄ discovered in our group. Its existence in sizable single-crystals and suitable energy scales allow full experimental access and make it an ideal candidate to explore the above scenario. We observed several field-induced quantum states and completely characterized its magnetic phase diagram. For each phase, we report comprehensive measurements of the ground state and excitations in a combination of bulk thermodynamics, neutron diffraction, inelastic neutron scattering, and terahertz spectroscopy experiments. In line with the above picture, we found scattering evidence of multiple incommensurate SDWs. Their connection with a commensurate m = 1/3 magnetization plateaux with up-up-down structure is discussed.