Quantum fluctuations in an easy axis spin-1/2 triangular lattice antiferromagnet*

In the search for quantum entangled states of matter, materials with triangular lattices of antiferromagnetically interacting magnetic ions remain a focal point. We report the magnetic phase diagram and excitations of a spin-½ easy axis triangular lattice antiferromagnet, K₂Co(SeO₃)₂.  While there is no sharp specific heat anomaly in zero field, there is a prominent magnetization plateau phase at 1/3 of the saturation magnetization [1]. For 0.96 T<B<21.2 T this plateau phase is marked by a second order phase boundary, which however, terminates at (T,B)=(4.4(2) K, 0.9(1) T). Neutron diffraction reveals persistent two-dimensional √3×√3<!--[if gte msEquation 12]> style='mso-bidi-font-style:normal'>√3×√3 magnetic order for T<12 K and B=0 T that can be described as an antiferromagnetic honeycomb lattice. Neutron spectroscopy uncovers low energy continuum scattering at zero field that we associate with the quantum disordered spins within honeycombs. In the plateau phase these spins become magnetized and the system supports coherent spin waves. From their dispersion relation we infer the spin Hamiltonian, which features nearest neighbor interactions with J_z=29.95(3) meV and J_xy=0.26(2) meV.