Fig. 1 |. Crystal structure and magnetic ($H$, $T$) phase diagram of NaYbO₂.

a, Refined NaYbO₂ structure (1.6 K, R3m) contains equilateral triangular layers of $D_{3d}$ YbO₆ distorted octahedra separated by 3.346 Å. Sodium cations refine to full occupation, creating a uniform chemical environment surrounding the triangular layers. Purple spheres, Yb atoms; black spheres, Na atoms; brown spheres, O atoms. b, Low-temperature phase boundary between quantum disordered and antiferromagnetic ordered states in NaYbO₂, plotted as a function of field and temperature, extracted from a.c. susceptibility and neutron-scattering experiments. The dashed line denotes the boundary of Zeeman-driven quenching of a minority fraction of free Yb moments under field, above which free moments are quenched. These free moments coexist with a quantum disordered ground state. Values in parentheses and error bars indicate one standard deviation. $k_{\mathrm{B}}$ is the Boltzmann constant, $H_{\mathbf{q}}$ denotes the onset temperature of the up–up–down ordered state and $H_{\text{Zeeman}}$ denotes the suppression in magnetic susceptibility observed due to the quenching of a small fraction of free Yb moments.