Experiments exploring the dynamics of KYbSe2 and NaYbSe2 in field near their magnetization plateaux.*

The AYbSe2 delafossite family is a near perfect realization of a triangular lattice Heisenberg model, with a ratio between the first and second neighbor in-plane exchanges J2/J1 very close to a theoretically predicted quantum spin liquid phase. In this talk I present neutron scattering measurements in zero field and the 1/3 magnetization plateau phase. In this "up-up-down" magnetic ordered phase we find coherent magnons which are describable via nonlinear spin wave theory, and can be fitted to a magnetic exchange model. The fittted J2/J1 of KYbSe2 lies within the 120 degree ordered phase, consistent with other measurements. However, the fitted J2/J1 of NaYbSe2 indicates larger J2/J1, potentially inside the theoretical quantum spin liquid phase, consistent with reports of no magnetic order in NaYbSe2. This indicates that the periodic table can be used to tune the triangular lattice across this phase diagram, and enter the long-sought quantum spin liquid phase in a controlled manner.