Nonlinear magnons and exchange Hamiltonians of the delafossite proximate quantum spin liquid candidates KYbSe2 and NaYbSe2

Quantum spin liquids (QSLs) are theoretical states of matter with long-range entanglement and exotic quasiparticles. However, they generally elude quantitative theory, rendering their underlying phases mysterious and hampering efforts to identify experimental QSL states. Here we study triangular-lattice resonating-valence-bond QSL candidate materials KYbSe2 and NaYbSe2. We measure the magnon modes in their 1/3 plateau phase, where quantitative theory is tractable, using inelastic neutron scattering and fit them using nonlinear spin wave theory. We also fit the KYbSe2 heat capacity using high-temperature series expansion. Both KYbSe2 fits yield the same magnetic Hamiltonian to within uncertainty, confirming previous estimates and showing the Heisenberg ratio J2/J1 to be an accurate model for these materials. Most importantly, comparing KYbSe2 and NaYbSe2 shows that the smaller A-site Na+ ion has a larger J2/J1 ratio. However, hydrostatic pressure applied to KYbSe2 increases the ordering temperature (a result consistent with density functional theory calculations), indicating that pressure decreases J2/J1. These results show how the periodic table and hydrostatic pressure can tune the AYbSe2 materials in a controlled way.