Polarized-Neutron Study of Spin Dynamics in the Kondo Insulator YbB₁₂

Inelastic neutron scattering experiments have been performed on the archetype compound YbB₁₂, using neutron polarization analysis to separate the magnetic signal from the phonon background. With decreasing temperature, components characteristic for a single-site spin-fluctuation dynamics are suppressed, giving place to specific, strongly Q-dependent, low-energy excitations near the spin-gap edge. This crossover is discussed in terms of a simple crystal-field description of the incoherent high-temperature state and a predominantly local mechanism for the formation of the low-temperature singlet ground state.