Magnetic excitations and quantum fluctuation in the covalent-chain antiferromagnet TlFeS2

Magnetic excitations in the covalent-chain antiferromagnet TlFeS2 with a low ordered spin Sz ≈ 0.9 were studied by time-of-flight neutron scattering. The scattering intensities are in agreement with a non-interacting multi-magnon model for intrinsic spins S ≈ 3/2 and a zero-point quantum reduction ΔS ≈ 0.6, confirming the ΔS predicted from the spin-wave energy dispersion. No evidence is found for charge fluctuation on the spin-wave energy scale. A local-moment Heisenberg Hamiltonian and conventional harmonic spin-wave theory thus apply to Neel-ordered TlFeS2, where the two-magnon signal allows to determine the unknown local spin S.