Copper(II) formate tetrahydrate (CFTH) is a model square S = 1/2 Heisenberg antiferromagnet with TN = 16.54±0.05 K. The dispersion of spin-waves in the magnetic layers of a fully deuterated sample of this material has been mapped at 4.3 K by inelastic neutron scattering from the zone centre at 0.38 meV to the zone boundary at 14.8 meV. The data are described well by spin-wave theory with a small correction for quantum fluctuations, yielding a nearest-neighbour in-plane exchange constant of 6.3±0.2 meV.
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