Quantum Exchange Magnon Gap in an Antiferromagnet with Dynamically Interacting Spin Subsystems

The authors have performed an inelastic neutron scattering experiment on the antiferromagnet Ca sub(3)Fe sub(2)Ge sub(3)O sub(12). This garnet has a nontrivial infinitely degenerate classical Neel ground state. They have found an energy gap of 0.033(4) THz in an acoustic spin wave branch at the antiferromagnetic zone centre. According to theoretical predictions this energy gap is caused by quantum spin fluctuations which lift the degeneracy of the ground state.