Dimerized antiferromagnets are systems that have a particularly strong antiferromagnetic interaction which couples the magnetic ions into pairs. These pairs of magnetic ions form units called dimers and the system can be described with a Heisenberg Hamiltonian where the ground state is singlet and is separated from a triplet excitated state by an energy gap.

The dimers can interact with each other through interdimer exchange interactions. If these interactions are weak, the ground state continues to be singlet, however the magnons become mobile, developing a dispersion, which is reflected in the appearance of an excitation bandwidth.

The excited quasiparticles are called magnons or triplons which are bosons. An intrinsic property of these triplons is the hard core constraint that excludes states with more than one quasiparticle per dimer. Systems where dimerization occur, maintain a singlet ground state down to absolute zero in temperature and never develop long-range magnetic order in the absence of a magnetic field. Applying a magnetic field lifts the triplet degeneracy by Zeeman splitting the excited state. The energy of one of the triplet excited states is reduced by the field and when the field is large enough, the system goes through a quantum critical point (QCP) and the magnons condense into the ground state producing different kinds of magnetic order. The type of order depends on the symmetry conditions and can be described as a Bose Einstein condensation (BEC), Wigner crystallization or supersolidity.

Sr₃Cr₂O₈ is a weakly coupled dimerized system with three dimensional magnetic interactions which has beem shown to undergo Bose-Einstein Condensation (A. A. Aczel et al., PRL 103, 207203 (2009)). It consist of a lattice of spin-1/2 Cr⁵⁺ ions, which form hexagonal bilayers and are paired into dimers by the dominant antiferromagnetic intrabilayer coupling, J0 (see Figure 1). The configuration of the Cr ions makes it a frustrated triangular lattice at room temperature. A Jahn-Teller distortion at 285 K lifts the frustration and leads to spatially anisotropic exchange interactions and to three monoclinic twins.

In order to estimate the intra and interdimer exchange couplings, the dispersion relations of the magnons in terms of energy and wavevector must be known. The best technique to investigate this is single crystal inelastic neutron scattering as it can be directly compared with theory to extract the exchange interactions. High quality single crystals of Sr3Cr2O8 were grown using the floating zone technique (for details on sample preparation see A.T.M.N Islam, Crys. Growth & Des. 10, 465 (2010)). The magnetic excitations were measured using the high resolution triple axis spectrometer V2-Flex. Energy scans were performed with fixed final wavevector along key directions in reciprocal space. These measurements revealed three gapped and dispersive singlet to triplet modes arising from the three twinned domains, see Figure 2. These data were fitted simultaneously to a random phase approximation dimer model. The extracted exchange interactions can successfully describe all powder and single crystal inelastic neutron data, DC susceptibility and magnetization. The good agreement with data is shown in Figure 3 (for further reading see D. L. Quintero-Castro, Phys. Rev. B 81, 014415 (2010)).