  In the lacunar spinel GaV4S8, the interplay of spin, charge, and orbital
degrees of freedom produces a rich phase diagram that includes an unusual
Neel-type skyrmion phase composed of molecular spins. To provide insight into
the interactions underlying this complex phase diagram, we study the spin
excitations in GaV4S8 through inelastic neutron scattering measurements on
polycrystalline and single-crystal samples. Using linear spin-wave theory, we
describe the spin-wave excitations using a model where V4 clusters decorate an
FCC lattice. The effective cluster model includes a ferromagnetic interaction
and a weaker antisymmetric Dzyaloshinskii-Moriya (DM) interaction between the
neighboring molecular spins. Our work clarifies the spin interactions in GaV4S8
and supports the picture of interacting molecular clusters.
