Dressed j-1/2 objects in mixed-valence lacunar spinel molybdates.

The lacunar-spinel chalcogenides exhibit magnetic centers in the form of transition-metal tetrahedra. On the basis of density-functional computations, the electronic ground state of an Mo tetrahedron has been postulated as single-configuration a e t, where a, e, and t are symmetry-adapted linear combinations of single-site Mo t atomic orbitals. Here we unveil the many-body tetramer wave-function: we show that sizable correlations yield a weight of only 62% for the a e t configuration. While spin-orbit coupling within the peculiar valence orbital manifold is still effective, the expectation value of the spin-orbit operator and the g factors deviate from figures describing nominal t j = 1/2 moments. As such, our data documents the dressing of a spin-orbit j = 1/2 object with intra-tetramer excitations. Our results on the internal degrees of freedom of these magnetic moments provide a solid theoretical starting point in addressing the intriguing phase transitions observed at low temperatures in these materials.