Orbitally driven spin pairing in the three-dimensional nonmagnetic Mott insulator ${\mathrm{BaVS}}_3:$ Evidence from single-crystal studies

Static electrical and magnetic properties of single crystal ${\mathrm{BaVS}}_3$ were measured over the structural ${(T}_S=240\mathrm{}\mathrm{K}),$ metal insulator ${(T}_{\mathrm{MI}}=69\mathrm{}\mathrm{K}),$ and suspected orbital ordering ${(T}_X=30\mathrm{}\mathrm{K})$ transitions. The resistivity is almost isotropic both in the metallic and insulating states. An anomaly in the magnetic anisotropy at $T_X$ signals a phase transition to an ordered low-T state. The results are interpreted in terms of orbital ordering and spin pairing within the lowest crystal-field quasidoublet. The disordered insulator at $T_X<T<\mathrm{}{T}_{\mathrm{MI}}$ is described as a classical liquid of nonmagnetic pairs.