One-dimensional orbital fluctuations and the exotic magnetic properties of YVO3

Starting from the Mott insulator picture for cubic vanadates, we derive and investigate the model of superexchange interactions between V3+ ions, with nearly degenerate t(2g) orbitals occupied by two electrons each. The superexchange interactions are strongly frustrated and demonstrate a strong interrelation between possible types of magnetic and orbital orders. We elucidate the prominent role played by fluctuations of yz and xz orbitals which generate ferromagnetic superexchange interactions even in the absence of Hund's exchange. In this limit, we find orbital valence bond state which is replaced either by C-type antiferromagnetic order with weak G-type orbital order at increasing Hund's exchange, or instead by G-type antiferromagnetic order when the lattice distortions stabilize C-type orbital order. Both phases are observed in YVO3, and we argue that a dimerized C-type antiferromagnetic phase with stronger and weaker ferromagnetic bonds alternating along the c axis may be stabilized by large spin-orbital entropy at finite temperature. This suggests a scenario which explains the origin of the exotic C-type antiferromagnetic order observed in YVO3 in the regime of intermediate temperatures and allows one to specify the necessary ingredients of a more complete future theory.