Structural Aspects of Magnetic Coupling in CaV4O9
W. E. Pickett, Phys. Rev. Lett. 79, 1746 (1997)
CaV4O9 was the first two-dimensional system to show spin gap behavior, which is characterized by a periodic array of ionic moments (V4+ d1 ion in this case) which refuse to order even at the lowest temperatures. The ions form a spin liquid singlet state, from which the lowest excitation involves an excitation to a triplet state, requiring about 10 meV of energy in the case of CaV4O9. This behavior has stimulated a great deal of research into the microscopic mechanism causing the spin gap behavior.

The strong corrugation of the V4O5 layer in the spin gap system CaV4O9 is examined in this paper for its impact on the exchange coupling constants between the spin 1/2 V ions. Local spin density calculations show that the V spin occupies a dx2-y2 orbital (x and y are the V-V directions) that would have vanishing 2nd order exchange coupling if the V4O5 layer were flat. The Kanamori-Goodenough superexchange rules, and the small calculated energy difference of ferromagnetic and antiferromagnetic alignments of spins for the real structure, indicate that 2nd neighbor couplings dominate over nearest neighbor. This suggests two coupled S=1/2 metaplaquette systems, each tending toward singlet formation. This new picture has been confirmed by neutron scattering measurements of the singlet-triplet excitation spectrum and by subsequent theoretical work.