title: Li2VO(Si,Ge)O4, a prototype of a two-dimensional frustrated quantum heisenberg antiferromagnet
abstract: NMR and magnetization measurements in Li2VOSiO4 and Li2VOGeO4 are reported. The analysis of the susceptibility shows that both compounds are two-dimensional S = 1/2 Heisenberg antiferromagnets on a square lattice with a sizable frustration induced by the competition between the superexchange couplings J1 along the sides of the square and J2 along the diagonal. Li2VOSiO4 undergoes a low-temperature phase transition to a collinear order, as theoretically predicted for J2/J1>0.5. Just above the magnetic transition the degeneracy between the two collinear ground states is lifted by the onset of a structural distortion.
authors.affiliation: Dipartimento di Fisica "A. Volta," Unita INFM di Pavia, 27100 Pavia, Italy.
authors.initials: R
authors.lastname: Melzi
authors.initials: P
authors.lastname: Carretta
authors.initials: A
authors.lastname: Lascialfari
authors.initials: M
authors.lastname: Mambrini
authors.initials: M
authors.lastname: Troyer
authors.initials: P
authors.lastname: Millet
authors.initials: F
authors.lastname: Mila