Li$_2$VO(Si,Ge)O$_4$, a prototype of a two-dimensional frustrated   quantum Heisenberg antiferromagnet
  NMR and magnetization measurements in Li$_2$VOSiO$_4$ and Li$_2$VOGeO$_4$ 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 sizeable frustration induced by the competition between the superexchange couplings $J_1$ along the sides of the square and $J_2$ along the diagonal. Li$_2$VOSiO$_4$ undergoes a low-temperature phase transition to a collinear order, as theoretically predicted for $J_2/J_1 > 0.5$. Just above the magnetic transition the degeneracy between the two collinear ground states is lifted by the onset of a structural distortion. 
R. Melzi, P. Carretta, A. Lascialfari, M. Mambrini, M. Troyer, P.   Millet and F. Mila