The Union Jack model: a quantum spin model with frustration on the square lattice

A new quantum spin model with frustration, the `Union Jack' model on the square lattice, is analyzed using spin-wave theory. For small values of the frustrating coupling $\alpha$, the system is N{\' e}el ordered as usual, while for large $\alpha$ the frustration is found to induce a canted phase. The possibility of an intermediate spin-liquid phase is discussed.