Phase Diagram Of The Frustrated Heisenberg Antiferromagnet On The Union Jack Lattice

We use series expansions to investigate the zero-temperature phase diagram of a recently proposed frustrated quantum spin-1/2 antiferromagnet on the two-dimensional union jack lattice with coupling ratio alpha. A single second-order phase transition is found between a Neel phase and a canted ferrimagnetic phase at a critical coupling alpha(c)=0.65(1). The Neel magnetization vanishes at the transition. Dispersion curves for magnon excitations are also obtained and compared with spin-wave predictions. Two Goldstone modes are found in the canted phase, one linear in momentum and the other quadratic, in accordance with theoretical predictions.