The planar pyrochlore antiferromagnet: A large-N analysis

We study possible quantum phases of the Heisenberg antiferromagnet on the planar pyrochlore lattice, also known as the checkerboard lattice or the square lattice with crossings. It is assumed that the exchange coupling on the square-lattice-links is not necessarily the same as those along the crossing links. When all the couplings are the same, this model may be regarded as a two dimensional analog of the pyrochlore antiferromagnet. The large-N limit of the Sp($N$) generalized model is considered and the phase diagram is obtained by analyzing the fluctuation effects about the $N \to \infty$ limit. We find a topologically-ordered $Z_2$-spin-liquid phase in a narrow region of the phase diagram as well as the plaquette-ordered and staggered spin-Peierls phases as possible quantum-disordered paramagnetic phases.