Prof. Philippe Sindzingre (Universite Pierre & Marie Curie)
"Spin-1/2 frustrated antiferromagnet on a spatially anisotopic square lattice: contribution of exact diagonalizations"
The phase diagram of a spin-1/2 J-J'-J_2 model is investigated by means of exact diagonalizations on finite samples with periodic boundaries. This model is a generalization of the J_1-J_2 model on the square lattice with two different nearest-neighbor couplings J,J' and may be also viewed as an array of coupled Heisenberg chains. The results indicate the occurence of a columnar dimer phase in the intermediate region between the N\'eel (\pi,\pi) and (\pi,0) phases. This columnardimer order could extend for J'\rightarrow J which correspond to the J_1-J_2 model. Moreover the dimer phase occurs for a finite number of coupled chains and may in fact already be present in the two legladder,as suggested by Starykh and Balents [Phys. Rev. Lett. 93, 157206 (2004)]. In this intermediate region, besides the columnar dimer states,the results also suggest that the resonnating valence bond state predicted by Nersesyan and Tsvelik [Phys. Rev. B 67, 024422 (2003)] for J_2=0.5J' << J could be realized, along a curve nearly coincident withthe line where the energy per spin is maximum and extend beyond the limit of small interchain coupling, This RVB state might then separate the intermediate region in two subregions that both display dimer order.