Series investigations of magnetically disordered ground states in two-dimensional frustrated quantum antiferromagnets

An extensive series-expansion study of the square-lattice, spin-1/2 Heisenberg antiferromagnet with nearest- and second-neighbor couplings has been carried out to investigate the nature of the ground state for ${\mathit{J}}_2$/${\mathit{J}}_1$≊1/2. In agreement with an earlier numerical study along similar lines, we find that the magnetically disordered phase exhibits columnar spontaneous dimerization, but the magnitude of such order may not be as large as had been suggested previously. Although the magnetically disordered phase is not critical, it appears to have both spin-spin and energy-energy correlations of range greater than a few lattice spacings. The possibility of ‘‘spin-nematic’’ order, as proposed by Chandra, Coleman, and Larkin, has not been directly ruled out, but the balance of circumstantial evidence weighs against it in this system.