Theory of two-dimensional quantum Heisenberg antiferromagnets with a nearly critical ground state

We present the general theory of clean, two-dimensional, quantum Heisenberg antiferromagnets which are close to the zero-temperature quantum transition between ground states with and without long-range Néel order. While some of our discussion is more general, the bulk of our theory will be restricted to antiferromagnets in which the Néel order is described by a three-vector order parameter. For Néel-ordered states, ``nearly critical'' means that the ground-state spin stiffness, ρ<sub>s</sub>, satisfies ρ<sub>s</sub><<J, where J is the nearest-neighbor exchange constant, while ``nearly critical'' quantum-disordered ground states have an energy gap, Δ, towards excitations with spin 1, which satisfies Δ<<J. The allowed temperatures, T, are also smaller than J, but no restrictions are placed on the values of k_BT/ρ_s or k_BT/Δ. Under these circumstances, we show that the wave vector and/or frequency-dependent uniform and staggered spin susceptibilities, and the specific heat, are completely universal functions of just three thermodynamic parameters. On the ordered side, these three parameters are ρ_s, the T=0 spin-wave velocity c, and the ground-state staggered moment N₀; previous works have noted the universal dependence of the susceptibilities on these three parameters only in the more restricted regime of k_BT<<ρ_s. On the disordered side the three thermodynamic parameters are Δ, c, and the spin-1 quasiparticle residue scrA. Explicit results for the universal scaling functions are obtained by a 1/N expansion on the O(N) quantum nonlinear σ model, and by Monte Carlo simulations. These calculations lead to a variety of testable predictions for neutron scattering, NMR, and magnetization measurements. Our results are in good agreement with a number of numerical simulations and experiments on undoped and lightly doped La_2-δSr_δCuO₄.