Excitation spectrum of the homogeneous spin liquid

We discuss the excitation spectrum of a disordered, isotropic, and translat ionally invariant spin state in the two-dimensional Heisenberg antiferromag net. The starting point is the nearest-neighbor resonating-valence-bond sta te which plays the: role of the vacuum of the theory, in a similar sense as the Neel state is the vacuum for antiferromagnetic spin-wave theory. We di scuss the elementary excitations of this state and show that these are not Fermionic spin-1/2 "spinons" but spin-1 excited dimers which must be modele d by bond bosons. We derive an effective Hamiltonian describing the excited dimers which is formally analogous to spin-wave theory. Condensation of th e bond bosoms at zero temperature into the state with momentum (rr, rr) is shown to be equivalent to antiferromagnetic ordering. The latter is a key i ngredient for a microscopic interpretation of Zhang's SO(5) theory of cupra te superconductivity. [S0163-1829(99)03721-2].