Susceptibility and excitation spectrum of (VO)2P2O7 in ladder and dimer-chain models.

We present numerical results for the magnetic susceptibility of a Heisenberg antiferromagnetic spin ladder as a function of temperature and the spin-spin interaction strengths [ital J][sub [perpendicular]] and [ital J][sub [parallel]]. These are contrasted with bulk-limit results for the dimer chain. A fit to the experimental susceptibility of the candidate spin-ladder compound vanadyl pyrophosphate, (VO)[sub 2]P[sub 2]O[sub 7], gives the parameters [ital J][sub [perpendicular]]=7.82 meV and [ital J][sub [parallel]]=7.76 meV. With these values we predict a singlet-triplet energy gap of [ital E][sub gap]=3.9 meV, and give a numerical estimate of the ladder triplet dispersion relation [omega]([ital k]). In contrast, a fit to the dimer-chain model leads to [ital J][sub 1]=11.11 meV and [ital J][sub 2]=8.02 meV, which predicts a gap of [ital E][sub gap]=4.9 meV.