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 J(perpendicular-to) and J(parallel-to). 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)2P2O7, gives the parameters J(perpendicular-to) = 7.82 meV and J(parallel-to) = 7.76 meV. With these values we predict a singlet-triplet energy gap of E(gap) = 3.9 meV, and give a numerical estimate of the ladder triplet dispersion relation omega(k). In contrast, a fit to the dimer-chain model leads to J1 = 11.11 meV and J2 = 8.02 meV, which predicts a gap of E(gap) = 4.9 meV.