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 Jperpendicular and Jparallel. 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 Jperpendicular=7.82 meV and Jparallel=7.76 meV. With these values we predict a singlet-triplet energy gap of Egap=3.9 meV, and give a numerical estimate of the ladder triplet dispersion relation ω(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 Egap=4.9 meV