The susceptibility and excitation spectrum of ( VO ) 2 P 2 O 7 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 ⊥ and J ||. These are contrasted with new bulk limit results for the dimer chain. A fit to the experimental susceptibility of the candidate spin-ladder compound vanadyl pyrophosphate, (VO) 2 P 2 O 7 , gives the parameters J ⊥ = 7.82 meV and J || = 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 ω(k). In contrast, a fit to the dimer chain model leads to J 1 = 11.11 meV and J 2 = 8.02 meV, which predicts a gap of E gap = 4.9 meV.