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

Abstract

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 ${\mathit{J}}_{\mathrm{\perp }}$ and ${\mathit{J}}_{\mathrm{\parallel }}$. 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${)}_2$ ${\mathrm{P}}_2$ ${\mathrm{O}}_7$, gives the parameters ${\mathit{J}}_{\mathrm{\perp }}$=7.82 meV and ${\mathit{J}}_{\mathrm{\parallel }}$=7.76 meV. With these values we predict a singlet-triplet energy gap of ${\mathit{E}}_{\mathrm{gap}}$=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 ${\mathit{J}}_1$=11.11 meV and ${\mathit{J}}_2$=8.02 meV, which predicts a gap of ${\mathit{E}}_{\mathrm{gap}}$=4.9 meV.