We report results from inelastic neutron-scattering experiments on powder samples of vanadyl pyrophosphate, (VO${)}_2$${\mathrm{P}}_2$${\mathrm{O}}_7$ (VOPO). We see evidence for three magnetic excitations, at 3.5, 6.0, and 14 meV. The 3.5 meV mode peaks at Q=0.8 ${\mathrm{Å}}^{\mathrm{-}1}$ and is consistent with the one-magnon gap mode reported previously. The 6.0 meV mode is inconsistent with expectations for magnetic excitations in the usual spin-ladder model. Our results imply that VOPO must be described by a different magnetic Hamiltonian than the simple Heisenberg antiferromagnetic spin ladder.

DOI:https://doi.org/10.1103/PhysRevB.55.3631