NEUTRON-SCATTERING STUDY OF MAGNETIC EXCITATIONS IN (VO)(2)P2O7

We report results from inelastic neutron-scattering experiments on pow der samples of vanadyl pyrophosphate, (VO)(2)P2O7 (VOPO). We see evide nce for three magnetic excitations, at 3.5, 6.0, and 14 meV. The 3.5 m eV mode peaks at Q=0.8 Angstrom(-1) and is consistent with the one-mag non gap mode reported previously. The 6.0 meV mode is inconsistent wit h expectations for magnetic excitations in the usual spin-ladder model . Our results imply that VOPO must be described by a different magneti c Hamiltonian than the simple Heisenberg antiferromagnetic spin ladder .