A Neutron Scattering Study of Magnetic Excitations in the Spin Ladder (VO)2P2O7

In this letter we report results from inelastic neutron scattering experiments on powder samples of vanadyl pyrophosphate, (VO)2P2O7. We see evidence for three magnetic excitations, at 3.5 meV, 6.0 meV and 14 meV. The intensity of the 3.5 meV mode is strong near Q=0.8 A-1, consistent with the one-magnon gap mode reported previously and predicted by the spin-ladder model. The magnetic scattering at 14 meV may be due to the top of the one-magnon band or the two-magnon continuum predicted in the ladder model. The 6.0 meV mode also peaks in intensity at Q=0.8 A-1. This mode has not been reported previously and was not anticipated by existing theoretical treatments of the spin ladder.