Magnetic Excitations in the S=1/2 Alternating Chain Compound (VO)_2P_2O_7

Magnetic excitations in an array of (VO)_2P_2O_7 single crystals have been measured using inelastic neutron scattering. Until now, (VO)_2P_2O_7 has been thought of as a two-leg antiferromagnetic Heisenberg spin ladder with chains running in the a-direction. The present results show unequivocally that (VO)_2P_2O_7 is best described as an alternating spin-chain directed along the crystallographic b-direction. In addition to the expected magnon with magnetic zone-center energy gap \Delta = 3.1 meV, a second excitation is observed at an energy just below 2\Delta. The higher mode may be a triplet two-magnon bound state.