EXCITATIONS AND POSSIBLE BOUND-STATES IN THE S=1 2 ALTERNATING CHAIN COMPOUND (VO)(2)P2O7/

Magnetic excitations in an array of (VO)(2)P2O7 single crystals have b een measured using inelastic neutron scattering. Until now, (VO)(2)P2O 7 has been thought of as a two-leg antiferromagnetic Heisenberg spin l adder with chains running in the a-direction. The present results show unequivocally that (VO)(2)P2O7 is best described as an alternating sp in-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 elicitation is observed at an energy just below 2 Delt a. The higher mode may be a triplet two-magnon bound state. Numerical results in support of bound modes are presented. Published by Elsevier Science B.V.