Magnetoelastic coupling and spin excitations in the spin-gap system (VO)(2)P2O7: A Raman scattering study

Single crystals of the spin-gap system (VO)(2)P2O7 have been investigated b y means of Raman scattering. At temperatures below T = 75 K approximate to 2 Delta(01)(k = 0), with Delta(01) the singlet-triplet gap, a shoulder appe ars at 47 cm(-1). An analysis of the polarization selection rules and the t emperature dependence of the scattering intensity leads to its identificati on as a singlet bound state with very small binding energy. Therefore in co ntrast to recent theoretical models the importance of frustration for this compound must be rejected. The observation of a strong anharmonicity of thr ee phonons with energies close to Delta(01) at the zone boundary and additi onal quasielastic light scattering, however, both point to a strong spin-ph onon coupling in this system. We propose this coupling to be the origin of the second bound triplet state observed in recent neutron-scattering experi ments.