Magnetoelastic coupling and spin excitations in the spin-gap system (VO){sub 2}P{sub 2}O{sub 7}: A Raman scattering study

Single crystals of the spin-gap system (VO){sub 2}P{sub 2}O{sub 7} have been investigated by means of Raman scattering. At temperatures below T=75 K (approx =)2{delta}{sub 01}(k=0), with {delta}{sub 01} the singlet-triplet gap, a shoulder appears at 47 cm{sup -1}. An analysis of the polarization selection rules and the temperature dependence of the scattering intensity leads to its identification as a singlet bound state with very small binding energy. Therefore in contrast to recent theoretical models the importance of frustration for this compound must be rejected. The observation of a strong anharmonicity of three phonons with energies close to {delta}{sub 01} at the zone boundary and additional quasielastic light scattering, however, both point to a strong spin-phonon coupling in this system. We propose this coupling to be the origin of the second bound triplet state observed in recent neutron-scattering experiments. (c) 2000 The American Physical Society.