Single crystals of the spin-gap system (VO)2P2O7 have been investigated by means of Raman scattering. At temperatures below T=75 K~≈2Δ01(k=0), with Δ01 the singlet-triplet gap, a shoulder appears at 47 cm−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 Δ01 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.
@article{arxiv.cond-mat/9912050,
title = {Magnetoelastic coupling and spin excitations in the spin-gap system (VO)$_2$P$_2$O$_7$: A Raman scattering study},
author = {M. Grove and P. Lemmens and G. Güntherodt and B. C. Sales and F. Büllesfeld and W. Assmus},
journal= {arXiv preprint arXiv:cond-mat/9912050},
year = {2009}
}