Magnetoelastic coupling and spin excitations in the spin-gap system (VO)${}_2$P${}_2$O${}_7$: A Raman scattering study

Abstract

Single crystals of the spin-gap system $(\mathrm{VO}{)}_2{\mathrm{P}}_2{\mathrm{O}}_7$ have been investigated by means of Raman scattering. At temperatures below T=75 K~$\approx 2{\Delta }_{01}(\mathbf{k}=0)$, with ${\Delta }_{01}$ 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 ${\Delta }_{01}$ 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.

6 pages, 9 figures