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

TL;DR

This Raman scattering study of (VO)$_2$P$_2$O$_7$ reveals a singlet bound state and strong spin-phonon coupling, challenging previous frustration-based models and providing insights into its spin-gap properties.

Contribution

The paper identifies a singlet bound state and demonstrates strong spin-phonon coupling in (VO)$_2$P$_2$O$_7$, offering new understanding of its magnetic interactions.

Findings

Identification of a singlet bound state at 47 cm$^{-1}$

Evidence of strong spin-phonon coupling

Rejection of frustration as a key factor

Abstract

Single crystals of the spin-gap system $\rm (VO)_2P_2O_7$ have been investigated by means of Raman scattering. At temperatures below T=75 K~$\approx 2\Delta_{01}({\bf k}=0)$, 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.