The Origin of Extra Modes in (VO)_2P_2O_7

TL;DR

This paper explains unexpected extra modes observed in neutron scattering experiments on (VO)_2P_2O_7 as shadows caused by umklapp scattering, supported by strong-coupling calculations.

Contribution

It proposes that the extra modes are shadows of known features due to umklapp scattering, providing a theoretical explanation supported by calculations.

Findings

Extra modes are shadows of known modes caused by umklapp scattering.

Elementary calculations support the shadow interpretation.

Experimental tests are suggested to verify the proposal.

Abstract

Recent inelastic neutron scattering experiments on (VO)_2P_2O_7 (VOPO) have seen previously unanticipated sharp peaks in the dynamic structure factor in addition to the pair of triplet modes observed earlier. We argue that the additional features are in essence `shadows' of the previously studied features arising due to umklapp scattering, and suggest experimental tests of this proposal. The basic point is illustrated by some elementary calculations within a strong-coupling expansion for the alternating chain with the right geometry (on which the matrix element for umklapp scattering depends sensitively) taken fully into account.