Magnetic Excitations in the S=1/2 Alternating Chain Compound (VO)_2P_2O_7

TL;DR

This study uses inelastic neutron scattering to reveal that (VO)_2P_2O_7 is best described as an alternating spin chain along the b-direction, showing two magnetic excitations including a potential two-magnon bound state.

Contribution

It redefines the magnetic structure of (VO)_2P_2O_7 from a two-leg ladder to an alternating spin chain based on experimental evidence.

Findings

Identification of an energy gap  3.1 meV for the magnon.

Observation of a second excitation below 2.1 meV.

Potential detection of a two-magnon bound state.

Abstract

Magnetic excitations in an array of (VO)_2P_2O_7 single crystals have been measured using inelastic neutron scattering. Until now, (VO)_2P_2O_7 has been thought of as a two-leg antiferromagnetic Heisenberg spin ladder with chains running in the a-direction. The present results show unequivocally that (VO)_2P_2O_7 is best described as an alternating spin-chain directed along the crystallographic b-direction. In addition to the expected magnon with magnetic zone-center energy gap \Delta = 3.1 meV, a second excitation is observed at an energy just below 2\Delta. The higher mode may be a triplet two-magnon bound state.