Localized magnetic excitations in the fully frustrated dimerized magnet Ba$_2$CoSi$_2$O$_6$Cl$_2$

TL;DR

This study used inelastic neutron scattering to investigate magnetic excitations in the frustrated dimerized magnet Ba$_2$CoSi$_2$O$_6$Cl$_2$, revealing localized triplet excitations and the role of frustration and undimerized spins.

Contribution

It provides the first direct experimental evidence of localized triplet excitations in a fully frustrated dimerized magnet, highlighting the effects of frustration and undimerized spins.

Findings

Five dispersionless excitation modes observed.

Different Q-dependences for low-lying excitations.

Localized triplet excitations due to frustration.

Abstract

Magnetic excitations of the effective spin $S$=1/2 dimerized magnet Ba$_2$CoSi$_2$O$_6$Cl$_2$ have been probed directly via inelastic neutron scattering experiments at temperatures down to 4 K. We observed five types of excitation at 4.8, 5.8, 6.6, 11.4, and 14.0 meV, which are all dispersionless within the resolution limits. The scattering intensities of the three low-lying excitations were found to exhibit different $Q$-dependences. Detailed analysis has demonstrated that Ba$_2$CoSi$_2$O$_6$Cl$_2$ is a two-dimensional spin dimer system described only by a single dimer site, where the triplet excitations are localized owing to the almost perfect frustration of the interdimer exchange interactions and the undimerized spins, even in small concentration, make an essential contribution to the excitation spectrum.