Localized Magnetic Excitations in the Almost Perfectly Frustrated Spin Dimer Quantum Magnet

Abstract

We have studied magnetic excitations of the effective spin S = 1/2 dimerized magnet Ba₂CoSi₂O₆Cl₂ directly via inelastic neutron scattering experiments. Observed five types of magnetic excitations are all dispersionless within the resolution limits. Unexpected observation is that the three low-lying excitations exhibit different Q dependencies of scattering intensities. Detailed analysis has demonstrated that Ba₂CoSi₂O₆Cl₂ is a two-dimensional spin dimer system where the triplet excitations are localized owing to the almost perfect frustration of the interdimer exchange interactions and the undimerized spins produced by vacancies in the crystals, even in small concentration, have an essential effect on the excitation spectra.