Ghost modes and continuum scattering in the dimerized distorted kagome lattice antiferromagnet Rb₂Cu₃SnF₁₂

High intensity pulsed neutron scattering reveals a new set of magnetic excitations in the pinwheel valence bond solid state of the distorted kagome lattice antiferromagnet Rb$_2$Cu$_3$SnF$_{12}$. The polarization of the dominant dispersive modes (2 meV $<\hbar\omega< $7 meV) is determined and found consistent with a dimer series expansion with strong Dzyaloshinskii-Moriya interactions ($D/J=0.18$). A weakly dispersive mode near 5 meV and shifted "ghosts" of the main modes are attributed to the enlarged unit cell below a $T=215$ K structural transition. Continuum scattering between 8 meV and 10 meV might be interpreted as a remnant of the kagome spinon continuum [T.-H. Han et al., Nature 492, 406 (2012)]