Two ubiquitous features of frustrated spin systems stand out: massive degeneracy of their ground states and flat, or dispersionless, excitation branches. In real materials, the former is frequently lifted by secondary interactions or quantum fluctuations, in favor of an ordered or spin-liquid state, but the latter often survive. We demonstrate that flat modes may precipitate remarkably strong quantum effects even in the systems that are otherwise written off as almost entirely classical. The resultant spectral features should be reminiscent of the quasiparticle breakdown in quantum systems, only here the effect is strongly amplified by the flatness of spin-excitation branches, leading to the damping that is not vanishingly small even at $S\gg 1$. We provide a theoretical analysis of the excitation spectrum of the $S=5/2$ iron jarosite to illustrate our findings and to suggest further studies of this and other frustrated spin systems.

• Received 14 May 2015
• Revised 13 August 2015

DOI:https://doi.org/10.1103/PhysRevB.92.094409