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Quasiparticle breakdown in a quantum spin liquid

Much of modern condensed matter physics is understood in terms of elementary excitations, or quasiparticles - fundamental quanta of energy and momentum. Various strongly-interacting atomic systems are successfully treated as a collection of quasiparticles with weak or no interactions. However, there are interesting limitations to this description: the very existence of quasiparticles cannot be taken for granted in some systems. Like unstable elementary particles, quasiparticles cannot survive beyond a threshold where certain decay channels become allowed by conservation laws - their spectrum terminates at this threshold. This regime of quasiparticle failure was first predicted for an exotic state of matter, super-fluid helium-4 at temperatures close to absolute zero - a quantum Bose-liquid where zero-point atomic motion precludes crystallization. Using neutron scattering, here we show that it can also occur in a quantum magnet and, by implication, in other systems with Bose-quasiparticles. We have measured spin excitations in a two dimensional (2D) quantum-magnet, piperazinium hexachlorodicuprate (PHCC) in which spin-1/2 copper ions form a non-magnetic quantum spin liquid (QSL), and find remarkable similarities with excitations measured in superfluid 4He. There is a threshold momentum beyond which the quasiparticle peak merges with the two-quasiparticle continuum. It then acquires a finite energy width and becomes indistinguishable from a leading-edge singularity, so that lowest excited states occupy a wide band of energy. Our findings have important ramifications for understanding phenomena involving excitations with gapped spectra in many condensed matter systems, including high-transition-temperature superconductors.