Quasiparticle breakdown in a quantum spin liquid

Matthew B. Stone, Igor A. Zaliznyak (), Tao Hong, Collin L. Broholm and Daniel H. Reich
Additional contact information
Matthew B. Stone: Oak Ridge National Laboratory
Igor A. Zaliznyak: Brookhaven National Laboratory
Tao Hong: The Johns Hopkins University
Collin L. Broholm: The Johns Hopkins University
Daniel H. Reich: The Johns Hopkins University

Nature, 2006, vol. 440, issue 7081, 187-190

Abstract: Abstract Much of modern condensed matter physics is understood in terms of elementary excitations, or quasiparticles—fundamental quanta of energy and momentum1,2. 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: in some systems the very existence of quasiparticles cannot be taken for granted. 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. Such quasiparticle breakdown was first predicted for an exotic state of matter—super-fluid 4He at temperatures close to absolute zero, a quantum Bose liquid where zero-point atomic motion precludes crystallization1,2,3,4. Here we show, using neutron scattering, that quasiparticle breakdown 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 quantum magnet, piperazinium hexachlorodicuprate (PHCC)5, in which spin-1/2 copper ions form a non-magnetic quantum spin liquid, and find remarkable similarities with excitations in superfluid 4He. We observe 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 excited states are no longer quasiparticles but occupy a wide band of energy. Our findings have important ramifications for understanding excitations with gapped spectra in many condensed matter systems, ranging from band insulators to high-transition-temperature superconductors6.

Date: 2006
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DOI: 10.1038/nature04593

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