Nature440, 187-190 (9 March 2006) | doi:10.1038/nature04593
Quasiparticle breakdown in a quantum spin liquid
Matthew B. Stone1, Igor A. Zaliznyak2, Tao Hong3, Collin L. Broholm3,4 and Daniel H. Reich3
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.
Condensed Matter Sciences Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, USA
Condensed Matter Physics and Material Science Department, Brookhaven National Laboratory, Upton, New York 11973, USA
Department of Physics and Astronomy, The Johns Hopkins University, Baltimore, Maryland 21218, USA
National Institute of Standards and Technology, Gaithersburg, Maryland 20899, USA
Correspondence to: Igor A. Zaliznyak2 Correspondence and requests for materials should be addressed to I.A.Z. (Email: email@example.com).