Quasiparticle breakdown in a quantum spin liquid

Abstract

Much of modern condensed matter physics is understood in terms of elementary excitations, or quasiparticles-fundamental quanta of energy and momentum 1,2 . 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 4 He at temperatures close to absolute zero, a quantum Bose liquid where zero-point atomic motion precludes crystallization 1-4 . 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 4 He. 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 superconductors 6 . Although of all the elements only liquid helium fails to crystallize at temperature T ¼ 0, quantum liquids are quite common in condensed matter. Metals host electron Fermi liquids, and superconductors contain Bose liquids of Cooper pairs. Trapped ultracold atoms can also form quantum liquids, and some remarkable new examples were recently identified in magnetic crystals 5,7-10 . The organometallic material PHCC is an excellent physical realization of a quantum spin liquid (QSL) in a two-dimensional (2D) Heisenberg antiferromagnet (HAFM). Its Cu 2þ spins are coupled through a complex network of orbital overlaps, and form an array of slightly skewed anisotropic spin-1/2 ladders 10 in the crystalline a-c plane with highly frustrated super-exchange interactions 5 . The spin excitations in PHCC have a spectral gap D s < 1 meVand nearly isotropic 2D dispersion in the (h0l) plane with a bandwidth slightly larger than D s . In the absence of a magnetic field, only the short-range dynamic spin correlations typical of a liquid exist: the spin gap precludes longrange magnetic order down to T ¼ 0. Here we explore magnetic excitations in PHCC via inelastic neutron scattering and compare the results with similar measurements in the quantum fluid 4 He, emphasizing the effects ...

Decay of quasiparticles in quantum spin liquids

“…We have verified the above assertion for several model dispersion curves 17,18 as well as for a few experimental fits. 5,6,19 The common feature of all analyzed cases is presence of two regimes: for a small total momentum p of a magnon pair the minimum energy corresponds to equal momenta of two quasiparticles, whereas for p near the Brillouin zone boundary two particles of the lowest-energy pair have different momenta. Such a bifurcation is closely related to appearance of bound pairs of magnons at large momenta for one-dimensional spin systems.…”

“…Upon entering two-magnon continuum, see Fig. 1, spontaneous (T = 0) decay of a magnon into a pair of quasiparticles becomes possible, which leads to a rapid decrease in the quasiparticle life-time in the former case 5 and complete disappearance of the single-particle branch in the latter system. 6 Quasiparticle instability is well documented for another type of quantum liquid-superfluid 4 He.…”

“…The lowenergy triplet of magnons can be split due to intrinsic anisotropies or under applied magnetic field. Recent neutron experiments 5,6 on two organic spin-gap materials PHCC and IPA-CuCl 3 have revealed drastic transformation of triplet quasiparticles undergoing at high energies. Upon entering two-magnon continuum, see Fig.…”

Higgs mode and its decay in a two-dimensional antiferromagnet

“…In particular, it is uncertain at this point whether the decay process considered above fully describes its dynamics. In addition to the multimagnon continuum [27][28][29][30] , other channels such as decays into vortex-like excitations are conceivable in two-dimensional planar magnets and require further investigation. It would be also interesting to compare the results presented herein with the spectra from resonant inelastic X-ray scattering, which can in principle access both the scalar and longitudinal susceptibilities.…”

Crystals for neutron scattering studies of quantum magnetism

“…The magnetic properties of pure PHCC have been extensively studied. The value of the spin gap ∆=1 meV, as well as all peculiarities of the spin excitation spectrum, are well established [15,16,54,55]. Armed with this knowledge, we set out to introduce disorder in PHCC by substituting a fraction of the nonmagnetic exchange interaction-mediating Cl − ions for the larger ionic radii Br − .…”