Physics Colloquium

Quasiparticles are the elementary excitations that carry energy in consensed matter systems much like photons and various elementary particles carry energy in the real world. Recent neutron scattering experiments demonstrate
how the quasiparticle description of the energy spectrum fails in magnetic crystals with non-magnetic ground states that can be identified as quantum spin liquids. The elementary excitations in such systems can be described as
a massive (i.e. having non-zero rest energy) uasiparticles, called magnons, which obey Bose statistics. It is known, however, that in a Bose quantum liquid the single-particle dispersion must terminate at a sufficiently high energy where the quasiparticle break-up into two excitations becomes allowed (i.e. the single-particle dispersion enters continuum of the two-particle states). The spectrum termination was originally predicted by L. Landau for
the superfluid helium-4, where it was subsequently extensively studied both theoretically and experimentally. A manifestation of this peculiar phenomenon in the case of a quantum spin liquid was recently observed in the spin dynamics of the Haldane-chain S=1 antiferromagnet CsNiCl3, where it was initially identified as a crossover from the single quasi-particle to a spin-continuum response. The quantum-spin-liquid (QSL) state of the two-dimensional (2D) S=1/2 Heisenberg antiferromagnet is of great current
interest as it may be relevant to the type of high-temperature superconductivity found in the cuprates. An organo-metallic material piperazinium hexachlorodicuprate (PHCC) is among the best known examples of the 2D QSL. Recent experiments indicate that a quasiparticle spectrum
termination point is also present in the excitation spectrum of the 2D quantum spin liquid existing in PHCC. It signals the failure of the Bose-quasiparticle description in an extended region of the system's phase space.