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Condensed Matter Seminars

ics Condensed Matter
Thursday, April 13, 2006
4:00 PM
Physics Building, Room 204
Note special time.
Note special room.
Igor Zaliznyak [Host: Seunghun Lee]
Brookhaven National Laboratory
"Quasiparticle Breakdown in a Quantum Spin Liquid"
Quasiparticles are the elementary excitations carrying energy and momentum quanta in consensed matter, much like photons and elementary particles carry energy and momentum in the Universe surrounding us. Recent neutron scattering experiments demonstrate how quasiparticle description of energy spectra fails in magnetic crystals with non-magnetic ground states that can be identified as "quantum spin liquids" (QSL). The elementary excitations in such systems can be identified with massive (i.e. having non-zero rest energy) quasiparticles, called magnons, which obey Bose statistics, forming a Bose liquid. In a Bose quantum liquid, however, the single-particle dispersion can terminate at an energy where quasiparticle break-ups into two excitations become allowed, i.e. where the single-particle dispersion enters a continuum of two-particle states. Such spectrum endpoint 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 found in the spin dynamics of the Haldane-chain S=1 antiferromagnet CsNiCl3, where it was initially identified as a crossover from the single quasiparticle to a spin-continuum response. More recently, signatures of the quasiparticle spectrum termination were observed in the excitation spectrum of the two-dimensional (2D) quantum spin liquid existing in the organo-metallic material piperazinium hexachlorodicuprate (PHCC), indicating a failure of the Bose-quasiparticle description of the QSL state of the 2D S=1/2 Heisenberg antiferromagnet in an extended region of its phase space. These findings are of great current interest as they might have important implications for the type of high-temperature superconductivity found in cuprates.

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