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 Condensed Matter Thursday, October 26, 2006 4:00 PM Physics Building, Room 204 Note special time. Note special room. Andrey Zheludev [Host: Seung-Hun Lee] Oak Ridge National Laboratory "Quantum phase transitions and magnon stability in gapped spin chains and ladders" ABSTRACT:The lowest energy excitations in quantum disordered spin chains and ladders are a triplet of massive magnons. The stability of these quasiparticles depends critically on the symmetries of the actual spin Hamiltonian. When the spin system undergoes a quantum phase transition, for example one induced by a strong external magnetic field, the symmetry of the ground state changes, and so do the excitations. I will present a comparative inelastic neutron scattering study of three distinct one-dimensional disordered spin systems: the S=1 quasi-1D bond-alternating antiferromagnet NTENP, the uniform anisotropic S=1-chain Haldane-gap compound NDMAP and the uniform isotropic composite'' Haldane spin chain IPA-CuCl3. For each material I will discuss the field-induced condensation of magnons, and analyze the spectra measured below, at and above the transition point. The high-field phase will be characterized either as a Bose-Einstein magnon condensate, with a cartelistic gapless Goldstone mode, or as a gapped "quantum spin solid". The issue of magnon stability in each phase will be addressed in detail.