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Solving Impurity Structures Using Inelastic Neutron Scattering

Solving Impurity Structures Using Inelastic Neutron Scattering. Collin Broholm* Johns Hopkins University and NIST Center for Neutron Research. Y 2-x Ca x Ba Ni O 5. Y 3+. Quantum Magnetism - Pure systems - vacancies - bond impurities Conclusions. Ca 2+.

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Solving Impurity Structures Using Inelastic Neutron Scattering

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  1. Solving Impurity Structures Using Inelastic Neutron Scattering Collin Broholm* Johns Hopkins University and NIST Center for Neutron Research Y2-xCaxBaNiO5 Y3+ Quantum Magnetism - Pure systems - vacancies - bond impurities Conclusions Ca2+ *supported by the NSF through DMR-0074571

  2. D. H. Reich, Guangyong Xu, and I. Zaliznyak Physics and Astronomy, Johns Hopkins University G. Aeppli, M. E. Bisher, and M. M. J. Treacy NEC Research Institute J. F. DiTusa Physics and Astronomy, Lousiana State University R. L. Paul National Institute of Standards and Technology C. D. Frost ISIS Facility Rutherford Appleton Laboratory T. Ito K. Oka Electrotechnical Laboratory, Japan H. Takagi ISSP, University of Tokyo Collaborators Collaborators

  3. Inelastic Neutron Scattering Nuclear scattering Magnetic scattering

  4. SPINS Cold neutron triple axis spectrometer at NCNR

  5. Simple example of “Quantum” magnet Cu(NO3)2.2.5D2O : dimerized spin-1/2 system Only Inelastic magnetic scattering

  6. Why dimerized chain is a spin liquid J • inter-dimer coupling allows triplet propagation as can be seen from the dispersion of the singlet triplet transition • A spin-1/2 pair with AFM exchange has a singlet - triplet gap:

  7. Types of Quantum magnets • Definition: small or vanishing frozen moment at low T: • Conditions that yield quantum magnetism • Low effective dimensionality • Low spin quantum number • geometrical frustration • dimerization • weak connectivity • interactions with fermions • Novel coherent states • impurities can induce magnetic polarons

  8. One dimensional spin-1 antiferromagnet Y2BaNiO5 Y2BaNiO5 : spin 1 AFM Ni 2+ Impure Pure 2

  9. Macroscopic singlet ground state of S=1 chain • Magnets with 2S=nz have a nearest neighbor singlet covering • with full lattice symmetry. • This is exact ground state for spin projection Hamiltonian • Excited states are propagating bond triplets separated from the • ground state by an energy gap Haldane PRL 1983 Affleck, Kennedy, Lieb, and Tasaki PRL 1987

  10. Coherence in a fluctuating system w ³ D h w = D h Short range G.S. spin correlations Coherent triplet propagation

  11. Impurities in Y2BaNiO5 Ca2+ Mg Pure • Mg2+on Ni2+ sites finite length chains • Ca2+ on Y3+ sites mobile bond defects Mg Ca2+ Ni Y3+ Kojima et al. (1995)

  12. Zeeman resonance of chain-end spins 20 g=2.16 hw (meV) 15 0 2 4 6 8 H (Tesla) 10 I(H=9 T)-I(H=0 T) (cts. per min.) 0 -5 0 0.5 1 1.5 2

  13. Form factor of chain-end spins Y2BaNi1-xMgxO5 x=4% Q-dependence reveals that resonating object is AFM. The peak resembles S(Q) for pure system. Chain end spin carry AFM polarization cloud of length x 7 Ni-Ni spacings »

  14. Sub gap excitations in Ca-doped Y2BaNiO5 Pure 9.5% Ca Y2-xCaxBaNiO5: • Ca-doping • creates states • below the gap • sub-gap states • have doubly • peaked structure • factor G. Xu et al. Science (2000)

  15. Incommensurate modulations in high TC superconductors YBa2Cu3O6.6 T=13 K E=25 meV La2-xSrxCuO4 k (rlu) h (rlu) Yamada et al. (1998) Hayden et al. (1998)

  16. Origin of incommensurate peaks in doped systems x q d µ • Charge ordering yields incommensurate spin order • Quasi-particle Quasi-hole pair excitations in Luttinger liquid • Single impurity effect dqindep. ofx

  17. Does d q vary with calcium concentration? dq not strongly dependent on x single impurity effect G. Xu et al. Science (2000)

  18. Bond Impurities in a spin-1 chain: Y2-xCaxBaNiO5 FM AFM Ni Ca2+ Y3+ O

  19. Form-factor for FM-coupled chain-end spins A symmetric AFM droplet Ensemble of independent randomly truncated AFM droplets

  20. Consistent description for all concentrations • Parameters: • background • amplitude • droplet size 7.6(5) G. Xu et al. Science (2000)

  21. Conclusions • Quantum Magnets • low dimensional frustrated and/or weakly connected • Coherent low T states rather than magnetic order • Challenging to describe because fluctuations are essential • Impurities in spin-1 chain • They create sup-gap composite spin degrees of freedom • Edge states have extended AFM wave function • Holes create AFM spin polaron with phase shift p • Probing impurities with neutrons • Spectroscopic separation yields unique sensitivity to impurity structures (in quantum magnets) through coherent diffuse inelastic neutron scattering

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