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Structure and dynamics of spin polarons induced by doping a Haldane spin-1 chain

Structure and dynamics of spin polarons induced by doping a Haldane spin-1 chain. Collin Broholm * Johns Hopkins University and NIST Center for Neutron Research. M. Kenzelmann JHU K. Oka Tsukuba T. G. Perring ISIS H. Takagi ISSP M. E. Bisher NEC M. M. J. Treacy NEC

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Structure and dynamics of spin polarons induced by doping a Haldane spin-1 chain

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  1. Structure and dynamics of spin polarons induced by doping a Haldane spin-1 chain Collin Broholm* Johns Hopkins University and NIST Center for Neutron Research M. Kenzelmann JHU K. Oka Tsukuba T. G. Perring ISIS H. Takagi ISSP M. E. Bisher NEC M. M. J. Treacy NEC R. Paul NIST Ying Chen LANL Guangyong Xu BNL G. Aeppli UCL J. F. DiTusa LSU I. A. Zaliznyak BNL C. D. Frost ISIS T. Ito Tsukuba Xu et al., Science 289, 419 (2000) Kenzelmann et al., PRL 90, 087202 (2003) * Supported by the National Science Foundation

  2. Outline • Introduction to pure Y2BaNiO5 • Site impurities (Mg2+↔ Ni2+) • Bond impurities (Ca2+↔ Y3+) • Conclusions and outlook

  3. ki kf Q SPINS cold neutron spectrometer at NCNR

  4. MAPS Spectrometer at ISIS in UK

  5. Haldane gap in Y2BaNiO5 60 hw(meV) 40 20 0 1.5 0 2 0.5 1 q (p) O2- MAPS (ISIS) Ni 1-cosq Y S(qw)~d(w-e(q)) e(q) Ba T. Ito, K. Oka and H. Takagi

  6. Inter-chain coupling and anisotropy Chain I(q,w) (1/meV) Dint. (cts per 15 min.) DaDbDc f I(q,w) (1/meV) Dintensity (coutns per 15 min.) Q

  7. Facts on Y2BaNiO5 Intra-chain exchange Anisotropy Inter-chain exchange

  8. Outline • Introduction to pure Y2BaNiO5 • Site impurities (Mg2+↔ Ni2+) • Bond impurities (Ca2+↔ Y3+) • Conclusions and outlook

  9. Cutting chains in Y2BaNi1-xMgxO5 • Mg2+on Ni2+ sites finite length chains Ni2+ 3 Mg2+ Mg2+ Pure Kojima et al. (1995) O2- Y3+

  10. Haldane gap with impurities Kenzelmann et al. PRL (2003)

  11. An ensemble of finite length chains 8% P(D)/max(P(D)) 8% P(L) 4% 4% 2% 2% Chain length L Probability spin in isolated segment of length L Gap from chain of length L Ensemble averaged scattering

  12. Haldane gap with impurities Kenzelmann et al. PRL (2003)

  13. ESR with neutrons Kenzelmann et al. PRL (2003)

  14. Minimal model for chain end spins L is even Affleck, Kennedy, Lieb, and Tasaki PRL (1987) L is odd C. D. Batista et al., PRB(1999) Singlet-triplet Single ion anisotropy Zeeman

  15. Chain-end composite spin Kenzelmann et al. PRL (2003)

  16. Form factor for chain-end spin Kenzelmann et al. PRL (2003)

  17. Outline • Introduction to pure Y2BaNiO5 • Site impurities (Mg2+↔ Ni2+) • Bond impurities (Ca2+↔ Y3+) • Conclusions and outlook

  18. Hole doping Y2-xCaxBaNiO5 • Ca2+ on Y3+ sites mobile bond defects FM Ca2+ Ca2+ Y3+ Pure Kojima et al. (1995) Ni O2-

  19. Transport in Ca doped Y2BaNiO5 1D conductivity, no Charge ordering T. Ito et al. PRL (2002)

  20. Gap modes in 4% Ca-doped Y2BaNiO5

  21. Does d q vary with calcium concentration? dq not strongly dependent on x single impurity effect Xu et al., Science 289, 419 (2000)

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

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

  24. Gap modes in 4% Ca-doped Y2BaNiO5 Excited state ? Quasi-elastic From deg. GS

  25. Minimal model of spin polaron

  26. Intra polaron dynamics in Ca-doped Y2BaNiO5 Clean gap Intra quartet ? Pure Normalized Intensity (Arb) 4% Ca 0 0 20 15 5 25 10

  27. Conclusions: • Experimental observation of increase in Haldane gap with decreasing chain length • Dilute impurities in the Haldane spin chain create sub-gap composite spin degrees of freedom. • Edge states have an AFM wave function that extends into the bulk over distances of order the Haldane length. • Holes in Y2-xCaxBaNiO5 are surrounded by AFM spin polaron with central phase shift of p • Low energy spin polaron excitations could be • oxygen spin flip between aligned chain-end spins • Or anisotropy-split quartet Viewgraphs and publications at http://www.pha.jhu.edu/~broholm/homepage

  28. Outlook • Determine spin polaron level assignment in field • What is localization length for spin polaron? • Spin polaron structure in frustrated magnets • Spin polarons in high TC superconductors? • Magnetism of interacting spin polarons Design by T. D. Pike http://www.pha.jhu.edu/~broholm/MACS

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