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 * Supported by the National Science Foundation Ying Chen LANL Guangyong XuBNL G. AeppliUCL J. F. DiTusaLSU I. A. ZaliznyakBNL C. D. FrostISIS T. ItoTsukuba M. KenzelmannJHU K. Oka Tsukuba T. G. PerringISIS H. TakagiISSP M. E. Bisher NEC M. M. J. TreacyNEC R. PaulNIST Xu et al., Science 289, 419 (2000) Kenzelmann et al., PRL 90, 087202 (2003)

2. Outline  Introduction to pure Y 2 BaNiO 5  Site impurities (Mg 2+ ↔ Ni 2+ )  Bond impurities (Ca 2+ ↔ Y 3+ )  Conclusions and outlook

3. Sendai 11/8/03 SPINS cold neutron spectrometer at NCNR kiki kfkf Q

4. Sendai 11/8/03 MAPS Spectrometer at ISIS in UK

5. Sendai 11/8/03 Haldane gap in Y 2 BaNiO 5 40 20 60 0 0 0.51 1.5 2 q (  ) h  (meV) S(q  ) ~  (  -  (q)) 1-cosq (q)(q) Ba Y O 2- Ni T. Ito, K. Oka and H. Takagi MAPS (ISIS)

6. Sendai 11/8/03  intensity (coutns per 15 min.) I(q,w) (1/meV)  a  b  c Inter-chain coupling and anisotropy I(q,w) (1/meV)  int. (cts per 15 min.)  Chain Q

7. Sendai 11/8/03 Facts on Y 2 BaNiO 5 Intra-chain exchange Anisotropy Inter-chain exchange

8. Sendai 11/8/03 Outline  Introduction to pure Y 2 BaNiO 5  Site impurities (Mg 2+ ↔ Ni 2+ )  Bond impurities (Ca 2+ ↔ Y 3+ )  Conclusions and outlook

9. Sendai 11/8/03 Cutting chains in Y 2 BaNi 1-x Mg x O5 Mg 2+ on Ni 2+ sites finite length chains Kojima et al. (1995) Pure Ni 2+ Mg 2+ 3 Y 3+ O 2-

10. Sendai 11/8/03 Haldane gap with impurities Kenzelmann et al. PRL (2003)

11. Sendai 11/8/03 An ensemble of finite length chains Chain length L P(L) 2% 4% 8% P(  )/ max (P(  )) 2% 4% 8% Probability spin in isolated segment of length L Gap from chain of length L Ensemble averaged scattering

12. Sendai 11/8/03 Haldane gap with impurities Kenzelmann et al. PRL (2003)

13. Sendai 11/8/03 ESR with neutrons Kenzelmann et al. PRL (2003)

14. Sendai 11/8/03 Minimal model for chain end spins L is odd L is even Singlet-triplet Single ion anisotropy Zeeman C. D. Batista et al., PRB (1999) Affleck, Kennedy, Lieb, and Tasaki PRL (1987)

15. Sendai 11/8/03 Chain-end composite spin Kenzelmann et al. PRL (2003)

16. Sendai 11/8/03 Form factor for chain-end spin Kenzelmann et al. PRL (2003)

17. Sendai 11/8/03 Outline  Introduction to pure Y 2 BaNiO 5  Site impurities (Mg 2+ ↔ Ni 2+ )  Bond impurities (Ca 2+ ↔ Y 3+ )  Conclusions and outlook

18. Sendai 11/8/03 Hole doping Y 2-x Ca x BaNiO 5 Ca 2+ on Y 3+ sites mobile bond defects Kojima et al. (1995) Pure Ca 2+ Y 3+ FM Ca 2+ Ni O 2-

19. Sendai 11/8/03 Transport in Ca doped Y 2 BaNiO 5 T. Ito et al. PRL (2002) 1D conductivity, no Charge ordering

20. Sendai 11/8/03 Gap modes in 4% Ca-doped Y 2 BaNiO 5

21. Sendai 11/8/03 Does  q vary with calcium concentration?  q not strongly dependent on x single impurity effect Xu et al., Science 289, 419 (2000)

22. Sendai 11/8/03 Bond Impurities in a spin-1 chain: Y 2-x Ca x BaNiO 5 Y3+Y3+ Ni O FM Ca 2+

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

24. Sendai 11/8/03 Gap modes in 4% Ca-doped Y 2 BaNiO 5 Excited state ? Quasi-elastic From deg. GS Quasi-elastic From deg. GS

25. Sendai 11/8/03 Minimal model of spin polaron

26. Sendai 11/8/03 Intra polaron dynamics in Ca-doped Y 2 BaNiO 5 0 5 10 15 20 25 0 Normalized Intensity (Arb) Pure 4% Ca Clean gap ? Intra quartet

27. Sendai 11/8/03 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 Y 2-x Ca x BaNiO 5 are surrounded by AFM spin polaron with central phase shift of   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. Sendai 11/8/03 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 T C superconductors?  Magnetism of interacting spin polarons Design by T. D. Pike http://www.pha.jhu.edu/~broholm/MACS