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  1. Y2-xCaxBaNiO5 One Dimensional Magnetic Systems Collin Broholm* Johns Hopkins University and NIST Center for Neutron Research Y3+ Strong Fluctuations in Condensed Matter Magnetism in one dimension Pure systems Doped systems Magnetized states Conclusions Ca2+ *supported by the NSF through DMR-0074571

  2. Collaborators G. Aeppli M. E. Bisher S.-W. Cheong J. F. DiTusa D. C. Dender C. D. Frost T. ItoP. R. Hammar T. H. KimS.-H. Lee K. Oka R. Paul W. Ratcliff III D. H. Reich H. Takagi M. M. J. Treacy G. XuI. A. Zaliznyak

  3. Inelastic Neutron Scattering Nuclear scattering Magnetic scattering

  4. SPINS Cold neutron triple axis spectrometer at NCNR

  5. Dynamic condensed matter: Phonons ZrW2O8 Al2O3 Weak connectivity Low energy “twist” modes Strong connectivity “Hard” spectrum Ernst el al (1998)

  6. Dynamic condensed matter: 1D antiferromag. Y2BaNiO5 : spin 1 AFM Ni 2+ Infrared divergence destabilizes Neel phase 2 Cooperative singlet ground state

  7. Dynamic Condensed matter: Magnetic Frustration ZnCr2O4 S.-H. Lee et al Weak connectivity triangular motif Interactions specify local order, not a critical Q vector

  8. Consequences of strong fluctuations Phonons : Thermal contraction Frustration : cooperative paramagnet c-1 Ernst et al (1998) 0 0 200 400 600 800 1000 T (K) 1D magnons : macroscopic singlet Ajiro et al. (1989)

  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. 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)

  11. 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

  12. 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 spin polarization of length x back into chain

  13. 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)

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

  15. Why is Y2-xCaxBaNiO5 incommensurate? x q d µ • Charge ordering yields incommensurate spin order • Quasi-particle Quasi-hole pair excitations in Luttinger liquid • Single impurity effect dqindep. ofx

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

  17. Bond Impurities in a spin-1 chain: Y2-xCaxBaNiO5 (b) Ca (c) (d) (e) (f) Y Ba (a) O Ni

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

  19. Gap-less continuum in spin-1/2 chain Copper Benzoate T=0.3 K 0 0.4 0.8 S=1/2 S=1

  20. Field induced incommensurate soliton lattice Dender et al. (1997) 0 2 4 6 8 H (T) Copper Benzoate T=0.3 K H=0 T H=7 T

  21. Conclusions • Weakly connected systems • Low energy fluctuations of composite degrees of freedom • Unusual bulk properties and impurity effects • 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 • Field effects in spin-1/2 chain • Gap-less RVB state, singlet formation on all length scales • Magnetization is carried by soliton defects that form dynamic incommensurate superstructure.