1. Solving Impurity Structures Using Inelastic Neutron Scattering Quantum Magnetism - Pure systems - vacancies - bond impurities Conclusions Collin Broholm* Johns Hopkins University and NIST Center for Neutron Research *supported by the NSF through DMR-0074571 Ca 2+ Y 3+ Y 2-x Ca x BaNiO 5

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. ItoK. Oka Electrotechnical Laboratory, Japan H. Takagi ISSP, University of Tokyo Collaborators

3. ACA 7/24/00 Inelastic Neutron Scattering Nuclear scattering Magnetic scattering

4. ACA 7/24/00 SPINS Cold neutron triple axis spectrometer at NCNR

5. ACA 7/24/00 Simple example of “Quantum” magnet Cu(NO 3 ) 2. 2.5D 2 O : dimerized spin-1/2 system Only Inelastic magnetic scattering Only Inelastic magnetic scattering

6. ACA 7/24/00  A spin-1/2 pair with AFM exchange has a singlet - triplet gap: 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

7. ACA 7/24/00 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. ACA 7/24/00 2 Y 2 BaNiO 5 : spin 1 AFM One dimensional spin-1 antiferromagnet Y 2 BaNiO 5 Ni 2+ Impure Pure

9. ACA 7/24/00 Macroscopic singlet ground state of S=1 chain This is exact ground state for spin projection Hamiltonian Magnets with 2S=nz have a nearest neighbor singlet covering with full lattice symmetry. 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. ACA 7/24/00 Coherence in a fluctuating system   Coherent triplet propagation   Short range G.S. spin correlations

11. ACA 7/24/00 Impurities in Y 2 BaNiO 5 Ca 2+ Y 3+ Mg 2+ on Ni 2+ sites finite length chains Ca 2+ on Y 3+ sites mobile bond defects Mg Ni Kojima et al. (1995) Mg Ca 2+ Pure

12. ACA 7/24/00 Zeeman resonance of chain-end spins I(H=9 T)-I(H=0 T) (cts. per min.) h  (meV) H (Tesla) 0 2 4 6 8 g=2.16 0 0.5 1 1.5 2 -5 0 10 15 20

13. ACA 7/24/00 Form factor of chain-end spins Q-dependence reveals that resonating object is AFM. The peak resembles S(Q) for pure system. 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  7 Ni-Ni spacings Chain end spin carry AFM polarization cloud of length  7 Ni-Ni spacings Y 2 BaNi 1-x Mg x O 5 x=4% 

14. ACA 7/24/00 Sub gap excitations in Ca-doped Y 2 BaNiO 5 Pure 9.5% Ca Ca-doping creates states below the gap sub-gap states have doubly peaked structure factor Y 2-x Ca x BaNiO 5 : G. Xu et al. Science (2000)

15. ACA 7/24/00 Incommensurate modulations in high T C superconductors Hayden et al. (1998) YBa 2 Cu 3 O 6.6 T=13 K E=25 meV h (rlu) k (rlu) Yamada et al. (1998) La 2-x Sr x CuO 4

16. ACA 7/24/00 Origin of incommensurate peaks in doped systems  Charge ordering yields incommensurate spin order  Quasi-particle Quasi-hole pair excitations in Luttinger liquid  Single impurity effect x q   q indep. of x

17. ACA 7/24/00 Does  q vary with calcium concentration?  q not strongly dependent on x single impurity effect G. Xu et al. Science (2000)

18. ACA 7/24/00 Bond Impurities in a spin-1 chain: Y 2-x Ca x BaNiO 5 Y3+Y3+ Ni O Ca 2+ FMAFM

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

20. ACA 7/24/00 Consistent description for all concentrations G. Xu et al. Science (2000) Parameters: background amplitude droplet size 7.6(5) Parameters: background amplitude droplet size 7.6(5)

21. ACA 7/24/00 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  Probing impurities with neutrons  Spectroscopic separation yields unique sensitivity to impurity structures (in quantum magnets) through coherent diffuse inelastic neutron scattering  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 