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Impurities and finite temperature effects in a one-dimensional spin-1 antiferromagnet

Impurities and finite temperature effects in a one-dimensional spin-1 antiferromagnet. Collin Broholm Johns Hopkins University and NIST Center for Neutron Research. Coherent excitations in Y 2 BaNiO 5 Loss of coherence for T>0 Chain-end spins in Y 2 BaNi 1-x Mg x O 5

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Impurities and finite temperature effects in a one-dimensional spin-1 antiferromagnet

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  1. Impurities and finite temperature effects in a one-dimensional spin-1 antiferromagnet Collin Broholm Johns Hopkins University and NIST Center for Neutron Research Coherent excitations in Y2BaNiO5 Loss of coherence for T>0 Chain-end spins in Y2BaNi1-xMgxO5 AFM droplets in Y2-xCaxBaNiO5 Conclusion Supported by the NSF through DMR-9453362

  2. Collaborators Guangyong Xu JHU -> University of Chicago G. Aeppli NEC J. F. DiTusa Louisiana State University I. A. Zaliznyak JHU -> BNL C. D. Frost ISIS T. Ito Electro-technical Lab Japan K. Oka Electro-technical Lab Japan H. Takagi ISSP and CREST-JST M. E. Bisher NEC M. M. J. Treacy NEC Experiments performed at NIST and ISIS

  3. Why does spin-1 AFM have a spin gap? • Magnets with 2S=nz have a nearest neighbor singlet covering. • For integer spin chains (S=n, z=2) this state is “close to” • the ground state • 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

  4. Magnetic Neutron Scattering The scattering cross section is proportional to the Fourier transformed dynamic spin correlation function

  5. Y2BaNiO5 on SPINS E<10 meV NIST cold neutron guide hall

  6. ISIS Experimental hall Y2BaNiO5 on MARI 5 meV<E<100 meV

  7. Low T excitations in spin-1 AFM chain Y2BaNiO5 T=10 K MARI chain ki • Points of interest: • Haldane gap D=8 meV • Coherent mode • S(q,w)->0 for Q->2np

  8. Probing anisotropy and inter-chain coupling in Y2BaNiO5 DaDbDc • Maintaining , we • Derive polarization by • rotating about chain • Look for inter-chain • coupling by varying Dintensity (coutns per 15 min.) I(q,w) (1/meV) Weak anisotropy: Highly one dimensional

  9. Sum rules and the single mode approximation The dynamic spin correlation function obeys sum-rules: When a coherent mode dominates the spectrum: Then sum-rules link S(q) and e(q)

  10. Propagating triplet in alternating spin-1/2 chain Cu(NO3)2.2.5D2O IRIS data SMA model 0.5 hw (meV) 0.4 Q/p 0 2 4 0 2 4 6 The “incommensurate” size of spin dimers yields different periods for dispersion relation and structure factor. An effect captured by the SMA.

  11. Two magnon excitations in an alternating spin chain Tennant, Nagler, Xu, Broholm, and Reich.

  12. Haldane mode in Y2BaNiO5 at finite T • Effects of heating: • Line-width increases • Effective D increases

  13. T-dependence of relaxation rate and “resonance” energy • Parameter free comparison: • Semi-classical theory of triplet • scattering by Damle and Sachdev æ ö D 3 k T ( ) ç ÷ G = - 0 B T exp ç ÷ p k T è ø B • Quantum non linear s model æ ö D ( ) ç ÷ D = D + p D - T 0 2 k T exp ç ÷ 0 0 B k T è ø B Derived from c = 0 ( T ) D ( ) x T Neglecting T-dependence of spin wave velocity c0

  14. Q-scans versus T: energy resolved and energy integrated w ³ D h Probing equal time correlation length w = D h Probing spatial coherence of Haldane mode

  15. Coherence and correlation lengths versus T Coherence length exceeds correlation length for kBT<D becoming very long as T 0 Equal-time correlation length saturates at x=8. (Solid line from Quantum non linear s model)

  16. Properties of pure Y2BaNiO5 • Anisotropy split Haldane gap: Da=7.5 meV, Db=8.6 meV, Dc=9.5 meV • No inter-chain coupling detected: |J’/J|<5.10-4 • Coherent mode described by SMA for T<<D/kB • Activated relaxation rate of q=p mode is described by semi-classical theory of interacting triplet wave packets. • Activated increase in resonance energy is significantly less than predicted by Qnls-model • Coherence length exceeds correlation length for T< D/kB and exceeds 40 lattice spacings for kBT/D=0.1

  17. Effects of finite chain length on Haldane mode q=p T=10 K • Mode shifts towards • J as in numerical • work on finite length • chains • Peak Broadens • because of chain • length distribution Pure 4% Mg

  18. 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 hw (meV)

  19. Structure factor of chain-end spins Q-dependence reveals that resonating object is AFM. The structure factor is indistinguishable from S(Q) for pure system. Chain end spin carry AFM spin polarization of length x back into chain

  20. Vacancy doping a Haldane spin chain • q=p mode shifts towards J • q=p mode broadens due to random chain length distribution • Applied field induces Zeeman resonance below Haldane gap • Resonating chain end spins have AFM form factor resembling S(q) for pure system.

  21. New 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

  22. Why a double ridge below the gap in Y2-xCaxBaNiO5 ? d q is single impurity prop. Indep. of • Charge ordering yields incommensurate spin order • Quasi-particle Quasi-hole pair excitations in a one dimensional hole liquid • Anomalous form factor for independent spin degrees of freedom associated with each donated hole x q d µ x

  23. Does dq vary with calcium concentration? 9.5% Ca dq 14% Ca dq is independent of [ ] Î x 0 . 095 ; 0 . 14 Double peak is predominantly a single impurity effect

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

  25. Form-factor for FM-coupled chain-end spins- AFM droplets 9.5% Ca A symmetric AFM droplet 14% Ca Ensemble of independent randomly truncated AFM droplets

  26. Calcium doping Y2BaNiO5 Experimental facts: • Ca doping creates sub-gap excitations with doubly peaked structure factor and bandwidth • The structure factor is insensitive to concentration and temperature for 0.095<x<0.14 and T<100 K Current interpretation: • Ca2+ creates FM impurity bonds which nucleate AFM droplets with doubly peaked structure factor • AFM droplets interact through intervening chain forming disordered random bond 1D magnet

  27. Broader Conclusions: • Dilute impurities in the Haldane spin chain create sub-gap composite spin degrees of freedom. • Composite spins have an AFM wave function that extends into the bulk over distances of order the Haldane length. • Neutron scattering can detect the structure of composite impurity spins in quantum magnets when the corresponding states exist at energies where the bulk magnetic density of states vanishes.

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