Contents

## Collaborators Inelastic Neutron Scattering Simple example of Quantum magnet Why dimerized chain is a spin liquid

Tindell, Lisa, News Editor has reference to this Academic Journal, PHwiki organized this Journal Solving Impurity Structures Using Inelastic Neutron Scattering Quantum Magnetism – Pure systems – vacancies – bond impurities Conclusions Collin Broholm Johns Hopkins University in addition to NIST Center as long as Neutron Research supported by the NSF through DMR-0074571 Ca2+ Y3+ Y2-xCaxBaNiO5 Collaborators G. Aeppli M. E. Bisher J. F. DiTusa C. D. Frost T. Ito T. H. Kim K. Oka R. Paul D. H. Reich H. Takagi M. M. J. Treacy G. Xu I. A. Zaliznyak Inelastic Neutron Scattering

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SPINS Cold neutron triple axis spectrometer at NCNR Simple example of Quantum magnet Cu(NO3)2.2.5D2O : dimerized spin-1/2 system Only Inelastic magnetic scattering A spin-1/2 pair has a singlet – triplet gap: inter-dimer coupling yields dispersive mode Why dimerized chain is a spin liquid J

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 low connectivity interactions with fermions Quantum magnets can display novel coherent states Why study Quantum magnets Coherent many body states are fascinating in addition to useful Superconductivity Fractional Quantum Hall effect Bose Condensation Quantum magnets without static order at T=0 Each phenomenon provides different experimental info about macroscopic quantum coherence Only in quantum magnets are dynamic correlations directly accessible (through neutron scattering) Why study impurities in quantum magnets Impurities are inevitable or even necessary to produce coherence Probing the response to impurities reveals the building blocks of a macroscopic quantum state. Impurities in quantum magnets can be explored at the microscopic level.

Dynamic condensed matter: 1D antiferromag. 2 Ni 2+ Y2BaNiO5 : spin 1 AFM Impure Pure Macroscopic singlet ground state of S=1 chain This is exact ground state as long as 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, in addition to Tasaki PRL 1987 Coherence in a fluctuating system D = w h Probing spatial coherence of Haldane mode D ³ w h Probing equal time correlation length

Impurities in Y2BaNiO5 Ca2+ Y3+ Mg2+on Ni2+ sites finite length chains Ca2+ on Y3+ sites mobile bond defects Mg Ni Kojima et al. (1995) Zeeman resonance of chain-end spins I(H=9 T)-I(H=0 T) (cts. per min.) hw (meV) H (Tesla) 0 2 4 6 8 g=2.16 0 0.5 1 1.5 2 -5 0 10 15 20 Form factor of chain-end spins Q-dependence reveals that resonating object is AFM. The peak resembles S(Q) as long as pure system. Chain end spin carry AFM spin polarization of length x back into chain Y2BaNi1-xMgxO5 x=4%

Sub gap excitations in Ca-doped Y2BaNiO5 Pure 9.5% Ca Ca-doping creates states below the gap sub-gap states have doubly peaked structure factor Y2-xCaxBaNiO5: G. Xu et al. Science (2000) Incommensurate modulations in high TC superconductors Hayden et al. (1998) Yamada et al. (1998) La2-xSrxCuO4 Why is Y2-xCaxBaNiO5 incommensurate Charge ordering yields incommensurate spin order Quasi-particle Quasi-hole pair excitations in Luttinger liquid Single impurity effect dq indep. of x

Does d q vary with calcium concentration dq not strongly dependent on x single impurity effect G. Xu et al. Science (2000) Bond Impurities in a spin-1 chain: Y2-xCaxBaNiO5 Ba Ni Y O (a) Form-factor as long as FM-coupled chain-end spins A symmetric AFM droplet Ensemble of independent r in addition to omly truncated AFM droplets

Conclusions Quantum Magnets low dimensional frustrated in addition to /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 p

## Tindell, Lisa News Editor

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