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Neutron scattering investigation of (TMTTF) 2 PF 6

Neutron scattering investigation of (TMTTF) 2 PF 6. P. Foury-Leylekian a , S. Petit b , B. Hennion b , A. Moradpour a and J.-P. Pouget a Laboratoire de Physique des Solides, CNRS-UMR 8502, Université Paris-sud, 91405 Orsay, France b. Laboratoire Léon Brillouin,

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Neutron scattering investigation of (TMTTF) 2 PF 6

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  1. Neutron scattering investigation of (TMTTF)2PF6 P. Foury-Leylekiana, S. Petitb, B. Hennionb, A. Moradpoura and J.-P. Pougeta Laboratoire de Physique des Solides, CNRS-UMR 8502, Université Paris-sud, 91405 Orsay, France b. Laboratoire Léon Brillouin, CEA-CNRS, UMR 12, 91191 Gif-sur-Yvette, France ECRYS Cargèse, Corse (F) August 25-29 2008

  2. Loc Phase diagram of (TMTCF)2X PF6(D12) O 4kF charge ordering at TCO=90K SP ground state

  3. Spin susceptibility measurements: (TMTTF)2X TCO Thermal behavior of a S=1/2 Heisenberg chain J~410-420K (Dumm et al PRB 62, 6510 (2000)) TSP no effect at TCO: spin – charge decoupling

  4. Ground states of the 1D Heisenberg chain AF 2kF SDW SP 2kF bond « CDW » (BOW) Spin-Peierls r=1 (1 electron per site) Mott-Hubbard localization case of CuGeO3 r=1/2 (1 electron per dimer) Case of (TMTTF)2X

  5. Spin-Peierls transition cS→0 at T=0: S=0 ground state

  6. (TMTTF)2PF6: superlattice reflexions below TSP h= ½ a* : 2a periodicity chain dimerization (pairing of S=1/2 units into S=0 singlet) Elastic neutron scattering (P. Foury-Leylekian et al PRB 7 R180405 (2004))

  7. Thermal dependence of the(TMTTF)2PF6 (D12) (1/2,1/2,1/2) superlattice peak intensity TSP=13K max dc/dT:TSP=12.9K

  8. Magnetic excitations in the SP ground state: T=0K Continuum of excitations gapped at 2Δσ Δσ spin-peierls gap q/qSP Triplet excitations of the SP dimer: S=1 magnonmode gapped at Δσ S=1 S=0 (Uhrig et Schulz PRB 54, R9624 (1996))

  9. Magnetic excitations in the SP ground state 1 - magnon mode: propagation of the triplet excitations (S=1) of the SP dimer dispersion: ħωM(q)= E(q) with E(q)=[Δ2(q)+ε2(q)]1/2 Δ(q)= Δcos2πq , ε(q)=Jeff sin2πq Jeff=J (πJ/2) for the XY(Heisenberg) chain (Bonner & Blote PRB 25, 6959 (1982)) 2 - continuum of double magnon excitations located in between: ħωl(q)= Δ +E(q) and ħωs(q)= 2E(q/2) (Uhrig & Schulz PRB 54, R9624 (1996))

  10. Inelastic neutron scattering study • performed with ~1cm3 of 98% deuterated (TMTTF)2 PF6 powder (A. Moradpour LPS) • 2T triple axis study at Orphée reactor (LLB Saclay)

  11. Simulation of powder average* 1D magnon collective mode 1D continuum Max of E(q): Jeff Max of ħωl(q):Δσ+Jeff Min of E(q): Δσ Min of ħωl(q):≥ 2Δσ Energy scan in QSP = (3/2,-1/2,1/2)= 1.66Å-1 * Simulation ignoring factor structure effects S. Petit

  12. neutron count variation inside the SP phase of (TMTTF)2 PF6 (d12): I(4K)-I(11K) evidence of two excitation energies Δmax narrow excitation ΔL broad excitation ΔU Δσ 2Δσ Double gap structure previously observed in CuGeO3 single crystals (Ain et al PRL 78, 1560, 1997) The continuum of excitationsis more peaked in(TMTTF)2PF6 than in CuGeO3 (magnetism more 1D in (TMTTF)2X than in CuGeO3?)

  13. Low T excitation gaps I (4K) – I(18K) reference of intensity 18-20K well above TSP=13K 0 ΔL ΔU negative intensity because of the formation of a gap in the excitations of the Heisenberg chain (reference of intensity above TSP) Powder ΔL~67KΔU~150K Single crystal ΔL=68K ΔL~ ΔU/2

  14. Spin-Peierls ground state: Singlet-Triplet splitting gap cT~exp-Ds/T (C. Coulon) PF6(D12): Ds = 75K = DU/2 ~DL

  15. Simulation of powder average* 1D magnon collective mode 1D continuum Max of E(q): Jeff Max of ħωl(q):Δσ+Jeff Min of E(q): Δσ Min of ħωl(q):≥ 2Δσ High energy scan * Simulation ignoring factor structure effects S. Petit

  16. High energy scan at Q = 3.4 Å-1 response at 55meV ~ Jeff (if Δ neglected) Jeff= πJ/2 leads to J~400K OK with J=410-420K obtained for the « Bonner et Fisher » fit of χspin(T)

  17. Thermal dependence of the magnetic excitations in the SP phasescan difference: I(T)-I(18K) Reference scan

  18. Only the « ΔU »continuum ofexcitationsisdetected at 7K (~TSP/2)! divergence of the density of state of the continuum of 1D magnetic excitations gapped excitations

  19. Reasons for the non–observation of the ΔL magnonpeak • Peak merges in the continuum? • Peak intensity vanishes? • Peak broadens? life time effect: efficient decay mode • Possible explanation:magnon modedecays intotwo bounded spinons on approachingTSP S=1 X X S=1/2 S=1/2 Binding due to unfavorable (out of phase) interchain coupling

  20. Decay of magnon into two bound spinons • Possible near TSP when the cost of interchain coupling is not large • Creation of bound spinons inside the SP phase of (TMTTF)2PF6 is possible because the 3D spin-Peierls distortion (i.e. SP satellite intensity) is very weak (P. Foury-Leylekian et al PRB 70, R180405 (2004)) • By this scenario one passes continuously (through a 2nd order transition) when Δ→0 from the excitations of the SP chain to those (only a continuum of free spinons) of the Heisenberg chain

  21. In the vicinity of TSP the intensity of the ΔUpeak drops and a larger gap in the excitations of the Heisenberg chain is revealed!

  22. A broad max of intensity at « ΔU » and a large gap in the excitations of the Heisenberg chain are still observed above TSP! I<0 below 20meV: pseudo-gap formation? difference with thermal correction

  23. Thermal evolution of the upper gap: ΔU 2Δχspin TSP Δu does not vanishes at TSP! (linear extrapolation to zero at ~35K)

  24. χRPEdu(TMTTF)2PF6 (D12)C. Coulon drops below ~ 40K vanishing of DU in thepseudo-gap region?

  25. Pseudo-gap built by SP 1D structural fluctuations:1D X-ray diffuse scattering observed above TSP in (TMTTF)2PF6and AsF6 1D structural SP fluctuations above TSP Pouget et al Mol. Cryst.Liq. Cryst. 79, 129 (1982)

  26. mean-field energy scale • 1D structural fluctuations detected until: ~60-80K PF6 (H12)- 40K AsF6 If one takes TSPMF~60K for the PF6 (D12), the BCS relationship gives: 2Δ1DMF~215K=18.5meV In this energy range inelastic neutron scattering reveals a drop of the magnetic excitation spectrum of the Heisenberg chain negative intensity in the scan difference I(T) - I(18K)

  27. Scan difference: 2ΔMF 2ΔMF

  28. Summary • This is the first time that magnetic excitations have been measured by neutron scattering in an organic conductor • The SP transition of (TMTTF)2PF6andof CuGeO3 differs: in (TMTTF)2PF6: - the magnon mode decay inside the SP phase - above TSP:there are pretransitional SP fluctuations and a pseudo gap formation (adiabatic limit) in CuGeO3 : - a sharp and intense magnon mode is followed until TSP where Δ vanishes abruptly - no pseudo gap effects are observed above TSP (non adiabatic limit) Crossover of S(q,ω) from the SP ground state (with magnon excitations) to the uniform Heisenberg chain (with spinon excitations) need to be calculated Chain fluctuations needed to be included in the treatment of excitations

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