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NEUTRON SCATTERING EVIDENCE FOR A SPIN-PEIERLS GROUND STATE IN (TMTTF) 2 PF 6

NEUTRON SCATTERING EVIDENCE FOR A SPIN-PEIERLS GROUND STATE IN (TMTTF) 2 PF 6. J.P. POUGET , P. FOURY-LEYLEKIAN, D. LE BOLLOC’H, V. CARDOSO, A. MORADPOUR Laboratoire de Physique des Solides –CNRS/ Université Paris-Sud B. HENNION Laboratoire Léon Brillouin - CNRS/CEA Saclay S. RAVY

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NEUTRON SCATTERING EVIDENCE FOR A SPIN-PEIERLS GROUND STATE IN (TMTTF) 2 PF 6

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  1. NEUTRON SCATTERING EVIDENCE FOR A SPIN-PEIERLS GROUND STATE IN (TMTTF)2PF6 J.P. POUGET, P. FOURY-LEYLEKIAN, D. LE BOLLOC’H, V. CARDOSO, A. MORADPOUR Laboratoire de Physique des Solides –CNRS/ Université Paris-Sud B. HENNION Laboratoire Léon Brillouin - CNRS/CEA Saclay S. RAVY Synchrotron SOLEIL - Saint Aubin C. COULON Centre de Recherche Paul Pascal - CNRS/Université Bordeaux I

  2. OUTLINE • earlier structural results • magnetic caracterisation of the samples used • neutron scattering detection of the SP superstructure • relationship with the charge ordering transition • the phase diagram of the TMTTF’s

  3. Loc O Phase diagram of (TMTCF)2X

  4. X-ray diffuse scattering evidence of a Spin-Peierls instability in (TMTTF)2PF6 1D diffuse scattering Satellite reflexion Pouget et al ICSM 1981: Mol. Cryst.Liq. Cryst. 79, 129 (1982)

  5. 1D SP instability develops below 80K and diverges

  6. (TMTTF)2PF6: SPIN PEIERLS GROUND STATE a* Very weak superlattice reflections in 1/2a* 10K X-ray pattern taken in 1981 But until now the structural counterpart of the SP ground state cannot be studied because of TMTTF X-ray damages!

  7. Growth of large crystals + synthesis of 98%deuterated TMTTFLPS (V. Cardoso and A. Moradpour) Volume ~20mm3! Low twinning ratio Both H12 and 98% D12 crystals

  8. SQUID and RPE measurements CRPP (C. Coulon)

  9. Spin-Peierls transition max dc/dT PF6(H12): TSP=16.4K PF6(D12):TSP=12.9K AsF6(H12):TSP=11.1K

  10. Spin-Peierls ground state: Singlet-Triplet splitting gap cT~exp-D/T PF6(H12): D = 79K PF6(D12): D = 75K AsF6(H12): D = 70K

  11. elastic neutron scattering evidence of SP superlattice reflexions in (TMTTF)2PF6 (H12) Laboratoire Léon Brillouin (-3.5,1.5,-1.5) reflexion  qSP=(1/2,1/2,1/2) But ISP/IB~1/10000 and ISP ~10% of the background ! Large background due to incoherent scattering coming from H P. Foury-Leylekian et al Phys. Rev. B70, 180405(R) (2004)

  12. then in (TMTTF)2PF6 (D12) (2.5,0.5,-0.5) reflexion ISP/IB~1 Deuteration diminishes the background by one order of magnitude!

  13. qSP=(1/2,1/2,1/2) Same qSP as for the Spin-Peierls transition of (BCPTTF)2X X : PF6 TSP = 36K X : AsF6 TSP = 32.5K (Q.Liu et al, Synth. Met. 56, 1840 (1993)) but in (TMTTF)2 PF6 ISP is weaker by more than 2 orders of magnitude! (OK with earlier X-ray diffuse scattering investigations) qSP: same wave vector as for the anion ordering transition of (TMTTF)2X with X=ReO4, ClO4, BF4 but in (TMTTF)2 PF6 ISP is weaker by 3 orders of magnitude Why the SP reflexions of (TMTTF)2PF6are so weak?

  14. Thermal dependence of the(TMTTF)2PF6superlattice peak intensity H12 D12 Very different of TSP=18+/-1K TSP=13K

  15. Charge ordering transitionfrom RPE measurements EPR lineshape asymmetry gives the effective conductivity (new analysis method: C. Coulon et al to be published) PF6(H12): TCO=69K PF6(D12):TCO=90K AsF6(H12):TCO=100K

  16. Tsp varies withTCO x x sp x from neutron scattering TSP ~ 30K – 0.19 TCO

  17. TSP ~ 30K – 0.19 TCO • For TCO = 0 TSP ~ 30K TSP > 30K in (BCPTTF)2PF6 and AsF6 • For TCO ~160K TSP ~ 0K but the ground state has shifted from SP to AF case of (TMTTF)2SbF6 and (TMTTF)2SCN

  18. Very large isotopic effect on the methyl groups (CH3 /CD3) Upon deuteration of (TMTTF)2PF6: TSP decreases by 30%! TCO increases by 25%! deuteration acts like a negative pressure: increase of the volume of themetyl group cavity? C-D bond length < C-H bond length by ~0.01Ǻ (Larger than in the AsF6 and the SbF6 salts; T.Nakamura et al)

  19. CO transition in the (TMTTF)2X’s Tco temperature of : charge disproportion /ferroelectric or antiferroelectric (X=SCN) transition abrupt change of the gap of charge (X= BF4,ClO4, NO3, Br)

  20. Mean-Field Peierls theory of the CO transition • Coupling between the structural degrees of freedom and the TMTTF charges via the 4kF electron hole response of the TMTTF For a spin-less fermion gas χ(4kF,T) ~ N(EF*) ln(EC*/kBT) • Mean-field 4kF Peierls transition kBTCO ≈ EC* exp-(2λ)-1 where: λ = N(EF*)g2/ħΩ with g : electron-phonon/electron-anion coupling Ω: phonon mode coupled to the charges for displacement of anions in the organic cavity (in the potential V of the cavity) Ω2=V’’/MX

  21. For a given 4kF electron hole response,TCO increases if: g increases (increase of the anion size) Ω decreases: V’’ decreases: anion potential flatter (large misfit between the anion shape and the methyl group cavity, deuteration) MXincreases (increase of the anion mass) g V’’ MX

  22. Ω V ’’ g

  23. However the structural degrees of freedom involved in the CO-ferroelectric transition remain to be identified! They have been detected at the CO-antiferroelectric transition of C. Coulon et al PRB26, 6322 (1982)

  24. Phase diagram of (TMTTF)2X T FS warping CO 4kF CDW ? SP (2kF BOW) SDW AF Br SbF6 SCN AsF6 PF6 pressure deuteration

  25. Left side of the phase diagram: 1D scenario 4kF site CDW (CO) 2kF BOW(SP instability) Effective spin-phonon coupling a quantum phase transition: SP spin fluid (AF)

  26. SPIN-PEIERLS Phase diagram at T=0°K kBTMF*=1.6 α²/πħw (Cross and Fischer) a: Spin-phonon coupling w: bare phonon frequency J: AF exchange integral Tco (no SP phase if a small) (TMTTF)2AsF6 (TMTTF)2 PF6 (BCPTTF)2PF6 *start of 2kF fluctuations MEM(TCNQ)2 CuGeO3 adiabatic regime (pseudogap) non adiabatic regime AF XY chain + site phonons (Caron and Moukouri PRL 76, 4050 (1996)) (J.P. Pouget Eur. Phys. J. B 20, 321 (2001); 24, 415 (2001))

  27. Right side of the phase diagram: Warping of the FS under pressure qnest Best nesting wave vector qnest ~ (1/2, 1/4,?) different of qSP= (1/2, 1/2, 1/2) « FS nesting » stabilizes the SDW ground state at the expense of the SP ground state

  28. SUMMARY • large (~ 20 mm3) deuterated single crystals of (TMTTF)2PF6 • and (TMTSF)2PF6 have been grown • elastic neutron scattering allows to detect very weak SP superlattice reflections in (TMTTF)2PF6 • next steps: • - inelastic neutron scattering investigation of the magnetic excitations in the SP ground state of (TMTTF)2PF6 • - elastic neutron scattering detection of SDW reflections in (TMTSF)2PF6

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