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NMR SB Fan Zhang also Ferenc Zamborszky Weiqiang Yu David Chow Pawel Wzietek (Orsay) Sylvie Lefebvre (Sherbrooke) Molecu

Bourbonnais and Jerome (1999) Charge order in quasi-1D organic conductors NMR SB Fan Zhang also Ferenc Zamborszky Weiqiang Yu David Chow Pawel Wzietek (Orsay) Sylvie Lefebvre (Sherbrooke) Molecules and crystals: Craig Merlic Andreas Baur Dean Tantillo Barakat Alavi Summary slide

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NMR SB Fan Zhang also Ferenc Zamborszky Weiqiang Yu David Chow Pawel Wzietek (Orsay) Sylvie Lefebvre (Sherbrooke) Molecu

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  1. Bourbonnais and Jerome (1999) Charge order in quasi-1D organic conductors NMR SB Fan Zhang also Ferenc Zamborszky Weiqiang Yu David Chow Pawel Wzietek (Orsay) Sylvie Lefebvre (Sherbrooke) Molecules and crystals: Craig Merlic Andreas Baur Dean Tantillo Barakat Alavi

  2. Summary slide • CO ubiquitous to ¼-filled CTS. Pressure can be used to tune interactions, ground states. What does this say about sequence of phase transitions in (TM)2X? • AsF6 salt: CO, SP order parameters repulsive • SbF6 salt: CO, AF order parameters attractive • New AF phase in SbF6; also CO (maybe different CO?) • Evidence thatcounterion potential softness plays a role in stabilizing intermediate CO phase? (Brazovskii, Poilblanc)

  3. 13C spectrum in (TMTTF)2AsF6, signature of CO is emergence of inequivalent sites… A B A B B at magic angle

  4. Clay, et al., PRB (2002) CO liquid 1D (or Q1D) Extended Hubbard model @ 1/4 filling, T=0 consistent with CO seen by experiments Seo and Fukuyama, JPSJ (1997): (mean-field approximation in higher dimension) Clay, et al., PRB (2002) Ground state AF with charge disproportionation

  5. Order parameters for two compounds: (TMTTF)2PF6, (TMTTF)2AsF6 Tco(PF6)~65K Tco(AsF6)=103K CO transition is probably continuous… Breaks inversion symmetry of unit cell (Monceau, et al., divergent low freq. susceptibility) SCN, ReO4, Br, PF6, AsF6, SbF6…: they’re insulating and they’re CO (Coulon, Monceau, Nad, Brown)

  6. T>TCO T<TCO Splitting of the C=C stretching mode results from 2:1 charge disproportionation From out T1: Charge disproportionation ratio approx. 3:1 ~.25. Fujiyama and Nakamura obtain 2:1 from NMR (cond-mat/0501063

  7. Clay, et al., PRB (2002) CO liquid 1D (or Q1D) Extended Hubbard model @ 1/4 filling, T=0 consistent with CO seen by experiments Seo and Fukuyama, JPSJ (1997): mean-field approximation in higher dimension Clay, et al., PRB (2002) Ground state AF with charge disproportionation • AsF6 • SbF6 pressure

  8. Competition between CO/SP phases in (TMTTF)2AsF6: high-pressure experiments

  9. CO D2 D1 CO CO+D D The appearance of the phase diagram is constrained by the order of the transitions… 2nd order boundary for CO/SP implies there is a coexistence region D=spin-Peierls

  10. c=0 c<0 c>0 b1b2<4c2 c>0 b1b2>4c2

  11. Dumm, et al., J. Phys. IV (2004) CO D2 D1 CO CO+D D CO D

  12. r/rRT AsF6,PF6 SbF6 TCO(SbF6) “structureless” transition, as in ReO4, SCN, SbF6 T(K) A puzzle: (TMTTF)2SbF6 with AF ground state C. Coulon, et al. *R. Laversanne, et al., J. Phys. Lett.45, L393 **C. Coulon, et al., PRB 33, 6235

  13. SbF6 salt CO at higher T AF (comm.) at lower T

  14. Applied pressure and the (TMTTF)2SbF6 phase diagram: CO, comm. AF order parameters ATTRACTIVE (GPa/10)

  15. P~0.6GPa ground state? c decreasing with T + equivalent intramolecular 13C, + broad spectrum singlet

  16. same AF? or different? Spectrum characteristics Peak separation ind. of B, as for AF, only weakly T-dependent Relative intensity of peaks grows smoothly on cooling, as for 1st order transition P=1.1GPa

  17. Jump in OP + smooth increase in AF volume fraction Similar to observations in SDW/AF first order phase boundary (Vuletic, et al., Lee, et al.) Conclude: new commensurate AF phase in SbF6 salt ??accompanied by charge disproportionation??

  18. Possible reason for suppression of CO: impeded motion of counterion (Monceau, Nad, Brazovskii, PRL 2001) SbF6 counterion broken symmetry (stops rotating) ambient pressure order parameter

  19. Riera & Poilblanc, PRB (2002) + Does

  20. Summary slide • CO ubiquitous to ¼-filled CTS • CO at high temperatures influences what further broken symm. observed at low T: AsF6 salts (CO vs. SP), AF in SbF6 • Different AF phase in SbF6, strongly first order character, different CO also? • Counterion potential softness plays a role in stabilizing intermediate CO phase (Brazovskii, Poilblanc): coincident crossovers in OP amplitude, motional narrowing associated with rotations + pressure effects

  21. View from crystallographic b-direction Pressure enhances interchain V Is the suppression of CO in (TMTTF)2SbF6 the result of a competition between these configurations?

  22. Papplied=0.5 GPa: No sign of splitting but lines are broad at higher temperatures T=10K At lower temperature, line broadens. 2D experiment demonstrates some molecules see no paramagnetism (somewhat like SP phase) T=4K

  23. H. Javadi, et al. (1988) CO is ubiquitous to TMTTF materials… ? Origin of metal-insulator (“structureless”) transition in (TMTTF)2SbF6

  24. pressure AsF6,PF6 SbF6 CO

  25. OP probably breaks inversion symmetry in MF6 salts… Divergence of real part of electric susceptibility ce’(q=0,w=0) observed; see Monceau, et al. (PRL, 2001) (Ising) symmetry-breaking OP that leads to divergent ce’(q=0)

  26. F. Zamborszky, et al., PRB 2002 Charge disproportionation ratio approx. 3:1 ~.25 Fujiyama and Nakamura obtain smaller rate ratio, about 4:1 (cond-mat/0501063)

  27. Organic D2X 2:1 charge-transfer salts: “½-” and “¼-filled” Hotta, JPSJ 72, 840 (TM)2X here (BEDT-TTF)2X (TM)2X ¼-filled systems susceptible to charge-disproportionation

  28. H. Javadi, et al. PRB (1988) CO ubiquitous to TMTTF salts: SCN, ReO4, Br, PF6, AsF6, SbF6… (Coulon, Monceau, Nad, ) What does phase diagram look like? What role does tendency for CO play in determining ground state?

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