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 Charge order in quasi-1D organic conductors Bourbonnais and Jerome (1999)

Summary slide 1.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) 2 X? 2.AsF 6 salt: CO, SP order parameters repulsive 3.SbF 6 salt: CO, AF order parameters attractive 4.New AF phase in SbF 6 ; also CO (maybe different CO?) 5.Evidence that counterion potential softness plays a role in stabilizing intermediate CO phase? (Brazovskii, Poilblanc)

13 C spectrum in (TMTTF) 2 AsF 6, signature of CO is emergence of inequivalent sites… B at magic angle A B

1D (or Q1D) Extended Hubbard 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 Clay, et al., PRB (2002) CO liquid

Order parameters for two compounds: (TMTTF) 2 PF 6, (TMTTF) 2 AsF 6 T co (PF 6 )~65K T co (AsF 6 )=103K CO transition is probably continuous… Breaks inversion symmetry of unit cell (Monceau, et al., divergent low freq. susceptibility) SCN, ReO 4, Br, PF 6, AsF 6, SbF 6 …: theyre insulating and theyre CO (Coulon, Monceau, Nad, Brown)

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

1D (or Q1D) Extended Hubbard 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 1.AsF 6 2.SbF 6 Clay, et al., PRB (2002) CO liquid pressure

Competition between CO/SP phases in (TMTTF) 2 AsF 6 : high-pressure experiments

The appearance of the phase diagram is constrained by the order of the transitions… CO D1D1 D2D2 CO+DD 2 nd order boundary for CO/SP implies there is a coexistence region D=spin-Peierls

c=0 c>0 b 1 b 2 >4c 2 c<0 c>0 b 1 b 2 <4c 2

CO D1D1 D2D2 D D CO+D Dumm, et al., J. Phys. IV (2004)

A puzzle: (TMTTF) 2 SbF 6 with AF ground state Salta (angstrom)*T CO (K) ( -cm) -1** Ground state (TMTTF) 2 PF K40Spin-Peierls (TMTTF) 2 AsF K25Spin-Peierls (TMTTF) 2 SbF K10AF SbF 6 AsF 6,PF 6 T CO (SbF 6 ) structureless transition, as in ReO 4, SCN, SbF 6 RT T(K) *R. Laversanne, et al., J. Phys. Lett.45, L393 **C. Coulon, et al., PRB 33, 6235 C. Coulon, et al.

SbF 6 salt CO at higher T AF (comm.) at lower T

Applied pressure and the (TMTTF) 2 SbF 6 phase diagram: CO, comm. AF order parameters ATTRACTIVE (GPa/10)

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

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

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 SbF 6 salt ??accompanied by charge disproportionation??

SbF 6 counterion broken symmetry (stops rotating) Possible reason for suppression of CO: impeded motion of counterion (Monceau, Nad, Brazovskii, PRL 2001) ambient pressure order parameter

Riera & Poilblanc, PRB (2002) Does +

Summary slide 1.CO ubiquitous to ¼-filled CTS 2.CO at high temperatures influences what further broken symm. observed at low T: AsF 6 salts (CO vs. SP), AF in SbF 6 3.Different AF phase in SbF 6, strongly first order character, different CO also? 4.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

Is the suppression of CO in (TMTTF) 2 SbF 6 the result of a competition between these configurations? Pressure enhances interchain V View from crystallographic b-direction

P applied =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

CO is ubiquitous to TMTTF materials… H. Javadi, et al. (1988) ? Origin of metal- insulator (structureless) transition in (TMTTF) 2 SbF 6

pressure CO SbF 6 AsF 6,PF 6

OP probably breaks inversion symmetry in MF 6 salts… Divergence of real part of electric susceptibility e (q=0, =0) observed; see Monceau, et al. (PRL, 2001) (Ising) symmetry-breaking OP that leads to divergent e (q=0)

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/ )

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

CO ubiquitous to TMTTF salts: SCN, ReO 4, Br, PF 6, AsF 6, SbF 6 … (Coulon, Monceau, Nad, ) What does phase diagram look like? What role does tendency for CO play in determining ground state? H. Javadi, et al. PRB (1988)