Bourbonnais and Jerome (1999)
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NMR SB Fan Zhang also Ferenc Zamborszky Weiqiang Yu David Chow Pawel Wzietek (Orsay) PowerPoint PPT Presentation


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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)

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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

  • 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)


13C spectrum in (TMTTF)2AsF6,

signature of CO is emergence of inequivalent sites…

A B A B

B at magic angle


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


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)


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


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


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


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


c=0

c<0

c>0

b1b2<4c2

c>0

b1b2>4c2


Dumm, et al., J. Phys. IV (2004)

CO

D2

D1

CO

CO+D

D

CO

D


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


SbF6 salt

CO at higher T

AF (comm.) at lower T


Applied pressure and the (TMTTF)2SbF6 phase diagram:

CO, comm. AF order parameters ATTRACTIVE

(GPa/10)


P~0.6GPa

ground state?

c decreasing with T

+ equivalent intramolecular 13C,

+ broad spectrum

singlet


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


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??


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


Riera & Poilblanc, PRB (2002)

+

Does


  • 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


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?


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


H. Javadi, et al. (1988)

CO is ubiquitous to TMTTF materials…

? Origin of metal-insulator (“structureless”) transition in (TMTTF)2SbF6


pressure

AsF6,PF6

SbF6

CO


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)


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)


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


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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