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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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) counterion (Monceau, Nad, Brazovskii, PRL 2001)

+

Does


  • Summary slide counterion (Monceau, Nad, Brazovskii, PRL 2001)

  • 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 counterion (Monceau, Nad, Brazovskii, PRL 2001)

Pressure enhances interchain V

Is the suppression of CO in (TMTTF)2SbF6 the result of a competition between these configurations?


P counterion (Monceau, Nad, Brazovskii, PRL 2001)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


H. Javadi, et al. (1988) counterion (Monceau, Nad, Brazovskii, PRL 2001)

CO is ubiquitous to TMTTF materials…

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


pressure counterion (Monceau, Nad, Brazovskii, PRL 2001)

AsF6,PF6

SbF6

CO


OP probably breaks inversion symmetry in MF counterion (Monceau, Nad, Brazovskii, PRL 2001)6 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, counterion (Monceau, Nad, Brazovskii, PRL 2001)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 D counterion (Monceau, Nad, Brazovskii, PRL 2001)2X 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) counterion (Monceau, Nad, Brazovskii, PRL 2001)

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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