NMRSBFan Zhang

alsoFerenc ZamborszkyWeiqiang YuDavid ChowPawel Wzietek (Orsay)Sylvie Lefebvre (Sherbrooke)

Molecules and crystals:Craig MerlicAndreas BaurDean TantilloBarakat 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)2X?

2. AsF6 salt: CO, SP order parameters repulsive3. SbF6 salt: CO, AF order parameters attractive

4. New AF phase in SbF6; also CO (maybe different CO?)

5. Evidence that counterion 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…

6

5

4

3

2

1

0

abso

rptio

n (a

.u.)

6040200

frequency (kHz)

46K

81K

94K

97K

102K

T=105K

99K

(TMTTF)2AsF6

B=9T

B at magic angle

A B A B

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

Clay, et al., PRB (2002)

COliquid

i

iii

iii

ii nnVnnUchaatH 11* .).(

Order parameters for two compounds: (TMTTF)2PF6, (TMTTF)2AsF6

Tco(PF6)~65K

Tco(AsF6)=103KCO 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)

Splitting of the C=C stretching mode results from 2:1 charge disproportionation

T>TCO

T<TCO

From out T1: Charge disproportionation ratio approx. 3:1 ~.25.

Fujiyama and Nakamura obtain 2:1 from NMR(cond-mat/0501063

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 dimensionClay, et al., PRB (2002)

Ground state AF with charge disproportionation

1. AsF6

2. SbF6

Clay, et al., PRB (2002)

COliquid

i

iii

iii

ii nnVnnUchaatH 11* .).(

pressure

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

The appearance of the phase diagram is constrained by the order of the transitions…

CO

D1

D2

CO

CO+D Drepulsive OPs indicates 00 dP

dT

dP

dT COCO

2nd order boundary for CO/SP implies there is a coexistence region D=spin-Peierls

22

21

422

411

222

2110 ),(),( QcQQbQbQTPaQTPaFF

c=0

c>0b1b2>4c2

c<0

c>0b1b2<4c2

CO

D1

D2

CO

D

D

CO

CO+D

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

A puzzle: (TMTTF)2SbF6 with AF ground state

Salt a (angstrom)* TCO (K) (-cm)-1** Ground state

(TMTTF)2PF6 7.154 65K 40 Spin-Peierls

(TMTTF)2AsF6 7.178 103K 25 Spin-Peierls

(TMTTF)2SbF6 7.195 156K 10 AF

SbF6

AsF6,PF6

TCO(SbF6) “structureless” transition,as in ReO4, SCN, SbF6

RT

T(K)

*R. Laversanne, et al., J. Phys. Lett.45, L393**C. Coulon, et al., PRB 33, 6235

C. Coulon, et al.

1

2

3

4

5

67

10

2

3

4

5

67

100

2

3

13T

1-1 (

s-1

)

3 4 5 6 7 8 9100

2 3

temperature ( K )

(TMTTF)2SbF6

B=9.00T

0.1

2

4

68

1

2

4

68

10

2

4

68

100

1 H T

1-1(s

-1)

3 4 5 6 7 8 910

2 3

temperature T(K)

(TMTTF)2SbF6

B=9T P=0

SbF6 salt

CO at higher TAF (comm.) at lower T

1

2

4

6

810

2

4

6

8100

2

tem

pera

ture

T(K

)

0.100.080.060.040.020.00

applied pressure P(GPa)

(TMTTF)2SbF6

?charge ordered

antiferromagnetic

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

CO, comm. AF order parameters ATTRACTIVE

(GPa/10)

P~0.6GPa

ground state?

decreasing with T+ equivalent intramolecular 13C,+ broad spectrumsinglet

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

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 SbF6 salt??accompanied by charge disproportionation??

SbF6 counterion broken symmetry(stops rotating)

Possible reason for suppression of CO: impeded motion of counterion (Monceau, Nad, Brazovskii, PRL 2001)

ambient pressure order parameter

150

100

50

0

tem

pera

ture

T(K

)

1.00.80.60.40.20.0

applied pressure P(GPa)

(TMTTF)2SbF6

TCO (c-axis transport)

TCA (121

Sb NMR)

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: AsF6 salts (CO vs. SP), AF in SbF6

3. Different AF phase in SbF6, 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)2SbF6 the result of a competition between these configurations?

Pressure enhances interchain V

View from crystallographic b-direction

1

2

4

6

810

2

4

6

8100

2

tem

pera

ture

T(K

)

0.100.080.060.040.020.00

applied pressure P(GPa)

(TMTTF)2SbF6

TCO (c-axis transport)

TCA (121

Sb NMR)

TAF (1H NMR)

?

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

60

50

40

30

20

10

0

abso

rptio

n (a

.u.)

6040200

frequency (kHz)

8.0K

12.2K

14.4K

T=17.3K

(TMTTF)2AsF6

B=9T

CO is ubiquitous to TMTTF materials…

H. Javadi, et al. (1988)

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

pressure

CO

SbF6

AsF6,PF6

OP probably breaks inversion symmetry in MF6 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)

field mean in 2

1 )(~

p

TTk cB

0.001

2

3

4

567

0.01

2

3

4

567

0.1

2

3

13C

T1-1

(ms-1

)

3 4 5 6 7 8 9100

2 3

temperature T(K)

(TMTTF)2AsF6

B=9.00T

F. Zamborszky, et al., PRB 2002limit) Mott the (indensity particle local the ,

field local gfluctuatin transverse the is ;

2

21

1 )()(cos2

n

hohthtdtT

Charge disproportionation ratio approx. 3:1 ~.25

Fujiyama and Nakamura obtain smaller rate ratio, about 4:1 (cond-mat/0501063)

¼-filled systems susceptible to charge-disproportionation

Organic D2X 2:1 charge-transfer salts: “½-” and “¼-filled”

(TM)2X here

(BEDT-TTF)2X (TM)2X

Hotta, JPSJ 72, 840

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?

H. Javadi, et al. PRB (1988)