nmr sb fan zhang also ferenc zamborszky weiqiang yu david chow pawel wzietek (orsay) sylvie lefebvre...
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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)
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