Synthetic Metals, 41-43 (1991) 1735-1739 17 3 5

NMR STUDY OF (TMTTF)2Br

P. Wzietek~ C. Bourbonnai$~'* F .Creuzet~ D.J@rSme~ K. Bechgaard $

* L a b o r a t o i r e de Physique des S o l i d e s , Univers i t@ Par i s -Sud , b~t .510, 91405 Orsay, France

*Centre de Recherche en Physique du So l ide , Univers i t@ de Sherbrooke, Sherbrooke, Quebec, Canada J1K-2R1

SH.C. Oers ted I n s t i t u t e , DK-2100 Copenhagen, Denmark

ABSTRACT

We present 13C spin-lattice relaxation time and EPR susceptibility measurements

on selectively enriched single crystals of (TMTTF)2Br over a wide temperature

domain. These are shown to satisfy ID dynamic scaling for the existence of ID

paramagnons and antiferromagnetic correlations up to room temperature. Applied to

bromide salt which undergoes a Mott-Hubbard localization under 100K, our approach

confirms the spin-charge separation predicted for the ID correlated electron gas

model and provides a qualitative evaluation of the amplitude of AF fluctuations.

Electronic correlations in organic conductors have already been investigated by

the use of nuclear relaxation rate measurements [1,2] which provide a local probe

for studying the nature of the magnetic excitations and their evolution with

temperature. In the Bechgaard salts series for example, where the electronic

contribution to the 77Se and 13C relaxation is well known to be dominant [2], the -I

relaxation rate T 1 is directly related to the imaginary part X[ of the retarded

transverse electronic susceptibility integrated over all wave vectors ~ [3], and

the main contributions to come from uniform or ferro- (q=O) and antiferro-

(~=~0 where ~0 is the AF modulation vector) spin fluctuations [1,2]. magnetic

These contributions have quite different dynamics and can be experimentally

distinguished by their characteristic temperature profiles [2]. Dynamic scaling - - i ~

have shown that the Tli(q=O) and T I (q=Qo) parts are respectively arguments

proportional to the temperature dependent uniform and staggered magnetic correla-

tion lengths to some characteristic powers which depend on the dimensionality of

0379-6779/91/$3.50 © Elsevier Sequoia~rinted in The Netherlands

1736

spln fluctuations [2a]. Followln E thls scheme, we have been able to characterize

both the precursor and the critlcal temperature domains of several members of the

selenide [TMTSF)2X and sulfur [TMTTF)2X series. This has been particularly illus-

trated for three prototype compounds: the spln-Pelerls system (TMTTF)2PF 8 [2a, d],

the antiferromagnet (TMTSF)2PF 8 [2a, c] and the superconductor (TMTSF]2CIO 4 [2a].

Indeed, for all the three compounds, the observatlon of the scaling relatlon

between the nuclear relaxation rate T11 and the spin susceptlbility xs(T) has

demonstrated that one-dlmenslonal paramagnons glve the dominant contrlbutlon to

both T11 and xs(T) in the high temperature domain. The case of (TMTTF)2Br at

ambient pressure is particularly Interestlng here slnce the ground state Is

antiferromagnetic with no lattlce softening precursors, in contrast to (TMTTF]2PF s

[4-6], and furthermore, the Mort-Hubbard charge locallzatlon occurs below the

resistivity minimum near T ~IOOK [4,7,8]. This compound appears thus to be interme- P

diate between the strongly correlated (TMTTF)2PF 8 [Tp~220K) and the itinerant

antlferromagnet [TMTSF)2PF 6 (Tp<TN~12K) [2el. The above assumption will be glven a

more quantltative aspect in the present work.

1 H Prevlous NMR experiments performed by Creuzet et al [7] have shown that T11

is singular at T N ~ 14K , confirming the magnetic nature of the ordered state.

However, the proton NMR in these organic salts is not a good probe of the electro-

n i c b e h a v i o r in t h e f u l l p a r a m a g n e t i c domain b e c a u s e i s h u g e l y enhanced by

methyl g r o u p r o t a t i o n e f f e c t s [71. T h e r e f o r e , we have measu red t h e 13C n u c l e i

r e l a x a t i o n . The e x p e r i m e n t has been p e r f o r m e d on s i n g l e - c r y s t a l s ample o f a 90Z 13C

e n r i c h e d (on t h e c e n t r a l C-C bond ) (TMTTF)2Br. Al l m e a s u r e m e n t s were c a r r i e d ou t

a t a f i x e d f i e l d o f 56kG. The 13C s p e c t r u m i s made up o f two we l l r e s o l v e d l i n e s

s e p a r a t e d by 100 ppm a t 30OK. The r e l a x a t i o n o f each l i n e h a s been found to be

e x p o n e n t i a l o v e r a decade o f m a g n e t i z a t i o n . We have a l s o p e r f o r m e d an EPR e x p e r i -

ment on t h e same sample i n o r d e r to e x t r a c t t h e t e m p e r a t u r e dependence o f t h e

u n i f o r m m a g n e t i c s u s c e p t i b i l i t y i n t h e t e m p e r a t u r e r a n g e 20-300K.

The t e m p e r a t u r e dependence o f i s shown i n F i g u r e 1. I n t h e v i c i n i t y o f t he

m a g n e t i c t r a n s i t i o n a t T N s 1SK, t h e r e l a x a t i o n i s s t r o n g l y enhanced by a n t i f e r r o -

m a g n e t i c c r i t i c a l f l u c t u a t i o n s . For a 3D t r a n s i t i o n , u s i n g t h e C a u s s i a n a p p r o x i m a -

t i o n f o r t h e AF s u s c e p t i b i l i t y , i t ha s been shown t h a t : TTI(q=Qo)~(T-TN ) - 1 / 2 _ n e a r

T N [9 ] . We have v e r i f i e d t h a t f o r b o t h NMR l i n e s , s u c h a power law d i v e r g e n c e i s

o b s e r v e d be low 2SK o r so ( s e e t h e i n s e t o f t h e f i g u r e ) . I n t h e p a r a m a g n e t l c domain,

o u r r e s u l t s a r e s i m i l a r t o t h o s e o b t a i n e d p r e v i o u s l y on p r o t o n up t o 30K : t h e

r e l a x a t i o n r a t e i s a l m o s t c o n s t a n t up t o 60K [7] . Above t h i s t e m p e r a t u r e , T[ 1

i n c r e a s e s w i t h T and t h e upward c u r v a t u r e a p p e a r s t o be a common f e a t u r e o f t he

n o n - o r d e r e d p h a s e s o f b o t h (TMTTF)2X and (TMTSF)2X s e r i e s .

A c c o r d i n g t o t h e s c a l i n g a r g u m e n t s p r e s e n t e d i n r e f s . [ 2 a , c ] , t h e t e m p e r a t u r e

T~ ' dependence of for a d=1 antlferromagnet below T o is of the form:

Tll = CoTZ:[T) + C 1 (1)

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12O

100 I ~ - ' ( S - ' ) ( T M T T F ) z B r

7 E ""~ ,o o 60 I . . . . 1 T - T . . ( K ) . . . . . . . 1,0

7 ~_ 40 .. " •

20 ~ • • . °

~.... • • 0 = I I / t I t I t I =

0 50 1 O0 150 200 250 300 T e m p e r a t u r e (K )

Fig. 1. Temperature dependence of the relaxation rate for two central carbon sites in (TMTTF)2Br at 56kG. The inset shows its power-law divergence near T N.

, , , , , ~ 140 o I00 200 go ..... (TMTTFhPF, T% / . TCX) (TMTTF)zBT 60 . . . . . (TMTTF)zBr / . W / D ~ / ~ ~ ' " 120 8 f X $ ( O : U . ) ' . . . : t • %° &

'°° : f . " ....... t Ten . . . . (TMTS

T " " ~ ~ 0 p(n=m) 60 *' )-- ~ F,

20 " . ~ ~ ~o -'kk ~ L _ - - I p : 40 ~" ~ - t o

• ] p 1 0 - 0 1 O0 200 30C a.....~ a L I = I = I I I I

o& I X s Z T ( a . u . ) 2 z °c I 2 XsZT .~(a.u.)4 s

Fig. 2. l e f t : Re laxa t ion data o f Fig. I p l o t t e d vs zIT i n t e r p o l a t e d from EPR measurements showing the scaling relation (I). In the inset: temperature dependence of the ErR susceptibility; right: Comparison of T[l(q=2kF) amplitudes (represented by C1 constant in (I)) in (TffrTF)2Br (triangles) and (TMTTF)2PF2 (squares, from the ref. [2d]). The dashed line sketches the corresponding result in their selenide analogs (ref.[2a]). In the inset: longitudinal resistivity of TMTTF salts vs. temperature (ref.[41).

where C O and C 1 are positive constants. Consequently, in order to confirm that the

increase of the relaxation rate scales with the magnetic susceptibility, we present

2 using Zs values in Fig. 2 (left) the relaxation data of Fig. l plotted versus TZs

interpolated from EPR measurements between T=30K (Tz:=2.3), where critical precur-

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sots ape essentially inexistent, and 290K [Tx~=4.3). A relation of the form (I) can

T~ ~ then be fitted fairly well. A constant contribution to (C I) which is visible in

Fig. 2a can be attributed to the one-dimensional antiferromagnetic part of the

relaxation, as it was in (TMTTF)2PF 6. The validity of the scaling analysis carried

out simultaneously for the two carbon sites supports the hypothesis about the

electronic origin of the relaxation. Indeed, the ratio T~i(gmO)/TTi(2kF ) _ _ is the

same for the both lines as it is expected if the difference arise only from

different hyperfine constants. It is now instructive to compare our data with

similar analysis performed for other compounds [2a, d]. In Fig. 2 (right), the

2 (in the fastest of the relaxation rates of 13C in (TMTTF)2Br is plotted versus TXs

same units as in Fig. 2a), together with the 13C relaxation data in (TMTTF)2PF 6

obtained under similar conditions [2d]. The results presented in Fig.2b agree with

the theoretical prediction, namely that its relative amplitude is gradually

suppressed when the characteristic localization temperature T is lowered. In P

fact, this is the amplitude of the 2k F magnetic susceptibility in the strong

coupling domain (T ~ T ) which is responsible for the temperature independent P

contribution C I. Thus, comparing the two values of Tp for both sulfur compounds

(see the inset of Fig. Rb), it is natural to observe a lower value of C l for the

bromide salt. This picture can be extrapolated to the high temperature region for

the TMTSF family (dashed line on Figure 2b), where the resistivity minimum

disappears (that is, Tp<TN). This implies a much lower value for the 2k F suscepti-

bility at high temperature with CI=O since a power law: Tli(2k F ) - ~ T -~+I with ~<i

in weak coupling is expected [8], and C I will thus decrease rapidly with T at low

temperature. Therefore, as far as the AF correlations are concerned, the low

T~ ~ temperature AF characteristics of under pressure (or the amplitude of z(q=Qo))

should scale towards those of selenlde series as T 90. P

On the other hand, one should note that the enhancement of the spin susceptibi-

lity Xs(T) with T is essentially the same in both families at ambient pressure. In

this respect, a quantitative analysis of vs T in several members of both series

under hydrostatic pressure has shown [I0] that, as far as the uniform spin fluctua-

tions are concerned, their amplitude fop the sulfur series does not scale towards

the one of selenides under pressure.

ACKNOWLEDGEMENTS

We wish to acknowledge H. Humdequint for allowing us to use the EPR

spectrometer. One of us (C.B) would llke to thank the C.N.R.S fop chercheur

associe position during his stay in Orsay. P. Wzietek thanks the Canadian Institute

of Advanced Research and the Centre de Recherche en Physique du Solide de l'Unlver-

sit@ de Sherbrooke for financial support during his stay at Sherbrooke. This work

was also partially supported by ESPRIT contract n.3121BRA.

1739

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