Laboratoire de Physique des Bât. Solides, de Université ... complet/Creuzet83.pdf · de* Chimie...

J O U R N A L D E P H Y S I Q U E

C o l l o q u e C 3 , supp lémen t au n°6, T o m e 44, j u i n 1983 page C3-1099

T R A N S P O R T P R O P E R T I E S U N D E R P R E S S U R E OF T H E O R G A N I C C O N D U C T O R S

( T M T T F ) 2 X ; X = P F 6 , C 1 0 4 , B r

F. Creuzet, S.S.P. Parkin, D. Jérôme, K. Bechgaard and J.M. Fabre*

Laboratoire de Physique des Solides, Bât. 510, Université de Paris-Sud, 91405 Orsay, France * Laboratoire de Chimie Organique Structurale, U.S.T.L., 34060 Montpellier, France

Résumé - D'après les m e s u r e s de c o n d u c t i v i t é , la famille ( T M T T F ) 2 X présente des similarités avec la famille homologue sélenié. Cependant, la p r e s s i o n c r i t i q u e , correspondant à la suppression de l'état fondamental isolant, 25kbar pour le com­posé brome,, est plus é l e v é e , p e u t - ê t r e en relation avec la plus forte d i m é r i s a t i o n des m o l é c u l e s o r g a n i q u e s le long de la c h a î ­n e . Nous avons étudié la p o s s i b i l i t é d'existence d'un état supraconducteur dans ( T M T T F ) 2 Br par des mesures de c o n d u c t i v i t é , de susceptibilité a l t e r n a t i v e , et de m a g n é t o r é s i s t a n c e . Bien que l'existence d'une transition supraconductrice autour de 3K ne soit pas complètement d é m o n t r é e , les fluctuations supracon-d u c t r i c e s apparaissent importantes à basse t e m p é r a t u r e .

Abstract - A c c o r d i n g to c o n d u c t i v i t y studies, the ( T M T T F ) 2 X séries shows some similarities w i t h the sélénium s é r i e s . However, the critical pressure, c o r r e s p o n d i n g to the suppression of the insulating ground state, 25kbar for the bromide sait, is much higher and may be related to the stronger dimerization of the organic m o l é c u l e s along the stack. We have examined the p o s s i -b i l i t y of s u p e r c o n d u c t i v i t y in ( T M T T F ) 2 Br by c o n d u c t i v i t y , AC susceptibility and m a g n é t o r é s i s t a n c e e x p e r i m e n t s . A l t h o u g h the existence of a superconducting t r a n s i t i o n around 3K is not yet fully d e m o n s t r a t e d , superconducting short range order may be important at low t e m p é r a t u r e .

INTRODUCTION

The d i s c o v e r y of s u p e r c o n d u c t i v i t y in several members of the organic linear chain conductors ( T M T £ F ) 2 X (1) has stimulated interest in this family. However it is not well understood which features make this family so p a r t i c u l a r , and e s p e c i a l l y why it is différent from the iso­structural sulphur family ( T M T T F ) 2 X. At ambient pressure ( 2 ) , the eiectronic properties are apparently quite différent : at T = 3C0K the D C - c o n d u c t i v i t y is one order of magnitude lower (except for X = Br) and a m i n i m u m of the résistance appears between 100-250K, w i t h a weakly activated behaviour at lower t e m p é r a t u r e .

The d é t e r m i n a t i o n of the temperature-pressure phase diagram of three ( T M T T F ) 2 X compounds may elucidate thèse d i f f é r e n c e s , which are possibly related to the stronger d i m e r i z a t i o n of the organic m o l é c u l e s . Further-more, a highly conducting metallic state can be stabilized under p r e s ­sure in ( T M T T F ) 2 B r .

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol/1983148

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E X P E R I M E N T A L RESULTS

R e s i s t i v i t y v e r s u s t e m p é r a t u r e m e a s u r e m e n t s

( T M T T F ) 2 PF6 For a p r e s s u r e of 15kbar, the m a x i m u m in c o n d u c t i v i t y seen at ambient p r e s s u r e near 230K is s u p p r e s s e d , and m e t a l l i c b e h a v i o u r is o b s e r v e d in the h i g h température r é g i o n down to 19K, the t e m p é r a t u r e at w h i c h the c o m p o u n d b e c o m e s an i n s u l a t o r . The p h a s e t r a n s i t i o n t e m p é r a t u r e

(defined as that t e m p é r a t u r e w h e r e ^jnffifjj n a s a m a x i m u m ) can be calcu-

lated and the T-P phase d i a g r a m is c o n s t r u c t e d . In the i n s u l a t i n g p h a s e , an e n e r g y gap is d e f i n e d , d e c r e a s i n a from 56K at 15kbar to 25K at 23kbar (Fig. 1 ) .

A l t h o u g h the r é s i s t a n ­ce ratio R ( 3 û O K ) / R m i n is only 5 at the highest p r e s s u r e m e a ­sured, we n o t e a c h a n g e in slope of the r é s i s ­tance curve near 12CK, almost i n d e p e n d e n t of p r e s s u r e , w h i c h could be due to some lattice t r a n s f o r m a t i o n .

(TMTTF) 2 C I O i, The r e s u l t s , for s e v e ­ral p r e s s u r e s , are summarized in figure 2. At low p r e s s u r e (P<22 k b a r ) , the r é s i s t a n c e i n c r e a s e s as t e m p é r a ­ture is d e c r e a s e d , w i t h no sign of a m i n i m u m . At higher p r e s s u r e , a m e t a l - i n s u l a t o r tran­sition a p p e a r s at low t e m p é r a t u r e and the T-P phase d i a g r a m is e a s i l y e v a l u a t e d . The low r é s i s t a n c e ratio and the lack of m i n i ­m u m in r e s i s t i v i t y are p r o b a b l y due to a poor q u a l i t y of the samples a n d / o r the e f f e c t of p r è s s u r i z a t i o n .

F i g . 1 - T e m p é r a t u r e - P r e s s u r e phase d i a g r a m N o t e a shoulder in the of ( T M T T F ) 2 P F 6 r é s i s t a n c e curve at

6kbar near 50K. This is c e r t a i n l y related to the a n i o n o r d e r i n g o b s e r v e d at 1bar near 7CK ( 3 ) , but the effect is small, and d o e s not drive the t r a n s i t i o n .

(TMTTF) 2Br The p r e v i o u s results on (TMTTF) 2 P F G and (TMTTF) 2 C 1 0 i , are s i m i l a r to those for ( T M T S F ) 2 X , but the c r i t i c a l p r e s s u r e (corresponding to the s u p p r e s s i o n of the i n s u l a t i n g phase) is m u c h h i g h e r , e s t i m a t e d around 4 0 k b a r . From the ambient p r e s s u r e r e s u l t s , ( T M T T F ) 2 B r is m o r e m e t a l l i c and the critical p r e s s u r e is e x p e c t e d to be lower ; we find Pc = 2 4 k b a r .

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Fig. 2 at v a r i o u

perature as shown

The phase b o u n d a r y (Fig. 3) is quite simi­lar to that of (TMTSF) 2X in the metallic phase, the c o n d u c t i v i t y reaches more than 1 0 6 ( R e m ) ~ 1 , w i t h a résistance ratio of the order of 400, indicating the same features as for the sélénium c o m p o u n d s near the c r i t i c a l p r e s ­sure . We are sure that there is no minimum of r e s i s t i v i t y above the t r a n s i t i o n , at least for pressure greater than 3kbar.

Two important d i f f é r e n ­ces are found between the samples synthetized in M o n t p e l l i e r (indexed by F) and in C o p e n h a a e n (indexed by B ) . Firstly, the B-samples have b e t -ter metallic p r o p e r t i e s , seen in the résistance ratio R(300K)/P.(4K) and in the absolute value of the conductivity, w h i c h are both much greater than in F-sam-p l e s . Secondly, the b e h a v i o u r of the r e s i s ­tivity in the low tem­pérature région (T<3CK) is différent ; for the B-samples the r e s i s t i ­v i t y saturâtes as tern­

is decreased, w h e r a s R decreases m o n o t i c a l l y for the F-samples, in Figure 4 for the same pressure (P = 2 5 k b a r ) .

200 Température(K)

Résistance curves for ( T M T T F ) 2 C 1 0 s p r e s s u r e s

S u p e r c o n d u c t i v i t y in ( T M T T F ) 2 B r ?

The first results reported (4)

Fig. 3 - Phase d i a g r a m for (TMTTF),Br

indicated a superconducting transition near 3K at 24kbar in this compound, but some p r o b l e m s with residual r e s i s ­tivity made this resuit not so clear. In order to check if there is such a t r a n s i t i o n or not, we have performed A C - s u s c e p t i b i l i t y m e a s u r e m e n t s at 2 5kbar ; no signal was observed down to 1.4K and thus we observe no bulk s u p e r c o n d u c t i v i t y in this range of t e m p é r a t u r e . We have m e a s u r e d the m a g n é t o r é s i s t a n c e (with magnetic field along c*) as shown in figure 5. At 10kbar, no effect is observed up to SCkG. By increasing p r e s s u r e , the m a g n é t o r é s i s t a n c e appears g r a d u a l -ly : the rate (p(30kG) - p(OkG) / p(OkG)) evaluated at T = 15K, is increasing

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from 0.31 at 16kbar to 1.22 at 22kbar. Thèse v a l u e s , rather large for a m é t a l , are in the same range than those o b s e r v e d in the ( T M T S F ) 2 X family ( 5 ) . In line w i t h the i n t e r p r é t a t i o n of the ( T M T S F ) 2 X s é r i e s , we suggest that this could arise from the s u p p r e s s i o n of short range s u p e r c o n -d u c t i v e f l u c t u a t i o n s .

D I S C U S S I O N

The ( T M T T F ) 2 X family has s i m i l a r p r o p e r ­t i e s as ( T M T S F ) 2 X , but higher p r e s s u r e is n e e d e d to o b s e r v e thèse e f f e c t s , as d e m o n s t r a t e d by the phase d i a g r a m s :

- in ( T M T T F ) 2 B r in the high p r e s s u r e m e t a l l i c r é g i m e , the r e s i s t i v i t y f o l -lows a p o w e r law d e p e n d e n c e , almost q u a d r a t i c for the B - s a m p l e s .

- the i n s u l a t i n g p h a s e in ( T M T T F ) 2 Br has b e e n shown to be a spin d e n s i t y w a v e state at ambient p r e s s u r e (6,7) and at 13kbar ( 7 ) . But in ( T M T T F ) 2 P F 6 ( f o r w h i c h the c r i t i c a l p r e s s u r e is v e r y high) at ambient p r e s s u r e , this state seems m o r e c o m p l i c a t e d w i t h both m a g n e t i c (7) and lattice d i s t o r t i o n ( 8 ) e f f e c t s .

- a large m a g n e t o r e s i s t a n c e is o b s e r v e d at low t e m p é r a t u r e for p r e s s u ­re b e l o w P c .

An i n t e r p r é t a t i o n of thèse p r o p e r t i e s of the two families of compound was given by Emery et al (9) and H o r o v i t z et al ( 1 0 ) . Due to the spécial crystal structure of the c o m p o u n d s , U m k l a p p s c a t t e r i n g can b e c o m e i m p o r t a n t . F o l l o w i n g Emery, this p r o c e s s is related to the d i m e r i z a t i o n of the o r g a n i c m o l é c u l e s a l o n g the s t a c k . A c c o r d i n g to the m o d e l s , the Umklapp s c a t t e r i n g p r o c e s s , d e s c r i b e d by the p a r a ­m e t e r g3 in the g-ology t h e o r y , s t a b i l i z e s an i n s u l a t i n g state (either spin d e n s i t y w a v e or s p i n - P e i e r l s state) and has to be r e d u c e d to let s u p e r c o n d u c t i v i t y a p p e a r . If we assume that p r e s s u r e reduces the d i m e ­rization, and c o n s e q u e n t l y g3, we r a t i o n a l i z e that the c r i t i c a l p r e s s u r e is large for the strongly d i m e r i z e d ( T M T T F ) 2 X f a m i l y . F u r t h e r -m o r e , as the p r e s s u r e is increased, the g 3 - p a r a m e t e r b e c o m e s irrelevant and the c o m p é t i t i o n b e t w e e n s u p e r c o n d u c t i v e and i n s u l a t i n g states is m o r e and m o r e e f f i c i e n t : the s u p e r c o n d u c t i v e f l u c t u a t i o n s may appear and w i t h those the strong m a g n e t o r e s i s t a n c e . F i n a l l y , the nature of the i n s u l a t i n g state in the m o r e strongly d i m e r i z e d c o m p o u n d s (like ( T M T T F ) 2 PF 6 at P = 1bar) m a y be r a t i o n a l i z e d in t e r m s of c o e x i s t e n c e of a s p i n - P e i e r l s s t a t e .

The d a t a o n (TMTTF) 2 ClOi, e m p h a s i z e that anion o r d e r i n g must be c o n s i d e r e d s i m u l t a n e o u s l y w i t h the m o l e c u l a r stack i n s t a b i l i t i e s . The c h a r a c t e r i s t i c p r e s s u r e n e e d e d to s u p p r e s s anion o r d e r i n g may be h i g h e r or lower than Pc, and d é p e n d s on the m a t e r i a l c o n s i d e r e d .

C O N C L U S I O N

The m e m b e r s of the ( T M T T F ) 2 X family b e h a v e similar to those of the

0 10 20 30 T e m p é r a t u r e (K)

Fig. 4 - C o m p a r i s o n of b e h a v i o u r of r e s i s t i v i t y of F and B samples in (TMTTF)jBr

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( T M T S F ) 2 X s é r i e s . A l t h o u g h this resuit is u n e x p e c t e d c o n s i d e r i n g the ambient p r e s s u r e data, it is not so surprising in v i e w of the closely r e l a t e d c r y s t a l s t r u c t u r e .

The m a i n resuit is the s t a b i l i z a t i o n of a m e t a l l i c state in a sulphur compound, w i t h a large r é s i s t a n c e ratio (>400) and a v e r y high c o n d u c t i v i t y (> 1 0 6 (ftcm) - 1 ) . The results also suggest that the c r i t i c a l p r e s s u r e is related to the degree of d i m e r i z a t i o n of both f a m i l i e s .

R E F E R E N C E S

(1) see for example : J E R O M E D. and SCHULZ H.J., Advances in P h y s i c s , V o l . 3 1 , N° 4, 299,490

(2) D E L H A E S P., C O U L O N C., A M I E L L J., F L A N D R O I S S., T O R E I L L E S E . , FABRE J.M. and GIRAL L., M o l . C r y s t . Liq. C r y s t . 5_0 (1979) 43.

(3) P O U G E T J.P., M O R E T R., C O M E S R., BECHGAARD K., F A B R E J.M., GIRAL L., M o l . C r y s t . Liq. C r y s t . 79 (1982) 129.

(4) PARKIN S.S.P., C R E U Z E T F., R I B A U L T M., JEROME D . , BECHGAARD K., F A B R E J.M., M o l . C r y s t . Liq. C r y s t . 79_ (19S2) 249.

(5) JACOBSEN C . S . , M O R T E N S E M K., W E G E R M., B E C H G A A R D K., S.S.C. 3_8 (1981) 423.

(6) PARKIN S.S.P., SCOTT J.C., T O R R A N C E J. and E N G L E R E.M., P h y s . Rev. B ( s u b m i t t e d ) .

(7) C R E U Z E T F., T A K A H A S H I T., JEROME D . and FABRE J.M., J. de Physique L e t t r e s 43 (1982) 755.

(8) POUGET J.P., C h e m i c a Scripta V7 (1981) 8 5 .

(9) EMERY V . J . , B R U I N S M A R. and B A R I S I C S., P h y s . Rev. L e t t . £ 8 (1982) 1039.

(10) H O R O V I T Z B., G U T F R E U N D H., W E G E R M., S.S.C. 39 (1981) 5 4 1 .