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Commensurability and fluctuating conductivity in the organic conductor TSF-TCNQ
Solid State Communications, Vol. 36, pp. 813-816. Pergamon Press Ltd. 1980. Printed in Great Britain. COMMENSURABILITY AND FLUCTUATING CONDUCTIVITY IN THE ORGANIC CONDUCTOR TSF-TCNQ J.F. Thomas Laboratoire de Physique des Solides, Universit~ Libre de Bruxelles, CP 233 - 1050 BruxeUes, Belgique and D. J~rome Laboratoire de Physique des Solides, Universit~ Paris-Sud, 91405 - Orsay, France (Received 30 June i 980 by A. Blandin)
We report the pressure dependence of the phase transition in TSF-TCNQ as determined by resistivity measurements. We find a narrow pressure domain centered on 6.25 kbar where the transition temperature peaks above a monotonous pressure dependence. We suggest that there is a commensurate x 3 superlattice in this pressure regime resulting from an increase in charge transfer from 0.63 under ambient pressure to 2/3. A drop of longitudinal conductivity related to the x 3 commensurability is visible in the same temperature region where X-ray diffuse scattering data show I-D features for the precursor fluctuations. We suggest that like TTF-TCNQ, the metallic conductivity of TSF-TCNQ is also influenced by the fluctuating Fr6hlich mode. THE CONDUCTING PROPERTIES of molecular conductors originate in the existence of partially filled energy bands, ttigh conductivity is observed in two groups of materials: the double and single stack substances [ 1]. The former is exemplified by TTF-TCNQ which is a simple one to one complex crystallizing in parallel segregated stacks of TTF and TCNQ molecules. The charge transfer which occurs between molecules of different nature in the solid is responsible for the existence of the conducting properties. However, the amount of charge transfer is not unity and stoechiometry con. siderations are unable to lead to its determination. Hopefully, the charge transfer p which is directly related to the Fermi wavevector measured in units of the reciprocal vector b ° in one dimensional conductors by 2kp/b ° = p/2 can be determined very accurately by X-ray diffuse scattering data [2]. The amount of charge transfer is determined by crystal stability conditions and it can be affected by several features (i) the substitution of sulphur with selenium in the donor molecules, (ii) the substitution of alkyl substituents on donor molecules and (iii) the application of a hydrostatic pressure increasing the electron bandwidth. The experimental evidences supporting the influence of high pressure on p have been given for TTF-TCNQ by the peculiar behaviour of the Peierls transition temperature at 19 kbar [3]. The sharp peaking of the transition temperature which occurs at 19 kbar has been interpreted by a commensurability effect, namely the wavelength of the periodic lattice distortion 813
becomes commensurate with tile underlying lattice. It is equivalent to say that 2kF/b* = I/3 or p = 2/3 at 19 kbar. Additional evidences for a shift of the charge transfer from 0.59 at ambient pressure and low temperature to 0.66 (2/3) under 19 kbar have been corroborated by prelinlinary neutron scattering experiments under pressure up to 6 kbar [4]. The possibility of achieving commensurability under pressure in TTF-TCNQ has been a clue for the understanding of the conduction mechanism in the metallic regime. A loss of longitudinal conductivity occurring around commensurability has proved the existence of a conduction channel involving the coherent sliding of fluctuating CDW's [5, 6]. Tile I-D character of the fiuc. tuating CDW's which is a necessary requirement for their contribution to conductivity is well established in T T F TCNQ, by X-ray diffuse scattering techniques in the temperature domain extending from 60 K up to 200 K [7]. A three dimensional ordering of the fluctuations builds up below 60 K, and finally the crystal undergoes a phase transition at 54 K. Structural precursor effects have been detected in several other organic conductors besides TTF-TCNQ and the investigation of T S F TCNQ (the selenium analogue of'FI'F-TCNQ) has been performed in details by two groups [8, 9]. TTF-TCNQ and TSF-TCNQ display great similarities as far as the structure is concerned: isomorphism and a unit cell only slightly larger in TSF-TCNQ than in its sulfur analogue. However, the electronic properties of these two conductors are somcwhat different. The low temperature
814
COMMENSURABILITY IN THE ORGANIC CONDUCTOR TSF-TCNQ ! A
i
8
l
C
.
10
60K
80K "'=4~ = 80K ~ 100K H
=K
1 0 0 K Y 1 2 S K Y 5
150K J
125K
125K Y
~ :
150K
150K f
.'". .
_
200K ,......,,.-.,~
200K
~SOK ~SK
|
36
34
10
Pressure (N~r)
Fig. !. Pressure dependence of the Peierls transition in TSF-TCNQ (bottom curves) and of the conductivity at different temperatures (upper curves), oo = 800 (I2 cm)-t in the determination of the absolute conductivity. charge transfer is 0.63 in TSF-TCNQ [10] while only 0.59 in TTF-TCNQ [2, 11 I. Precursor structural effects with wave vectors 2kF and 4kF are observed in T T F TCNQ by X-ray scattering, whereas only 2k e scattering is observed for TSF-TCNQ [10]. The sequence of phase transitions at low temperature is somewhat complicated for TTF-TCNQ since three anomalies are detected by transport measurements under ambient pressure [ 1] (at 54, 49 and 38 K). TSF-TCNQ [ 12] undergoes a metal to semiconductor transition at 29 K and no other anomalies are noticed on the resistivity on further cooling. The transverse periodictiy of the distortion is 2a in TSF-TCNQ [ 10] below 29 K and 4a in TTF-TCNQ below 38 K [7]. The precursor structural effects are I-D in TTF-TCNQ in almost the entire temperature domain where they are observed. The picture is different in TSF-TCNQ since short range 3-D coupling is observed between 29 and 50 K and limited 2-D coupling extending over 2 or 3 interstack spacings is noticed up to 200 K [8]. The dominant role played by the fluctuating CDW's on the conduction of the metallic state of TTF-TCNQ has been clearly established from high pressure studies [6, 13]. The possibility of a pressure induced commensurability triggered a thorough investigation of both the phase diagram and conduction properties of T S F TCNQ. The conductivity of TSF-TCNQ single crystals was
Vol. 36, No. 9
measured along the b-axis, with the usual 4-probes low frequency lock.in technique up to 12 kbar in the temperature domain 300-25 K. Hydrostatic pressure was provided by compressed isopentane. Its value derived from the reading of a manganine gauge located in the pressure intensifier at room temperature. During cooling and wamdng, pressure was stabilized by means of a pressure stabilizer within + 150 bar of the nominal room temperature pressure. The temperature inside the pressure cell was measured by a copper-constantan thermocouple. The stability of the sample's resistance was rather satisfying since usually 5 or 6 different pressure runs were performed with a given sample. In agreement with previous works on TSF-TCNQ a single phase transition was detected at low temperature using resistive measurements. The temperature of the transition was given by the peaking of 5 IogR/ST at Tp [14]. Figure 1 displays the pressure dependence of Tp for three samples. An overall increase of Tp is noticed under pressure in agreement with previous results [! 5l but a weak peaking is clearly observed for all studied samples in the vicinity of 6 kbar. A maximum of TO is observed at 32.5-33 K for samples A and B under 6 kbar whereas the peaking occurs for sample C at a pressure of 6.5 kbar. tlowever, we do not believe that the 500 bar pressure difference observed between samples A or B and sample C bears much significance since it may be explained by tile limitation imposed by the manganine gauge calibration. From now on we consider that the phase transition peaking occurs at Pc = 6.25 -+0.25 kbar. An important feature displayed on the upper part of Fig. 1 is the nonmonotonous behaviour of the conductivity versus pressure in the vicinity of P c. A dip of conductivity occurs at Pc and amounts to about 10% for samples A and B in the temperature domain 100-125 K. Such a dip is not observed for sample C which instead displays a monotonous decrease of the conductivity under pressure. The peaking of the phase transition can be interpreted following the same lines as for TTF-TCNQ [3], using only simple scaling arguments. Provided the energy difference between the first unoccupied molecular level of TCNQ and the last occupied level of TSF (or TTF) is not influenced by high pressure in a significant way, the charge transfer is increased by the pressure induced band broadening. Within the lowest order approximation the charge transfer enhancement Ap/p is proportional to the bandwidth increase which in turn is linearly related to the change of lattice parameter along the high conductivity axis. In Fig. 2 we have plotted the pressure dependence of the b parameter in TI'F-TCNQ as determined by neutron scattering experiments on powders up to 20 kbar [161. It is known that for TTF-TCNQ a pressure of
COMIV.ENSURABI LITY IN THE ORGANIC CONDUCTOR TSF-TCNQ
Vol. 36, No. 9
of
/.
TTF - TC,~
o
Y
x
t
0.., TSF
2
- TCtxCl
1 0
I
0
5
!
10
: 0 15 20 Pressure /(kbarl
Fig. 2. b-axis compressibility of TTF-TCNQ, following reference [ 16]. The right hand scale gives the pressure induced shift of the charge transfer, knowing that 19 kbar leads to commensurability in TTF-TCNQ. 19 kbar increases the charge transfer by Ap = 0.07 in order to achieve commensurability. Therefore, the use of the vertical scale on the right indicates that an enhance. ment of Ap = 0.03 (which is needed for the achievement of commensurability starting from p = 0.63 in T S F TCNQ) requires a pressure of about 6 kbar. Admittedly the comparison between T T F - T C N Q and TSF-TCNQ is somewhat crude since we have assumed similar compressibility in the two compounds and have neglected possible differences in the pressure responses of donor and acceptor baqds, llowever, we feel confident that the peaking of Tc at 6.25 kbar in TSF-TCNQ can be attributed to a (x 3) commensurability. For a charge transfer 0.66 the wavelength of the CDW (rt/kF) is three times the lattice constant in the chain direction. In tiffs case third-order invariants are allowed to appear in the Landau expansion of the tree energy. In the mean field approximation the third-order terms make the transition from the metallic state into tile commensurate phase first order with a higher transition temperature Tel than the second order transition temperature into the incommensurate state Tin. The estimate of Tt, t -- Ttn is about 2 K from Fig. 1, leading to a relative peaking ATt,/Tt, ~ 6%. This value is actually small compared to the ~ ! 8% increase observed in T T F - T C N Q at 19 kbar [17]. This suggests that the tldrd order terms (.pinning interaction) are somewhat smaller in T S F - T C N Q than in TTF-TCNQ. Another argument supporting the weak pinning picture is the fact that contrary to "ITF-TCNQ, no 1st order character is noticed for the phase transition at commensurability. If the charge transfer deviates slightly from its commensurate value 0.66 there is a competition between the
815
third order terms which favour the commensurate state and the elastic energy associated with the driving of the incommensurate value of the CDW wavelength towards the commensurate value 3b [13]. The commensurability domains defined by the half height widths of the temperature peaks amount to 4 kbar and I kbar for "II'F-TCNQ and TSF-TCNQ respectively. We believe that there are several reasons explaining this large difference: (i) the increment of charge transfer is not pressure independent; a numerical derivation [ ! 8] gives a decrease of dp/dP by a factor 2 for T T F - T C N Q between ambient pressure and 19 kbar (ii) the b-axis compressibility is also known to decrease by a factor 2 in the same pressure domain [16] and finally (iii) for a given elastic energy of the CDW, the weakening of the pinning potential favours the narrowing of the commensurability domain. The loss of conductivity observed within the commensurability domain can be explained by the same token as in TTF-TCNQ, namely tile existence of a contribution to the conduction provided by the friction-less motion of fluctuating CDW's. When commensurability is realized under pressure, tile pinning of the CDW's diminishes their contribution to the conduction [19]. Tile temperature dependence of tile conductivity given by tile fluctuating CDW's is dominated by that of the mean square average of the order parameter, (IAI 2) which can be calculated exactly by functional integral methods [201. If no interchain coupling is present the conductivity is thus diverging like (T - T~,)-t/2 at the mean-field temperature in a 1-D mean-field theory. If interchain coupling is introduced, o,¢ no longer diverges at Tp but can still tend towards large values. Well above the Peierls transition, there is not much difference between the amplitude of the fluctuation conductivity in TSF-TCNQ and TTF-TCNQ. For example, if the depth of the conduction loss at commensurability is taken as a lower limit estimate to the fluctuation conductivity (because of possible thermal depinning) we notice that oF is fairly similar in TTF-TCNQ [13] and TSF-TCNQ (sample A) at T = 3Tp(aF ~ 400(I2-cm)-t). The situation is however very different for the two parent compounds as Te is approached from above. Fig. 1 shows that the contribution of the CDW's to conduction is maximum in the temperature domain 125lOOK. Towards high temperatures the vanishing of the fluctuation conductivity is in fairly good agreement with the observed temperature dependence of the 2kF X-ray scattering intensity, decreasing by a factor 5 (or so) from 100 to 200 K [8]. llowever towards low temperatures the amplitude of oF clearly does not follow the monotonous increase of tile X-ray scattering intensity. We believe that the low temperature decrease ofo,v can be
816
COMMENSURABILITY IN THE ORGANIC CONDUCTOR TSF-TCNQ
related to the establishment of short range interchain coherence between fluctuations as demonstrated by X-ray scattering data [8]. Close to the phase transition the short range 3-D coupling which is observed in a relatively large temperature domain should decrease the efficiency of CDW's for current production since along the a-direction CDW's alternate in sign. The interchain Coulomb coupling may increase under pressure for two reasons, (i) increase of the charge transfer and (ii) the diminishing of the interstack distance. We believe that it is the origin for the only partial recovery under pressure of the fluctuation conductivity above commensurability. A similar effect has been noticed for "Iq'F-TCNQ above 19 kbar. The behaviour of samples A, B and C in Fig. 1 is probably representative of decreasing purity since not only the commensurability dip becomes smaller going from A to C, but the value of the conduction itself at commensurability decreases steadily as long as the temperature is lower than 200 K. In a weak pinning potential situation, the fluctuation conductivity may not be fully destroyed at commensurability [ 19], especially at elevated temperatures and therefore the decrease o f f (6.25 kbar, T) noticed between A and C can be explained by an increase of the impurity pinning. Actually, it has been shown recently with transport properties studies on TTF-TCNQ samples that irradiation, while drastically affecting the fluctuation conductivity, leaves the single particle channel hardly changed [21 ]. It is interesting to notice that the conduction dip at commensurability is observed in TSF-TCNQ indepe ndent of the absence of 4k~ scatterings [ 10]. This rules out the explanation of the conmlensurability dip in terms of a strong 4RF scattering (observed in TTFTCNQ [22]) increasing the single particle scattering rate,. since when 2kF = b*/3, 2kF and 4kF scatterings become equivalent (modulo a reciprocal lattice vector). tligh pressure studies have revealed some common features in the electronic properties of the isotostruc. tural compounds TTF-TCNQ and TSF-TCNQ. Several differences have yet not been understood: the interchain Coulomb coupling which is stronger in TSF-TCNQ in the metallic regime and the different behaviour of the two compounds as far as the distorted low temperature phase is concerned. Moreover fluctuating conduction via CDW's seems to be an important channel of conduction in quasi I-D organic conductors as long as X-ray diffuse scattering experiments do not show strong transverse coherence.
Vol. 36, No. 9
Acknowledgements - We acknowledge the help of G. Benedek and A. Andrieux at various stages of the experiment. We would like to thank E.M. Engler who provided us with TSF-TCNQ crystals. J.F.T. wishes to acknowledge the Fonds National de la Recherche Scientifique for its f'mancial support. REFERENCES 1.
2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21.
22.
For general references on Quasi One Dimensional Molecular conductors, see for example, Chemistry and Physics o f One Dimensional Metals (Edited by HJ. Keller). Plenum, New York (1977) and Molecular Metals (Edited by W.E. Hatfield). Plenum, New York (1979). F. Denoyer, R. Com6s, A.F. Garito & AJ. Heeger, Phys. Rev. Lett. 35,445 (1975). R.H. Friend, M. Miljak & D. Jdrome, Phyz Rev. Lett. 40, 1048 (1978). S. Megtert, R. Com6s, C. Vettier, R. Pynn & A.F. Garito, Solid State Commun. 31,977 (1979). A. Andrieux, HJ. Schulz, D. J6rome & K. Bechgaard,J. Phys. Lett. 43,227 (1979). A. Andrieux, H.J. Schulz, D. J~rome & K. Bechgaard, J. Phys. Lett. 40, L-385 (1979). R. Com6s, Chemistry and Physics o f One DimensionaIMetals (Edited by HJ. Keller). Plenum, New York (1977). S. Megtert, J.P. Pouget & R. Com~s, Molecular Metals (Edited by W.E. Hatfield). Plenum, New York (1979). S Kagoshima, Molecular Metals (Edited by W.E. Hat field). Plenum, New York (1979). C. Weyl, E.M. Engler, K. Bechgaard, G. Jehanno & S. Etemad, Solid State Commun. 19,925 (1976). S. Kagoshima, T. Ishiguro & H. Anzai, J. Phys. Soc. Japan 41, 2061 (1976). S. Etemad, T. Penney, EaM. Engler, B.A. Scott & P.E. Selden, Phys. Rev. Lett. 34, 741 (1975). D. J6rome & HJ. Schulz, To be published in Linear Chain Compounds, (Edited by J.S. Miller). Plenum Press, New York. S. Etemad,Phys. Rev. BI3, 2254 (1976). J.R. Cooper, D. J6rome, S. Etemad & E.M. Engler, Solid State Commun. 22,257 (1977). D. Debray, R. Millet, D. J~rome, S. Barisic, L. Giral & J.M. Fabre, J. Phyz Lett. 38, L-227 (1977). D. J6rome, Proc. NA TO-ASI, Summer School, Tomar, Portugal 1979, (Edited by L. Alcacer), Plenum Press, New York (1979). E.M. Conwell, SolidState Commun. 33, 17 (1980. HJ. Schulz, Solid State Commun. 34,455 (1980). DJ. Scalapino, M. Sears & R.A. Ferrel, Phys. Rev. B6, 3409 (1972). S. Bouffard (to be published). J.P. Pouget, S.K. Khanna, F. Denoyer, R. Com6s, A.F. Garito & AJ. Heeger,Phys. Rev. Lett. 37, 437 (1976).
We report the pressure dependence of the phase transition in TSF-TCNQ as determined by resistivity measurements. We find a narrow pressure domain centered on 6.25 kbar where the transition temperature peaks above a monotonous pressure dependence. We suggest that there is a commensurate x 3 superlattice in this pressure regime resulting from an increase in charge transfer from 0.63 under ambient pressure to 2/3. A drop of longitudinal conductivity related to the x 3 commensurability is visible in the same temperature region where X-ray diffuse scattering data show I-D features for the precursor fluctuations. We suggest that like TTF-TCNQ, the metallic conductivity of TSF-TCNQ is also influenced by the fluctuating Fr6hlich mode. THE CONDUCTING PROPERTIES of molecular conductors originate in the existence of partially filled energy bands, ttigh conductivity is observed in two groups of materials: the double and single stack substances [ 1]. The former is exemplified by TTF-TCNQ which is a simple one to one complex crystallizing in parallel segregated stacks of TTF and TCNQ molecules. The charge transfer which occurs between molecules of different nature in the solid is responsible for the existence of the conducting properties. However, the amount of charge transfer is not unity and stoechiometry con. siderations are unable to lead to its determination. Hopefully, the charge transfer p which is directly related to the Fermi wavevector measured in units of the reciprocal vector b ° in one dimensional conductors by 2kp/b ° = p/2 can be determined very accurately by X-ray diffuse scattering data [2]. The amount of charge transfer is determined by crystal stability conditions and it can be affected by several features (i) the substitution of sulphur with selenium in the donor molecules, (ii) the substitution of alkyl substituents on donor molecules and (iii) the application of a hydrostatic pressure increasing the electron bandwidth. The experimental evidences supporting the influence of high pressure on p have been given for TTF-TCNQ by the peculiar behaviour of the Peierls transition temperature at 19 kbar [3]. The sharp peaking of the transition temperature which occurs at 19 kbar has been interpreted by a commensurability effect, namely the wavelength of the periodic lattice distortion 813
becomes commensurate with tile underlying lattice. It is equivalent to say that 2kF/b* = I/3 or p = 2/3 at 19 kbar. Additional evidences for a shift of the charge transfer from 0.59 at ambient pressure and low temperature to 0.66 (2/3) under 19 kbar have been corroborated by prelinlinary neutron scattering experiments under pressure up to 6 kbar [4]. The possibility of achieving commensurability under pressure in TTF-TCNQ has been a clue for the understanding of the conduction mechanism in the metallic regime. A loss of longitudinal conductivity occurring around commensurability has proved the existence of a conduction channel involving the coherent sliding of fluctuating CDW's [5, 6]. Tile I-D character of the fiuc. tuating CDW's which is a necessary requirement for their contribution to conductivity is well established in T T F TCNQ, by X-ray diffuse scattering techniques in the temperature domain extending from 60 K up to 200 K [7]. A three dimensional ordering of the fluctuations builds up below 60 K, and finally the crystal undergoes a phase transition at 54 K. Structural precursor effects have been detected in several other organic conductors besides TTF-TCNQ and the investigation of T S F TCNQ (the selenium analogue of'FI'F-TCNQ) has been performed in details by two groups [8, 9]. TTF-TCNQ and TSF-TCNQ display great similarities as far as the structure is concerned: isomorphism and a unit cell only slightly larger in TSF-TCNQ than in its sulfur analogue. However, the electronic properties of these two conductors are somcwhat different. The low temperature
814
COMMENSURABILITY IN THE ORGANIC CONDUCTOR TSF-TCNQ ! A
i
8
l
C
.
10
60K
80K "'=4~ = 80K ~ 100K H
=K
1 0 0 K Y 1 2 S K Y 5
150K J
125K
125K Y
~ :
150K
150K f
.'". .
_
200K ,......,,.-.,~
200K
~SOK ~SK
|
36
34
10
Pressure (N~r)
Fig. !. Pressure dependence of the Peierls transition in TSF-TCNQ (bottom curves) and of the conductivity at different temperatures (upper curves), oo = 800 (I2 cm)-t in the determination of the absolute conductivity. charge transfer is 0.63 in TSF-TCNQ [10] while only 0.59 in TTF-TCNQ [2, 11 I. Precursor structural effects with wave vectors 2kF and 4kF are observed in T T F TCNQ by X-ray scattering, whereas only 2k e scattering is observed for TSF-TCNQ [10]. The sequence of phase transitions at low temperature is somewhat complicated for TTF-TCNQ since three anomalies are detected by transport measurements under ambient pressure [ 1] (at 54, 49 and 38 K). TSF-TCNQ [ 12] undergoes a metal to semiconductor transition at 29 K and no other anomalies are noticed on the resistivity on further cooling. The transverse periodictiy of the distortion is 2a in TSF-TCNQ [ 10] below 29 K and 4a in TTF-TCNQ below 38 K [7]. The precursor structural effects are I-D in TTF-TCNQ in almost the entire temperature domain where they are observed. The picture is different in TSF-TCNQ since short range 3-D coupling is observed between 29 and 50 K and limited 2-D coupling extending over 2 or 3 interstack spacings is noticed up to 200 K [8]. The dominant role played by the fluctuating CDW's on the conduction of the metallic state of TTF-TCNQ has been clearly established from high pressure studies [6, 13]. The possibility of a pressure induced commensurability triggered a thorough investigation of both the phase diagram and conduction properties of T S F TCNQ. The conductivity of TSF-TCNQ single crystals was
Vol. 36, No. 9
measured along the b-axis, with the usual 4-probes low frequency lock.in technique up to 12 kbar in the temperature domain 300-25 K. Hydrostatic pressure was provided by compressed isopentane. Its value derived from the reading of a manganine gauge located in the pressure intensifier at room temperature. During cooling and wamdng, pressure was stabilized by means of a pressure stabilizer within + 150 bar of the nominal room temperature pressure. The temperature inside the pressure cell was measured by a copper-constantan thermocouple. The stability of the sample's resistance was rather satisfying since usually 5 or 6 different pressure runs were performed with a given sample. In agreement with previous works on TSF-TCNQ a single phase transition was detected at low temperature using resistive measurements. The temperature of the transition was given by the peaking of 5 IogR/ST at Tp [14]. Figure 1 displays the pressure dependence of Tp for three samples. An overall increase of Tp is noticed under pressure in agreement with previous results [! 5l but a weak peaking is clearly observed for all studied samples in the vicinity of 6 kbar. A maximum of TO is observed at 32.5-33 K for samples A and B under 6 kbar whereas the peaking occurs for sample C at a pressure of 6.5 kbar. tlowever, we do not believe that the 500 bar pressure difference observed between samples A or B and sample C bears much significance since it may be explained by tile limitation imposed by the manganine gauge calibration. From now on we consider that the phase transition peaking occurs at Pc = 6.25 -+0.25 kbar. An important feature displayed on the upper part of Fig. 1 is the nonmonotonous behaviour of the conductivity versus pressure in the vicinity of P c. A dip of conductivity occurs at Pc and amounts to about 10% for samples A and B in the temperature domain 100-125 K. Such a dip is not observed for sample C which instead displays a monotonous decrease of the conductivity under pressure. The peaking of the phase transition can be interpreted following the same lines as for TTF-TCNQ [3], using only simple scaling arguments. Provided the energy difference between the first unoccupied molecular level of TCNQ and the last occupied level of TSF (or TTF) is not influenced by high pressure in a significant way, the charge transfer is increased by the pressure induced band broadening. Within the lowest order approximation the charge transfer enhancement Ap/p is proportional to the bandwidth increase which in turn is linearly related to the change of lattice parameter along the high conductivity axis. In Fig. 2 we have plotted the pressure dependence of the b parameter in TI'F-TCNQ as determined by neutron scattering experiments on powders up to 20 kbar [161. It is known that for TTF-TCNQ a pressure of
COMIV.ENSURABI LITY IN THE ORGANIC CONDUCTOR TSF-TCNQ
Vol. 36, No. 9
of
/.
TTF - TC,~
o
Y
x
t
0.., TSF
2
- TCtxCl
1 0
I
0
5
!
10
: 0 15 20 Pressure /(kbarl
Fig. 2. b-axis compressibility of TTF-TCNQ, following reference [ 16]. The right hand scale gives the pressure induced shift of the charge transfer, knowing that 19 kbar leads to commensurability in TTF-TCNQ. 19 kbar increases the charge transfer by Ap = 0.07 in order to achieve commensurability. Therefore, the use of the vertical scale on the right indicates that an enhance. ment of Ap = 0.03 (which is needed for the achievement of commensurability starting from p = 0.63 in T S F TCNQ) requires a pressure of about 6 kbar. Admittedly the comparison between T T F - T C N Q and TSF-TCNQ is somewhat crude since we have assumed similar compressibility in the two compounds and have neglected possible differences in the pressure responses of donor and acceptor baqds, llowever, we feel confident that the peaking of Tc at 6.25 kbar in TSF-TCNQ can be attributed to a (x 3) commensurability. For a charge transfer 0.66 the wavelength of the CDW (rt/kF) is three times the lattice constant in the chain direction. In tiffs case third-order invariants are allowed to appear in the Landau expansion of the tree energy. In the mean field approximation the third-order terms make the transition from the metallic state into tile commensurate phase first order with a higher transition temperature Tel than the second order transition temperature into the incommensurate state Tin. The estimate of Tt, t -- Ttn is about 2 K from Fig. 1, leading to a relative peaking ATt,/Tt, ~ 6%. This value is actually small compared to the ~ ! 8% increase observed in T T F - T C N Q at 19 kbar [17]. This suggests that the tldrd order terms (.pinning interaction) are somewhat smaller in T S F - T C N Q than in TTF-TCNQ. Another argument supporting the weak pinning picture is the fact that contrary to "ITF-TCNQ, no 1st order character is noticed for the phase transition at commensurability. If the charge transfer deviates slightly from its commensurate value 0.66 there is a competition between the
815
third order terms which favour the commensurate state and the elastic energy associated with the driving of the incommensurate value of the CDW wavelength towards the commensurate value 3b [13]. The commensurability domains defined by the half height widths of the temperature peaks amount to 4 kbar and I kbar for "II'F-TCNQ and TSF-TCNQ respectively. We believe that there are several reasons explaining this large difference: (i) the increment of charge transfer is not pressure independent; a numerical derivation [ ! 8] gives a decrease of dp/dP by a factor 2 for T T F - T C N Q between ambient pressure and 19 kbar (ii) the b-axis compressibility is also known to decrease by a factor 2 in the same pressure domain [16] and finally (iii) for a given elastic energy of the CDW, the weakening of the pinning potential favours the narrowing of the commensurability domain. The loss of conductivity observed within the commensurability domain can be explained by the same token as in TTF-TCNQ, namely tile existence of a contribution to the conduction provided by the friction-less motion of fluctuating CDW's. When commensurability is realized under pressure, tile pinning of the CDW's diminishes their contribution to the conduction [19]. Tile temperature dependence of tile conductivity given by tile fluctuating CDW's is dominated by that of the mean square average of the order parameter, (IAI 2) which can be calculated exactly by functional integral methods [201. If no interchain coupling is present the conductivity is thus diverging like (T - T~,)-t/2 at the mean-field temperature in a 1-D mean-field theory. If interchain coupling is introduced, o,¢ no longer diverges at Tp but can still tend towards large values. Well above the Peierls transition, there is not much difference between the amplitude of the fluctuation conductivity in TSF-TCNQ and TTF-TCNQ. For example, if the depth of the conduction loss at commensurability is taken as a lower limit estimate to the fluctuation conductivity (because of possible thermal depinning) we notice that oF is fairly similar in TTF-TCNQ [13] and TSF-TCNQ (sample A) at T = 3Tp(aF ~ 400(I2-cm)-t). The situation is however very different for the two parent compounds as Te is approached from above. Fig. 1 shows that the contribution of the CDW's to conduction is maximum in the temperature domain 125lOOK. Towards high temperatures the vanishing of the fluctuation conductivity is in fairly good agreement with the observed temperature dependence of the 2kF X-ray scattering intensity, decreasing by a factor 5 (or so) from 100 to 200 K [8]. llowever towards low temperatures the amplitude of oF clearly does not follow the monotonous increase of tile X-ray scattering intensity. We believe that the low temperature decrease ofo,v can be
816
COMMENSURABILITY IN THE ORGANIC CONDUCTOR TSF-TCNQ
related to the establishment of short range interchain coherence between fluctuations as demonstrated by X-ray scattering data [8]. Close to the phase transition the short range 3-D coupling which is observed in a relatively large temperature domain should decrease the efficiency of CDW's for current production since along the a-direction CDW's alternate in sign. The interchain Coulomb coupling may increase under pressure for two reasons, (i) increase of the charge transfer and (ii) the diminishing of the interstack distance. We believe that it is the origin for the only partial recovery under pressure of the fluctuation conductivity above commensurability. A similar effect has been noticed for "Iq'F-TCNQ above 19 kbar. The behaviour of samples A, B and C in Fig. 1 is probably representative of decreasing purity since not only the commensurability dip becomes smaller going from A to C, but the value of the conduction itself at commensurability decreases steadily as long as the temperature is lower than 200 K. In a weak pinning potential situation, the fluctuation conductivity may not be fully destroyed at commensurability [ 19], especially at elevated temperatures and therefore the decrease o f f (6.25 kbar, T) noticed between A and C can be explained by an increase of the impurity pinning. Actually, it has been shown recently with transport properties studies on TTF-TCNQ samples that irradiation, while drastically affecting the fluctuation conductivity, leaves the single particle channel hardly changed [21 ]. It is interesting to notice that the conduction dip at commensurability is observed in TSF-TCNQ indepe ndent of the absence of 4k~ scatterings [ 10]. This rules out the explanation of the conmlensurability dip in terms of a strong 4RF scattering (observed in TTFTCNQ [22]) increasing the single particle scattering rate,. since when 2kF = b*/3, 2kF and 4kF scatterings become equivalent (modulo a reciprocal lattice vector). tligh pressure studies have revealed some common features in the electronic properties of the isotostruc. tural compounds TTF-TCNQ and TSF-TCNQ. Several differences have yet not been understood: the interchain Coulomb coupling which is stronger in TSF-TCNQ in the metallic regime and the different behaviour of the two compounds as far as the distorted low temperature phase is concerned. Moreover fluctuating conduction via CDW's seems to be an important channel of conduction in quasi I-D organic conductors as long as X-ray diffuse scattering experiments do not show strong transverse coherence.
Vol. 36, No. 9
Acknowledgements - We acknowledge the help of G. Benedek and A. Andrieux at various stages of the experiment. We would like to thank E.M. Engler who provided us with TSF-TCNQ crystals. J.F.T. wishes to acknowledge the Fonds National de la Recherche Scientifique for its f'mancial support. REFERENCES 1.
2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21.
22.
For general references on Quasi One Dimensional Molecular conductors, see for example, Chemistry and Physics o f One Dimensional Metals (Edited by HJ. Keller). Plenum, New York (1977) and Molecular Metals (Edited by W.E. Hatfield). Plenum, New York (1979). F. Denoyer, R. Com6s, A.F. Garito & AJ. Heeger, Phys. Rev. Lett. 35,445 (1975). R.H. Friend, M. Miljak & D. Jdrome, Phyz Rev. Lett. 40, 1048 (1978). S. Megtert, R. Com6s, C. Vettier, R. Pynn & A.F. Garito, Solid State Commun. 31,977 (1979). A. Andrieux, HJ. Schulz, D. J6rome & K. Bechgaard,J. Phys. Lett. 43,227 (1979). A. Andrieux, H.J. Schulz, D. J~rome & K. Bechgaard, J. Phys. Lett. 40, L-385 (1979). R. Com6s, Chemistry and Physics o f One DimensionaIMetals (Edited by HJ. Keller). Plenum, New York (1977). S. Megtert, J.P. Pouget & R. Com~s, Molecular Metals (Edited by W.E. Hatfield). Plenum, New York (1979). S Kagoshima, Molecular Metals (Edited by W.E. Hat field). Plenum, New York (1979). C. Weyl, E.M. Engler, K. Bechgaard, G. Jehanno & S. Etemad, Solid State Commun. 19,925 (1976). S. Kagoshima, T. Ishiguro & H. Anzai, J. Phys. Soc. Japan 41, 2061 (1976). S. Etemad, T. Penney, EaM. Engler, B.A. Scott & P.E. Selden, Phys. Rev. Lett. 34, 741 (1975). D. J6rome & HJ. Schulz, To be published in Linear Chain Compounds, (Edited by J.S. Miller). Plenum Press, New York. S. Etemad,Phys. Rev. BI3, 2254 (1976). J.R. Cooper, D. J6rome, S. Etemad & E.M. Engler, Solid State Commun. 22,257 (1977). D. Debray, R. Millet, D. J~rome, S. Barisic, L. Giral & J.M. Fabre, J. Phyz Lett. 38, L-227 (1977). D. J6rome, Proc. NA TO-ASI, Summer School, Tomar, Portugal 1979, (Edited by L. Alcacer), Plenum Press, New York (1979). E.M. Conwell, SolidState Commun. 33, 17 (1980. HJ. Schulz, Solid State Commun. 34,455 (1980). DJ. Scalapino, M. Sears & R.A. Ferrel, Phys. Rev. B6, 3409 (1972). S. Bouffard (to be published). J.P. Pouget, S.K. Khanna, F. Denoyer, R. Com6s, A.F. Garito & AJ. Heeger,Phys. Rev. Lett. 37, 437 (1976).