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Fluctuating and single particles conductivity channels in TSF-TCNQ
Solid State Communications, Vol. 42, No. 8, pp. 587-589, 1982. Printed in Great Britain.
0038- ] 098/82/200587-03 $03.00/0 Pergamon Press Ltd.
FLUCTUATING AND SINGLE PARTICLES CONDUCTIVITY CHANNELS IN TSF-TCNQ J.F. Thomas Physique des Solides, CP 233, Universit6 Libre du Bruxelles, B-1050 Bruxelles, Belgium
(Received 12January 1982 by S. Amelinck:x) We measured the conductivity of TSF-TCNQ in the pressure domain surrounding the commensurability point. We extracted a temperature dependent quantity related to the fluctuating part of the conductivity which appears to follow the 2kv distortion development, except below 100 K where pinning due to 2-D couplings appears. We suggest a decomposition of the co~iductivity which shows that the fluctuating channel plays an important but, in contrast with TTF-TCNQ, non-dominant role in the conductivity peak of TSF-TCNQ. Tetraselenofulvalene-tetracyanoquinodimethane (TSF-TCNQ) is an organic crystal made up of segregated stacks in which overlapping of molecular orbitals gives rise to bands formation. This compound has conducting properties directly related to partial filling of the bands due to charge transfer between the TSF and the TCNQ molecules [ 1 ]. The amount of charge transfer, p, which is not an integer, may be experimentally determined by X-rays diffuse scattering, as it is related to the Fermi wavevector kF and thus to the Peierls distortion wavelength Xp by the expression: p 2
2kF b*
b Xv'
I
I
I
/- '1
~4 x
'<3
2
0
(1)
wherein b is the lattice parameter along the chains. At low temperature, p = 0.63 (electron/molecule) in TSF-TCNQ and 0.59 in the isomorphous and well known quasi-one-dimensional conductor TFF-TCNQ [2, 3 ]. It is now well established that p may be enhanced by high hydrostatic pressure and a summary of the at the present time available information on this pressure dependence is shown in Fig. 1. In the lowest order approximation, the charge transfer enhancement Ap/p (left scale) is proportional to the bandwidth increase which is in its turn linearly related to the b-axis compressibility Ab/b (right scale). This compressibility has been established in TTF-TCNQ by neutron scattering experiments (solid curve) [4]. The dashed curve on Fig. 1 is a tight-binding calculation of the p(P) function by Conwell [5 ]. Dots are direct pressure measurements of P(P) by neutron scattering experiments [6]; the square and the triangle are the commensurability points observed by transport properties analysis for respectively TTF-TCNQ and TSF-TCNQ [7, 8]. These points correspond to the situation in which p = 2/3. Under this circumstance, the Fr6hlich charge density wavelength 587
/ 0
I
I
5
10
I
0
15P(kbors) 20
Fig. 1. Pressure dependence of the charge transfer in TTF-TCNQ and TSF-TCNQ. Solid curve: b-axis compressibility [4]. Dashed curve: Conwell's calculation [5]. Dots: neutron measurements [6]. Square and triangle: commensurability points [7, 8]. (the Peierls distortion wavelength) is, according to equation (1), exactly three times the lattice parameter b and the second order metal to insulator transition becomes a first order one [9]. This induces an increase of the transition temperature, observed in both the salts TTF-TCNQ and TSF-TCNQ [8, 10]. Moreover, commensurability is experimentally characterized by a dip in the constant temperature o(P) curves [7, 8]. This effect indicates clearly that an important current carrying channel is at least reduced in the commensurability situation. On the contrary, no dip was observed in the transverse conductivity [ 11 ]. Thus, the conductivity channel reduced when p = 2/3 has a strict one-dimensional character and has been identified as the fluctuating conductivity initially suggested by Bardeen [12]. Theoretical developments [13] show indeed that in a third order (Xp = 3b) commensurability situation the
588
CONDUCTIVITY CHANNELS IN TSF-TCNQ 1.6
I
I
I
% 1.4--
1-61/T 1.4
I"1
1.2-
IO- (.(7.cm)-I I.
Vol. 42, No. 8
I
I
I
10
1.2 8.10
1 6.10 0-8
).8
4.10 Cl
0.6
•:..
0.4
0.6
:.:::...
2.10
0-4
.'~.
0
0.2
•
I 50
I 100
0.2
I"1
I
~'.o.O....~.or' t
150
0 100 200 300 Fig. 3. Decomposition of the total conductivity a r into the fluctuating channel crF and the single particles one olp for TSF-TCNQ.
200 T(K)
Fig. 2. oF(T) obtained by the relation (2) procedure and compared with the 2kF X-rays intensity [ 15 ]. Each of both the quantities is normalized at its own 50 K value. free energy of a charge density wave (CDW) system includes a cubic term which produces, additionally to the increase of the metal-insulator transition temperature, a pinning of the fluctuations. In a weak pinning situation, the commensurability pressure domain is expected to be very narrow, as observed on TSF-TCNQ: the dip is centered on 6.25 kbar and is neighboured by a temperature dependent conductivity maximum at 5.5 kbar [8]. Thus, measurements at those two pressures allow to estimate directly the fluctuating part of the TSF-TCNQ conductivity as
trv(T) cx o(T; P = 5.5 kbar) -- o(T; P = 6.25 kbar). (2) We took these data with the Orsay's high pressure equipment. Experimental details were previously reported [8]. In Fig. 2, we have plotted, according to the idea expressed in relation (2), the values (dots) of oF as a function of temperature. Fluctuating conductivity being, from a theoretical point of view, directly related to the order parameter of the Peierls distortion and thus to the X-rays 2kF diffusion intensity [8, 14], we found
it interesting to plot on the same temperature scale this intensity (Megter's data [ 15, 16 ]) corrected by the Boltzmann's population factor. Three interesting features are revealed by the comparison: (i) Above 100 K, the fluctuating conductivity aF follows quite well the 2kF X-rays intensity, what is the expected behaviour for a fully unpinned CDW's current. (ii) Between 80 and 100 K, a clear drop, X-rays unobserved, occurs in the oF(T) curve. Thus, the 2ke distortion continues to grow when the temperature is decreased, but the associated conductivity is partly pinned. On the other hand, 100 K is the temperature at which 2-D couplings are known to appear in T S F TCNQ [17]. Those couplings correspond to a weak twodimensional ordering of the 2kF distortion along the chain which is itself, from a structural point of view, a small periodic sliding of the molecules almost along their own plane [ 18 ]. This multi-dimensional ordering must, of course, induce a Coulombian pinning of the sliding CDW's and we suggest that this is the origin of the weak dip observed in the aF(T) curve. (iii) In spite of this weak pinning, the fluctuating conductivity continues to increase at low temperature and peaks at 40 K, just like the total conductivity does [19]. Consequently and as it is the case for T T F TCNQ, the CDW's sliding mode is an important contribution to the conductivity peak of TSF-TCNQ.
Vol. 42, No. 8
CONDUCTIVITY CHANNELS IN TSF-TCNQ
More quantitative information may be obtained from the following considerations. In TSF-TCNQ, the enhancement ATe of the metal to insulator transition temperature, produced by the third order commensurability, is about 2 K [8]; what leads to a relative peaking ATe[Tc = 6% compared to the 18% increase observed on TTF-TCNQ [10]. Moreover, the half-height width of the Te peak amounts respectively to 1 kbar in T S F TCNQ and to 4 kbar in TTF-TCNQ. From both the points, it may be deduced that the commensurability pinning potential is lower in TSF-TCNQ. Using numerical calculations from the third order commensurability pinning theory [13, 14, 20] one may estimate that the fluctuating channel is, at low temperature, twice more pinned in TTF-TCNQ than in TSF-TCNQ [21]. On the other hand, from a detailed comparison between the longitudinal and the transverse conductivities around the TTF-TCNQ commensurability point, it was shown that half of the fluctuating channel was pinned at this point [10]. Thus, the quantity extracted by the relation (2) procedure is roughly the fourth part of the total fluctuating conductivity of TSF-TCNQ. The other channel, i.e. the single particles one, may be obtained by subtraction of OF from the total conductivity of the crystal. The results which are valid in the low pressure domain are plotted on Fig. 3. In this decomposition it appears that the CDW's current is an important charge transport mechanism but that it never clearly dominates the single particles one. This is contrasting with T T F TCNQ where aF has been found to exceed 80% of the total conductivity at low temperature [10]. As a concluding remark we suggest that this difference could be the reason why the CPR, i.e. the ratio of the conductivity peak maximum to the room temperature value, is by about a factor 2 lower in TSF-TCNQ than in TTF-TCNQ. Acknowledgements - I acknowledge helpful discussions with D. J6rome.
589
REFERENCES 1.
2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21.
For general references on quasi-one-dimensional conductors, see for example, The Physics and Chemistry of L o w Dimensional Solids (Edited by L. Alcacer). D. Reidel, Dordrecht (1980). F. Denoyer, R. Com~s, A.F. Garito & A.J. Heeger, Phys. Rev. Lett. 35,445 (1975). C. Weyl, E.M. Engler, K. Bechgaard, G. Jehanno & S. Etemad, Solid State Commun. 19,925 (1976). D. Debray, R. Millet, D. J6rome, S. Barisic, L. Giral and J.M. Fabre, J. Phys. Lett. 38, L-227 (1977). E.M. Conwell, Solid State Commun. 33, 17 (1980). S. Megtert, R. Com6s, C. Vettier, R. Pynn & A.F. Garito, Solid State Commun. 31,977 (1979). A. Andrieux, H. J. Schulz, D. J6rome & K. Bechgaard, Phys. Rev. Lett. 43,227 (1979). J.F. Thomas & D. J6rome, Solid State Commun. 36,813 (1980). D. J6rome, Molecular Metals (Edited by W.E. Hatfield). Plenum, New York (1979). D. J6rome in the same book as [1]. A. Andrieux, H.J. Schulz, D. J6rome & K. Bechgaard,J. Phys. Lett. 40, L-385 (1979). J. Bardeen, Solid State Commun. 13,357 (1973). H.J. Schulz,Solid State Commun. 34,455 (1980). D. J6rome & H.J. Schulz, Extended Linear Chain Compounds II (Edited by J.S. Miller). Plenum, New York (in press). S. Megtert, J.P. Pouget & R. Com6s, in the same book as [9]. S. Megtert, Unpublished Ph.D. Thesis, Orsay (1978). S. Kagoshima, in the same book as [9]. K. Yamaji, S. Megtert & R. Com~s, J. Phys. (Paris) 42, 1327 (1981). S. Etemad, T. Penney, E.M. Engler, B.A. Scott & P.E. Seiden, Phys. Rev. Lett. 34, 741 (1975). H.J. Schulz, Unpublished Ph.D. Thesis, Hamburg (1980). J.F. Thomas, Unpublished Ph.D. Thesis, Brussels (1981).
0038- ] 098/82/200587-03 $03.00/0 Pergamon Press Ltd.
FLUCTUATING AND SINGLE PARTICLES CONDUCTIVITY CHANNELS IN TSF-TCNQ J.F. Thomas Physique des Solides, CP 233, Universit6 Libre du Bruxelles, B-1050 Bruxelles, Belgium
(Received 12January 1982 by S. Amelinck:x) We measured the conductivity of TSF-TCNQ in the pressure domain surrounding the commensurability point. We extracted a temperature dependent quantity related to the fluctuating part of the conductivity which appears to follow the 2kv distortion development, except below 100 K where pinning due to 2-D couplings appears. We suggest a decomposition of the co~iductivity which shows that the fluctuating channel plays an important but, in contrast with TTF-TCNQ, non-dominant role in the conductivity peak of TSF-TCNQ. Tetraselenofulvalene-tetracyanoquinodimethane (TSF-TCNQ) is an organic crystal made up of segregated stacks in which overlapping of molecular orbitals gives rise to bands formation. This compound has conducting properties directly related to partial filling of the bands due to charge transfer between the TSF and the TCNQ molecules [ 1 ]. The amount of charge transfer, p, which is not an integer, may be experimentally determined by X-rays diffuse scattering, as it is related to the Fermi wavevector kF and thus to the Peierls distortion wavelength Xp by the expression: p 2
2kF b*
b Xv'
I
I
I
/- '1
~4 x
'<3
2
0
(1)
wherein b is the lattice parameter along the chains. At low temperature, p = 0.63 (electron/molecule) in TSF-TCNQ and 0.59 in the isomorphous and well known quasi-one-dimensional conductor TFF-TCNQ [2, 3 ]. It is now well established that p may be enhanced by high hydrostatic pressure and a summary of the at the present time available information on this pressure dependence is shown in Fig. 1. In the lowest order approximation, the charge transfer enhancement Ap/p (left scale) is proportional to the bandwidth increase which is in its turn linearly related to the b-axis compressibility Ab/b (right scale). This compressibility has been established in TTF-TCNQ by neutron scattering experiments (solid curve) [4]. The dashed curve on Fig. 1 is a tight-binding calculation of the p(P) function by Conwell [5 ]. Dots are direct pressure measurements of P(P) by neutron scattering experiments [6]; the square and the triangle are the commensurability points observed by transport properties analysis for respectively TTF-TCNQ and TSF-TCNQ [7, 8]. These points correspond to the situation in which p = 2/3. Under this circumstance, the Fr6hlich charge density wavelength 587
/ 0
I
I
5
10
I
0
15P(kbors) 20
Fig. 1. Pressure dependence of the charge transfer in TTF-TCNQ and TSF-TCNQ. Solid curve: b-axis compressibility [4]. Dashed curve: Conwell's calculation [5]. Dots: neutron measurements [6]. Square and triangle: commensurability points [7, 8]. (the Peierls distortion wavelength) is, according to equation (1), exactly three times the lattice parameter b and the second order metal to insulator transition becomes a first order one [9]. This induces an increase of the transition temperature, observed in both the salts TTF-TCNQ and TSF-TCNQ [8, 10]. Moreover, commensurability is experimentally characterized by a dip in the constant temperature o(P) curves [7, 8]. This effect indicates clearly that an important current carrying channel is at least reduced in the commensurability situation. On the contrary, no dip was observed in the transverse conductivity [ 11 ]. Thus, the conductivity channel reduced when p = 2/3 has a strict one-dimensional character and has been identified as the fluctuating conductivity initially suggested by Bardeen [12]. Theoretical developments [13] show indeed that in a third order (Xp = 3b) commensurability situation the
588
CONDUCTIVITY CHANNELS IN TSF-TCNQ 1.6
I
I
I
% 1.4--
1-61/T 1.4
I"1
1.2-
IO- (.(7.cm)-I I.
Vol. 42, No. 8
I
I
I
10
1.2 8.10
1 6.10 0-8
).8
4.10 Cl
0.6
•:..
0.4
0.6
:.:::...
2.10
0-4
.'~.
0
0.2
•
I 50
I 100
0.2
I"1
I
~'.o.O....~.or' t
150
0 100 200 300 Fig. 3. Decomposition of the total conductivity a r into the fluctuating channel crF and the single particles one olp for TSF-TCNQ.
200 T(K)
Fig. 2. oF(T) obtained by the relation (2) procedure and compared with the 2kF X-rays intensity [ 15 ]. Each of both the quantities is normalized at its own 50 K value. free energy of a charge density wave (CDW) system includes a cubic term which produces, additionally to the increase of the metal-insulator transition temperature, a pinning of the fluctuations. In a weak pinning situation, the commensurability pressure domain is expected to be very narrow, as observed on TSF-TCNQ: the dip is centered on 6.25 kbar and is neighboured by a temperature dependent conductivity maximum at 5.5 kbar [8]. Thus, measurements at those two pressures allow to estimate directly the fluctuating part of the TSF-TCNQ conductivity as
trv(T) cx o(T; P = 5.5 kbar) -- o(T; P = 6.25 kbar). (2) We took these data with the Orsay's high pressure equipment. Experimental details were previously reported [8]. In Fig. 2, we have plotted, according to the idea expressed in relation (2), the values (dots) of oF as a function of temperature. Fluctuating conductivity being, from a theoretical point of view, directly related to the order parameter of the Peierls distortion and thus to the X-rays 2kF diffusion intensity [8, 14], we found
it interesting to plot on the same temperature scale this intensity (Megter's data [ 15, 16 ]) corrected by the Boltzmann's population factor. Three interesting features are revealed by the comparison: (i) Above 100 K, the fluctuating conductivity aF follows quite well the 2kF X-rays intensity, what is the expected behaviour for a fully unpinned CDW's current. (ii) Between 80 and 100 K, a clear drop, X-rays unobserved, occurs in the oF(T) curve. Thus, the 2ke distortion continues to grow when the temperature is decreased, but the associated conductivity is partly pinned. On the other hand, 100 K is the temperature at which 2-D couplings are known to appear in T S F TCNQ [17]. Those couplings correspond to a weak twodimensional ordering of the 2kF distortion along the chain which is itself, from a structural point of view, a small periodic sliding of the molecules almost along their own plane [ 18 ]. This multi-dimensional ordering must, of course, induce a Coulombian pinning of the sliding CDW's and we suggest that this is the origin of the weak dip observed in the aF(T) curve. (iii) In spite of this weak pinning, the fluctuating conductivity continues to increase at low temperature and peaks at 40 K, just like the total conductivity does [19]. Consequently and as it is the case for T T F TCNQ, the CDW's sliding mode is an important contribution to the conductivity peak of TSF-TCNQ.
Vol. 42, No. 8
CONDUCTIVITY CHANNELS IN TSF-TCNQ
More quantitative information may be obtained from the following considerations. In TSF-TCNQ, the enhancement ATe of the metal to insulator transition temperature, produced by the third order commensurability, is about 2 K [8]; what leads to a relative peaking ATe[Tc = 6% compared to the 18% increase observed on TTF-TCNQ [10]. Moreover, the half-height width of the Te peak amounts respectively to 1 kbar in T S F TCNQ and to 4 kbar in TTF-TCNQ. From both the points, it may be deduced that the commensurability pinning potential is lower in TSF-TCNQ. Using numerical calculations from the third order commensurability pinning theory [13, 14, 20] one may estimate that the fluctuating channel is, at low temperature, twice more pinned in TTF-TCNQ than in TSF-TCNQ [21]. On the other hand, from a detailed comparison between the longitudinal and the transverse conductivities around the TTF-TCNQ commensurability point, it was shown that half of the fluctuating channel was pinned at this point [10]. Thus, the quantity extracted by the relation (2) procedure is roughly the fourth part of the total fluctuating conductivity of TSF-TCNQ. The other channel, i.e. the single particles one, may be obtained by subtraction of OF from the total conductivity of the crystal. The results which are valid in the low pressure domain are plotted on Fig. 3. In this decomposition it appears that the CDW's current is an important charge transport mechanism but that it never clearly dominates the single particles one. This is contrasting with T T F TCNQ where aF has been found to exceed 80% of the total conductivity at low temperature [10]. As a concluding remark we suggest that this difference could be the reason why the CPR, i.e. the ratio of the conductivity peak maximum to the room temperature value, is by about a factor 2 lower in TSF-TCNQ than in TTF-TCNQ. Acknowledgements - I acknowledge helpful discussions with D. J6rome.
589
REFERENCES 1.
2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21.
For general references on quasi-one-dimensional conductors, see for example, The Physics and Chemistry of L o w Dimensional Solids (Edited by L. Alcacer). D. Reidel, Dordrecht (1980). F. Denoyer, R. Com~s, A.F. Garito & A.J. Heeger, Phys. Rev. Lett. 35,445 (1975). C. Weyl, E.M. Engler, K. Bechgaard, G. Jehanno & S. Etemad, Solid State Commun. 19,925 (1976). D. Debray, R. Millet, D. J6rome, S. Barisic, L. Giral and J.M. Fabre, J. Phys. Lett. 38, L-227 (1977). E.M. Conwell, Solid State Commun. 33, 17 (1980). S. Megtert, R. Com6s, C. Vettier, R. Pynn & A.F. Garito, Solid State Commun. 31,977 (1979). A. Andrieux, H. J. Schulz, D. J6rome & K. Bechgaard, Phys. Rev. Lett. 43,227 (1979). J.F. Thomas & D. J6rome, Solid State Commun. 36,813 (1980). D. J6rome, Molecular Metals (Edited by W.E. Hatfield). Plenum, New York (1979). D. J6rome in the same book as [1]. A. Andrieux, H.J. Schulz, D. J6rome & K. Bechgaard,J. Phys. Lett. 40, L-385 (1979). J. Bardeen, Solid State Commun. 13,357 (1973). H.J. Schulz,Solid State Commun. 34,455 (1980). D. J6rome & H.J. Schulz, Extended Linear Chain Compounds II (Edited by J.S. Miller). Plenum, New York (in press). S. Megtert, J.P. Pouget & R. Com6s, in the same book as [9]. S. Megtert, Unpublished Ph.D. Thesis, Orsay (1978). S. Kagoshima, in the same book as [9]. K. Yamaji, S. Megtert & R. Com~s, J. Phys. (Paris) 42, 1327 (1981). S. Etemad, T. Penney, E.M. Engler, B.A. Scott & P.E. Seiden, Phys. Rev. Lett. 34, 741 (1975). H.J. Schulz, Unpublished Ph.D. Thesis, Hamburg (1980). J.F. Thomas, Unpublished Ph.D. Thesis, Brussels (1981).