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# One dimensional organic superconductors: Beyond the Fermi liquid description

Denis Jerome

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## To cite this version:

Denis Jerome. One dimensional organic superconductors: Beyond the Fermi liquid description. Journal de Physique IV Proceedings, 2000, 10, 10.1051/jp4:2000307. hal-05019784

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# One dimensional organic superconductors: Beyond the Fermi liquid description

D. Jérome¹

¹ Laboratoire de Physique des Solides, UMR 8648, Université Paris-Sud, 91405 Orsay, France

## Abstract

Organic Superconductivity has been discovered for the first time in 1979 in the one dimensional organic conductor (TMTSF)₂PF₆. The extensive studies carried on over the last 20 years have revealed the existence of a vast and prototype family of isostructural compounds (TM)₂X, exhibiting signatures of non-Fermi behavior at high temperature and a variety of ground states at low temperature.

## 1 THE PREHISTORICAL PERIOD

Looking backwards upon the discovery of organic superconductivity in December of 1979 [1] it is illuminating to have a closer look at the early period starting in 1973. This was the time where the first organic molecular compound, tetrathiafulvalene-tetracyanoquinodimethane (TTF-TCNQ) exhibiting a large and truly metal-like conductivity had been discovered [2, 3].

In the early seventies, the research on organic conductors was boosted to a very large extent by the hope to find in these new materials superconductivity at a temperature higher than what was existing in usual intermetallic alloys. More specifically, Little [4] suggested in 1964 that the arrangement of chain conductors in a polarizable medium could open the way to superconductivity at room temperature and even above. The prerequisite for the formation of a molecular conductor is first the requirement for having charged molecules in a solid and second to allow their charge to delocalize between molecular entities. Charging the molecules is achieved in TTF-TCNQ by a transfer of charge between the donor molecule TTF and the acceptor molecule TCNQ. TTF-TCNQ is a material comprising segregated parallel stacks of TTF and TCNQ molecules. The intermolecular overlap is thus optimized along the stacks of molecules and the π orbitals of the HOMO (LUMO) molecular orbitals (for TTF and TCNQ respectively) give rise to two conduction bands. The overlap of molecular orbitals being largest along the stacks (bandwidth of about 0.5 to 1 eV) and much weaker between them makes the electron dispersion one dimensional. The fact that the charge can delocalize in TTF-TCNQ shows that the on-site Coulomb-Hubbard repulsion U does not overcome the energy $4t_‖$ which is gained by the formation of energy bands in the solid, i.e., the bare molecular Coulomb repulsion ($\approx 5$ eV) experiences a strong dielectric screening by the environment which reduces it by an order of magnitude, making it about 0.3 – 0.5 eV [5]. The announcement in 1973 of a conductivity peaking at a value of $5.10^4 – 10^5 (\Omega cm)^{-1}$ at $T = 60$ K [2] just above the transition towards an insulating ground state triggered a tremendous interest although the precise amplitude of the conductivity peak was actually questioned by several authors
[6]. This early work suggested that the metal-insulator transition could be the manifestation of the instability due to the spontaneous distortion of the lattice with a wave vector $2k_F$ predicted by Peierls in 1957 for a one dimensional conductor [7]. Since, electron-electron and electron-hole pairings are known to compete in a 1-D conductor on the same footing, the enormous conductivity at 60 K was ascribed to superconducting fluctuations. It was thus believed that the softening of a phonon mode at the wave vector $2k_F$ could possibly enhance the superconducting fluctuations in the high temperature metallic regime [2]. Making the conductor less one dimensional via the increase of the overlap between stacks (applying an hydrostatic for exemple) was considered as one possible remedy to suppress the one-dimensional Peierls instability [8, 9] and let the superconductivity instability (less sensitive to the 1-D character) develop at high temperature.

Although very brittle, organic samples like TTF-TCNQ and all subsequently discovered materials (TM)₂X, etc., have been studied rather easily under hydrostatic pressure using the pressure equipment available at Orsay up to about 35 kbar. High pressure measurements on TTF-TCNQ have been extremely productive. They have shown that the metal-insulator transition (confirmed to be a Peierls instability by X-ray diffuse scattering experiments [10]) cannot be suppressed under pressure [11]. Instead, the Peierls transition temperature due to the TCNQ chain ($T_Q$ = 53 K at 1 bar) increases steadily up to 35 kbar and a very sharp peaking of the transition temperature is observed between 15 and 20 kbar, Figure 1. In the same pressure window, the longitudinal conductivity measured at constant temperature reveals a pronounced drop superimposed to a monotonous increase under pressure [12, 13]. The loss of conductivity becomes smaller at high temperature and becomes practically unobservable above 250 K. As far as the transverse transport is concerned, no anomaly can be detected in the same 15–20 kbar domain [5]. The behavior of TTF-TCNQ under pressure can be summarized by the following results: 1) it is the transverse Coulomb coupling and not the single particle hopping which stabilizes the Peierls state in TTF-TCNQ. Consequently, stabilizing superconductivity under pressure in TTF-TCNQ and parent compounds appears to be hopeless below 35 kbar. 2) The phase diagram can be understood in terms of a band filling being incommensurate ($\rho$ = 0.55 carrier/unit cell at 1 bar) increasing under pressure and becoming commensurate ($\rho$ = 0.66 or 2/3) in the pressure window 15–20 kbar [14]. 3) The conductivity peak at 60 K ($\sigma$(60 K)/$\sigma$(300 K) = 15–20) is not as large as it was first claimed but still up to 80% of its value can be ascribed to long-lived 1-D charge density wave (CDW) fluctuations becoming large in the proximity of the Peierls instability, Figure 2. The optical properties do not follow the usual Drude law and instead the far-infrared (FIR) conductivity displays a sharp DC collective mode (1/$\tau$ = 4 cm⁻¹) with a pseudo-gap of about 1000 cm⁻¹ [15, 16].

## 2 THE TURNING POINT

The study of the two-chain charge transfer compound TMTSF-DMTCNQ (TM-DM) where the donor is the tetramethyl selenide derivative of TTF has been an important milestone in the quest of organic superconductivity [17]. This 1-D conductor undergoes a Peierls transition at 42 K [18] where a distortion occurs simultaneously on both chains [19]. Furthermore, the Peierls transition is suppressed under pressure and the conductivity remains metal-like reaching $10^5 (\Omega cm)^{-1}$ under 10 kbar at the temperature of liquid helium [20], Figure 3. In addition, several other results have triggered our attention. X-ray experiments had shown that the charge transfer is only $\rho$ = 0.5 leading to a quarter-filled band situation [19]. Transport and thermopower data emphasized the dominant role played by the TMTSF chain in the mechanism driving the Peierls transition and also in its contribution to the conduction at high temperature [21]. The conducting state of TM-DM was also remarkable through its huge
<figure><img src="image_4.png" /><figcaption>**Fig. 1.** TTF-TCNQ: schematic band structure (top right), phase diagram under pressure (top left), the pressure dependence of the longitudinal transport presents a drop in the commensurability domain (bottom left) whereas no dip is observed for the transverse conduction since the Frölich conductivity contributes only to the longitudinal conduction (bottom right).</figcaption></figure>

magnetoresistance displayed below 50 K in a transverse magnetic field [22]. Since all these phenomena, were new and unexpected to us after the extensive study of the TTF-TCNQ series we decided to concentrate our effort on a structure made of only one organic stack: an organic salt of the molecule TMTSF. Such a family of salts was already known from the early work of the Montpellier group who had synthesized and studied the series of isostructural (TMTTF)₂X salts [23] where TMTTF is the sulfur analog of TMTSF and X is a monoanion such as ClO₄⁻, BF₄⁻ or SCN⁻ etc. All these compounds exhibit a conductivity of the order of 10 to 100(Ωcm)⁻¹ at T = 300 K with a small maximum in-between 200 and 100 K [24] but they all turn into strong insulators at low temperature. This is the reason why they did not attract much interest until recently when they have been practically rediscovered after twenty years of studies which were devoted to the selenide series (TMTSF)₂X.
<figure><img src="image_5.png" /><figcaption>Fig. 2. Decomposition of the longitudinal conduction into single particle and collective contributions.</figcaption></figure>

<figure><img src="image_6.png" /><figcaption>Fig. 3. Phase diagram of TM-DM. The pressure hysteresis seems to be related to a phase transition occurring in the high temperature regime.</figcaption></figure>
<figure><img src="image_7.png" /><figcaption>Fig. 4. Typical structure for (TM)₂X.</figcaption></figure>

### 3 THE TM₂X ERA

Shortly after the synthesis of the (TMTSF)₂X salts by Bechgaard *et al*. in 1979 it became clear that some new physics was going on in this 1-D series, Figure 4. The conductivity is large ($\sigma = 500(\Omega cm)^{-1}$), at $T = 300$ K and increases up to $10^5(\Omega cm)^{-1}$ at 12 K in (TMTSF)₂PF₆ where an abrupt metal-insulator transition takes place [25]. Furthermore, the absence of any noticeable lattice modulation in X-ray measurements [19, 26], as well as the results of NMR susceptibility measurements [27, 28] showed that the metal-insulator transition is driven by the onset of a spin density wave modulation [29]. These unusual properties stimulated an investigation of (TMTSF)₂X under pressure which allowed a stabilization of a metallic state down to liquid helium temperature at a pressure of about 9 kbar. The finding of a very small and still non-saturating resistivity at 1.3 K extrapolating linearly to a practically zero value at $T = 0$ K was a sufficient motivation to trigger its study under pressure in a dilution refrigerator and a zero resistance state was stabilized below 1 K [1]. Shortly after the discovery of superconductivity in (TMTTF)₂PF₆ many other members of the same series with a variety of anions have also been found superconducting in the vicinity of 1 K in the 10 kbar pressure domain [30]. (TMTSF)₂ClO₄ is the only member of the (TMTSF)₂X series to show superconductivity under atmospheric pressure [31]. The discovery of superconductivity in the (TMTSF)₂X family was a very exciting phenomenon since that was the first time such an instability could be stabilized in an organic compound. This was 15 years after the publication of Littles suggestion and 10 years after the holding of an international symposium organized by Little at Hawaii on the Physical and Chemical Problems of Possible Organic Superconductors. Probably the most surprising feature of superconductivity in (TMTSF)₂X is the common border existing between antiferromagnetic (SDW) and superconducting phases with the region of maximum $T_c$ for superconductivity located right at the border. It is this interesting behavior which initiated a reinvestigation of the sulfur-analog series (TMTTF)₂X, with the hope to stabilize superconductivity in this series as well. This has been successfully achieved only recently with (TMTTF)₂Br under 26 kbar [32] and (TMTTF)₂PF₆ at 47 kbar [33]. It is clear that we have
<figure><img src="image_8.png" /><figcaption>Fig. 5. Unified phase diagram.</figcaption></figure>

now reached a certain level of clarification in understanding the physical properties of these 1-D conductors namely (TMTTF)₂X and (TMTSF)₂X belong to the same (TM)₂X family and pressure is a relevant parameter which allows to move a given compound throughout a unified (T-P) phase diagram [34], Figure 5.

The salient feature of the (TM)₂X diagram is first the existence of a non ordered phase in the temperature domain between room temperature and ordering temperatures around 20 K or so. This non-ordered phase is insulating for all sulfur compounds with an activation of the conductivity which amounts to $Δ_ρ = 450$ K in (TMTTF)₂PF₆ at 1 bar. Moving right across the (TM)₂X phase diagram, the activation energy decreases and for selenium compounds, such as (TMTSF)₂PF₆ a very strongly metallic behavior is observed down to the onset of the insulating SDW phase at 12 K. Secondly, the diagram reveals a succession of ground states usually stable below 15–20 K, Spin-Peierls, Néel antiferromagnetism, SDW phase with an incommensurate magnetic modulation and ultimately superconductivity [35]. Notice that the compound at the extreme left in the (TM)₂X diagram, (TMTTF)₂PF₆ is the only one which can be taken through the entire series of ground states under high pressure. Given the unified phase diagram established experimentally and the recent proposal of theoretical ideas in 1-D physics applying specifically to 1-D conductors with a commensurate band filling we have reached a possible theoretical description which we intend to summarize very briefly in the rest of this article [36].

According to the band calculations, the transverse overlap $t_⊥$ along the *b*-direction is of order 120 K and 200 K for sulfur and selenium compounds respectively. Therefore, it is quite natural to expect first 1-D theory to govern the physics of these quasi 1-D conductors, at least in the high temperature regime when $T > t_⊥$ and second, to observe a cross-over towards higher dimensionality physics below room temperature. What 1-D physics means is that instead of the usual description of the low lying excitations in terms of quasi particle states in the Landau-Fermi liquid model, a collective mode description with decoupled spin and charge modes is a more appropriate approach [37, 38]. Such a model for 1D conductors has been proposed starting from a linearized energy spectrum for excitations close to the Fermi level and adding
<figure><img src="image_9.png" /><figcaption>Fig. 6. Temperature dependence of the Hall constant and $ρ_c$, $ρ_‖$ in (TMTSF)₂PF₆ at 1 bar.</figcaption></figure>

the relevant Coulomb repulsions which are responsible for electron scattering with momentum transfer $2k_F$ and 0. This is the popular Luttinger model for a 1-D conductor in which the spatial variation of all correlation functions exhibit a power law decay at large distance, characterized by a non-universal exponent $K_ρ$ which is a function of the microscopic coupling constants [39]. However (TM)₂X conductors are rather peculiar systems since the stoichiometry imposes half a carrier (hole) per TM molecule independent of the applied pressure. If the molecules are uniformly spaced along the stacking axis (a situation which is actually met in (TMTSF)₂ClO₄ the unit cell contains 1/2 carrier, i.e. the conduction band is quarter-filled. The existence of a slight dimerization of the molecules along the stacks in sulfur compounds could contribute to make them half-filled band compounds. The commensurate band filling allows a new scattering channel (Umklapp scattering) of carriers between both sides of the Fermi surface. This leads to important modifications of the gapless Luttinger liquid which in turn becomes a Mott-Hubbard insulator with a gap in the charge sector [40] subjected to the condition $K_ρ < 0.25$ or $< 1$ for 1/2 and 1/4 filling respectively where $K_ρ = 1$ refers to non-interacting electrons but still no gap in the spin channel. The gap in the density of states at the Fermi energy reads: $2Δ_ρ ≈ W(\frac{gU}{W})^{1/(2-2n^2K_ρ)}$ and $g_U$ is the coupling constant $U\frac{Δ}{W}W(\frac{U}{W})^3$ where $n = 1, 2$ respectively in the Hubbard limit ($Δ$ is the dimerization gap).

The striking behavior of the transport properties emerges from a comparison between the temperature dependence of longitudinal and transverse components of the resistivity [41], as shown in Figure 6. The longitudinal resistivity of (TMTSF)₂PF₆ is metal-like down to the SDW transition at 12 K, varying like $T^{0.93}$ from 300 to 150 K and more like $T^2$ below 150 K once the constant volume correction is taken into account. However, the behavior of the resistance along the direction of weakest coupling *i.e.* along the *c*-axis displays an insulating character with a maximum around 100 K and becomes metallic below although still orders of magnitude above the critical Mott value. The temperature corresponding to the maximum resistance moves up to room temperature under 10 kbar. Hence, a strongly compound and pressure dependent cross-over temperature is revealed by $ρ_c(T)$.

The Hall effect is also revealing a peculiar behavior with a cross-over at low temperature [42], see Figure 6. Although the amplitude of the Hall constant ($H//c$) is never very far from its tight binding 1/4 filled band value it is definitely temperature dependent and displays a minimum around 125 K with a subsequent huge drop followed by a sign change attributed to the SDW transition around 12 K.
Both, the transport and the Hall constant temperature dependences have been taken as signatures of a non-Fermi liquid behavior in the high temperature domain. An interpretation in terms of a 1-D Mott-Hubbard (M-H) insulator is indeed plausible.

The longitudinal transport is expected to vary according to a power law $\rho_\parallel \approx T^{4n^2K_\rho-3}$ at $T > \Delta_\rho$ [40], whereas the transport along $c$ being incoherent should probe the density of quasiparticle states in the (a-b) planes. In the high $T$ regime, the picture of non-coupled chains is approached. Therefore, the density of quasiparticle states should resemble the situation which is expected to prevail in a Luttinger liquid namely, $N(E) \approx |\omega|^\alpha$ where $\alpha$ is related to $K_\rho$ by $\alpha = \frac{1}{4}(K_\rho + 1/K_\rho - 2)$. At energies $\omega > \Delta_\rho$, the actual density of states of the (M-H) state should be reminiscent of the density of states of the gapless Luttinger liquid. Therefore, a temperature dependence of the transport $\rho_c(T) \approx T^{-2\alpha}$ is expected in the 1-D regime ($T > T^*$, the cross over temperature) as long as the condition $T > \Delta_\rho$ is fulfilled. The data of $\rho_\parallel$ and $\rho_c(T)$ lead to $K_\rho = 0.23$ in the 1-D regime assuming $n=2$ [41]. We now have to check that the condition $T > \Delta_\rho$ is fulfilled in the same temperature regime namely, $\Delta_\rho < T^*$.

The far infrared (FIR) conduction of $(TMTSF)_2PF_6$ exhibits also very unusual properties. There exists a FIR gap (which amounts to $\Delta_\rho = 200 \text{ cm}^{-1}$ in $(TMTSF)_2PF_6$) which has been taken as the reminiscence of the (M-H) gap. The experimental data and the theoretical power law dependence $\sigma(\omega) \approx \omega^{4n^2K_\rho-5}$ at $\omega > 2\Delta_\rho$ also lead to $K_\rho = 0.23$ [43]. This value for $K_\rho$ would fit the 1/4 filled scenario with $W = 12000$ K (from band structure calculations and plasma edge measurements), $\Delta_\rho = 200 \text{ cm}^{-1}$ (from FIR data) and $U/W = 0.7$ (which is in fair agreement with the enhancement of the spin susceptibility at low temperature) [36]. Since the (M-H) gap varies exponentially with $K_\rho$ only a small variation of the ratio between the Coulomb interaction and the bandwith under pressure would be sufficient to explain a decrease of $K_\rho$ moving left in the diagram. Both optical and transport data give $2\Delta_\rho = 1000$ K in $(TMTTF)_2PF_6$ at ambient pressure [44]. The difference between $K_\rho$ for selenium and sulfur compounds where ($K_\rho = 0.18$) can be explained by the difference of their bare bandwidths.

While the data of Hall effect are still waiting for a theory the power law dependence of the Hall constant in the 1-D regime ($R_H \approx T^{0.7}$) may be tentatively related to the temperature dependence of the density of quasiparticle states at the Fermi level which also follows the same power law according to the Luttinger liquid theory and the determination of the exponent from $\rho_c(T)$. In the $T < T^*$ domain, $R_H$ should progressively evolve towards the 1-D band value when a 2-D Fermi liquid behavior is recovered at low temperature since the value of $t_\perp$ cancels out in the Fermi liquid derivation of the Hall constant of a quasi 1-D conductor with open Fermi surfaces.

Apparently, the finite dimerization of the intrastack bonds is not large enough to make the 1/2-filled Umklapp scattering a pertinent mechanism in the formation of the 1-D (M-H) phase. As far as the selenium compounds are concerned this feature could be related to the transverse $b$-direction bandwidth ($W_b \approx 1500$ K) being larger than the dimerization gap (900 K) and thus smearing the role of the 1-D dimerization gap. This argument may still be valid for the sulfur series in which the dimerization gap and the transverse bandwidth are both in the same energy range.

The strong pressure dependence of the cross-over temperature $T^*$ is also a remarkable feature of the $TM_2X$ physics. According to the pressure data of $\rho_c$, the pressure dependence of $T^*$ is about ten times as large as the dependence for $t_\perp$ being typically $2\% \text{ kbar}^{-1}$ [36]. This is a feature suggesting that $T^*$ is actually a renormalized version of $t_\perp$ due to the 1-D confinement via intrastacks electron-hole interactions. However, the value for $K_\rho = 0.23$ would lead to a cross-over temperature of the order of 10 K instead of 150 K following the theory of Bourbonnais and Caron [45], namely $T^* = t_\perp (\frac{t_\perp}{E_F})^{\frac{1-K_\rho}{K_\rho}}$ and using the calculated
<figure><img src="image_10.png" /></figure>

**Fig. 7.** Schematic experimental phase diagram showing the evolution of the M-H gap, the bare transverse coupling and the cross-over temperature throughout the phase diagram.

band parameters for (TMTSF)₂PF₆. Admittedly, the theory of the dimensionality cross-over in a Q-1-D conductor will require some refinements taking also into account the role of the (M-H) gap which has been overlooked so far [46].

To summarize, the picture emerging for the TM₂X phase diagram is that of a (M-H) insulating phase with a correlation gap decreasing from sulfur to selenium compounds, Figure 7. As long as the M-H gap is large (say larger than 300 K) the 1-D confinement is quite active and the single particle interstack hopping is meaningless despite an unrenormalized value of the transverse coupling which is still of the order of 100 K. Moving towards the right in the phase diagram, the M-H gap decreases (because of the band broadening) and the renormalization of the transverse hopping integral become less severe. (TMTSF)₂PF₆ is a compound where T* and 2Δρ are both of the same order of magnitude (≈ 150 – 200 K), Figure 7.

Increasing pressure raises T* up to room temperature (this is also the situation encountered in (TMTSF)₂ClO₄ under ambient pressure). One dimensional features in DC transport properties are observed at T > T* but the 2-D Fermi liquid model is recovered only in the low temperature domain below T*. However, 1-D features can still be observed even at low temperature and manifest themselves by a gap in the charge excitations as revealed by the FIR conductivity [43].

## 4 WHAT SHOULD BE NEXT?

The present state of the art has revealed the existence of a broad cross-over regime in the diagram starting around 150 K but extending down to 10 K. In this temperature domain, the physics evolves progressively from a 1-D Luttinger-like one to a 2-D Fermi liquid at low temperature. The DC transport in the latter temperature domain is governed by a collective mode becoming extremely narrow at low temperature. This phenomenon will require more experimental studies, especially under magnetic field and also a proper theoretical treatment. Similarly, a new study of the NMR properties in this cross-over regime would be very useful.

The recent discovery of superconductivity in (TMTTF)₂PF₆ [33] has reemphasized the close interplay between superconductivity and antiferromagnetism and brings a strong support for a non-phonon mediated pairing mechanism. However, more experimental work in the superconducting phase (tunneling, role of perfectly aligned magnetic field) should be very rewarding in order to establish the proper symmetry of the order parameter. 1-D organic conductors with a non-commensurate band filling are not very numerous. But the series of
TTF[Ni(dmit)$_2$]$_2$ [47] is one of them and should deserve a renewed interest besides the early transport [48] and NMR [49] measurements since no (M-H) localization can be anticipated in these incommensurate conductors. Finally, a bridge should be established between the confinement observed in 1-D organics and the bound hole pair formation in 1-D cuprate spin-ladders where some signs of deconfinement under pressure have also been suggested after NMR [50,51] and transport studies under pressure [52].

In conclusion, the discovery of superconductivity in (TMTSF)$_2$PF$_6$ 20 years ago has had an impact which went much beyond the finding of one additional compound in the already long list of existing superconducting materials. Evidences for new concepts in physics have been obtained. It has led to the discovery of a wealth of new phenomena in the physics of low dimensional conductors such as the cross-over between the Mott-Hubbard-Luttinger limit and the Fermi liquid theory, the interplay between a magnetic field and the quasi-one dimensionality and finally the possibility for a new exotic mechanism for superconductivity, both topics which we could not approach in this short article.

This short article is a unique opportunity to thank very warmly all my co-workers who have contributed at Orsay to the development of the physics and chemistry of organic conductors. They were PhD students and also several came from all over the world as visiting scientists, postdocs and students. Several among them went back to their countries where they initiated a very active work in that field. Organic conductors would not have developed so successfully at Orsay without the remarkably sensible approach of J. Friedel to basic research mixing theory and sophisticated experimental tools. H.J. Schulz who was a very close friend and colleague left us much too early. His contribution has had a great impact on the development of low dimensional physics.

## References

[1] D. Jérome, A. Mazaud, M. Ribault and K. Bechgaard, *J. Phys. Lett.* **41** (1980) L-95.

[2] L.B. Coleman, M.H. Cohen, D.J. Sandman, F.G. Yamagishi, A.F. Garito and A.J. Heeger, *Solid. St. Comm.* **12** (1973) 1125.

[3] J. Ferraris, D.O. Cowan, V. Walatka and J.H. Perlstein, *J. Am. Chem. Soc.* **95** (1973) 948.

[4] W.A. Little, *Phys. Rev. A* **134** (1964) 1416.

[5] D. Jérome and H.J. Schulz, for a review *Adv. Phys.* **31** (1982) 299.

[6] G.A. Thomas et al., *Phys. Rev.* **13** (1978) 5105.

[7] R. Peierls, Quantum Theory of Solids (Oxford University Press, London, 1955) p. 108.

[8] H. Gutfreund, B. Horovitz and M. Weger, *Phys. Rev. B* **12** (1975) 3174.

[9] D. Jérome, W. Müller and M. Weger, *J. Phys. Lett.* **35** (1974) L-77.

[10] F. Dénoyer, R. Comès, A.F. Garito and A.J. Heeger, *Phys. Rev. Lett.* **35** (1975) 445.

[11] R.H. Friend, M. Miljak and D. Jérome, *Phys. Rev. Lett.* **40** (1978) 1048.

[12] A. Andrieux, H. Schulz, D. Jérome and K. Bechgaard, *J. Phys. Lett.* **40** (1979) L-385.

[13] A. Andrieux, H. Schulz, D. Jérome and K. Bechgaard, *Phys. Rev. Lett.* **430** (1979) 227.

[14] S. Megtert, R. Comès, C. Vettier, R. Pynn and A.F. Garito, *Solid. St. Comm.* **37** (1981) 875.

[15] D.B. Tanner, C.S. Jacobsen, A.F. Garito and A.J. Heeger, *Phys. Rev. Lett.*

[16] C.S. Jacobsen, D.B. Tanner, A.F. Garito and A.J. Heeger, *Phys. Rev. Lett.* **33** (1974) 1559.

[17] J.R. Andersen, K. Bechgaard, C.S. Jacobsen, G. Rindorf, H. Soling and N. Thorup, *Acta Cryst.* **B 34** (1978) 1901.

[18] C.S. Jacobsen, K. Mortensen, J.R. Andersen and K. Bechgaard, *Phys. Rev. B* **18** (1978) 905.

[19] J.P. Pouget, *Chem. Scr.* **17** (1981) 85.

[20] A. Andrieux, C. Duroure, D. Jérome and K. Bechgaard, *J. Phys. Lett.* **40** (1979) L-381.

[21] Y. Tomkiewicz, J.R. Andersen and A.R. Taranko, *Phys. Rev. B* **17** (1978) 1579.

[22] A. Andrieux, P.M. Chaikin, C.D. Duroure, D. Jérome, D. Weyl, K. Bechgaard and J.R. Andersen, *J. Phys. France* **40** (1979) 1199.

[23] G. Brun, S. Peytavin, B. Liautard, M. Maurin, E. Torreilles, J.M. Fabre and L. Giral, *C. R. Acad. Sci.* **284** (1977) 211.
[24] C. Coulon, P. Delhaes, S. Flandrois, E. Bonjour, R. Lagnier and J.M. Fabre, *J. Phys. France* **43** (1982) 1059.

[25] K. Bechgaard, C.S. Jacobsen, K. Mortensen, H.J. Pedersen and N. Thorup, *Solid. St. Comm.* **33** (1980) 1119.

[26] The very weak signature of a CDW has been reported recently by J.P. Pouget and S. Ravy, *Synth. Met.* **85** (1997) 1523.

[27] A. Andrieux, D. Jérome and K. Bechgaard, *J. Phys. Lett.* **42** (1981) L-87.

[28] H.J. Pedersen, J.C. Scott and K. Bechgaard, *Solid St. Comm.* **35** (1980) 207.

[29] K. Mortensen, Y. Tomkiewicz and K. Bechgaard, *Phys. Rev. B* **25** (1982) 3319.

[30] S.S. Parkin, M. Ribault, D. Jérome and K. Bechgaard, *J. Phys. C* **14** (1981) L-445.

[31] K. Bechgaard, K. Carneiro, M. Olsen and F.B. Rasmussen, *Phys. Rev. Lett.* **46** (1981) 852.

[32] L. Balicas, K. Behnia, W. Kang, E. Canadell, P. Auban-Senzier, D. Jérome, M. Ribault and J.M. Fabre, *J. Phys. I France* **4** (1994) 1539.

[33] D. Jaccard *et al.* (2000) to be published.

[34] D. Jérome, *Science* **252** (1991) 1509.

[35] D. Jérome, for a review in Organic Conductors, edited by J.P. Farges (M. Dekker Inc, New-York, 1991) p. 405.

[36] C. Bourbonnais and D. Jérome, *Adv. Synth. Met.*, edited by P. Bernier, S. Lefrant and G. Bidan (Elsevier, 1999) p. 206.

[37] H.J. Schulz, *Phys. Rev. Lett.* **64** (1990) 2831.

[38] J. Voit, *Rep. Prog. Phys.* **58** (1995) 977.

[39] J. Solyom, *Adv. Phys.* **28** (1979) 201.

[40] T. Giamarchi, *Physica B* **230** (1997) 975.

[41] J. Moser, M. Gabay, P. Auban-Senzier, D. Jérome, K. Bechgaard and J.M. Fabre, *Eur. Phys. J. B* **1** (1998) 39.

[42] J. Moser, J.R. Cooper, D. Jérome, B. Alavi, S.E. Brown and K. Bechgaard, *Phys. Rev. Lett.* (2000) submitted.

[43] Schwartz, M. Dressel, G. Grüner, V. Vescoli, L. Degiorgi and T. Giamarchi, *Phys. Rev. B* **58** (1998) 1261.

[44] P. Auban-Senzier, D. Jérome and J. Moser, in Physical Phenomena at High Magnetic Fields III, edited by Z. Fisk, L. Gork'ov and R. Schrieffer (World Scientific, 1999), p. 211.

[45] for a review of the cross-over see, C. Bourbonnais, in Strongly interacting fermions and high $T_c$ superconductivity, edited by B. Douçot and J. Zinn-Justin (Elsevier, 1995) p. 307.

[46] We thank A. Georges and T. Giamarchi for pointing out this problem.

[47] M. Bousseau, L. Valade, J.P. Legros, P. Cassoux, M. Garbauskas and L.V. Interrante, *J. Am. Chem. Soc.* **108** (1986) 1908.

[48] L. Brossard, M. Ribault, M. Bousseau, L. Valade and P. Cassoux, *C. R. Acad. Sci.* **302** (1986) 205.

[49] C. Bourbonnais, P. Wzietek, D. Jérome, F. Creuzet, L. Valade and P. Cassoux, *Europhys. Lett.* **6** (1988) 177.

[50] H. Mayaffre, P. Auban-Senzier, M. Nardone, D. Jérome, D. Poilblanc, C. Bourbonnais, U. Ammerahl, G. Dhalenne and A. Revcolevschi, *Science* **279** (1998) 345.

[51] Y. Piskunov, D. Jérome, P. Auban-Senzier, P. Wzietek, U. Ammerahl and A. Revcolevschi, *Eur. Phys. J. B* **13** (2000) 417.

[52] T. Nagata, M. Uehara, J. Goto, J. Akimitsu, N. Motoyama, H. Eisaki, S. Uchida, H. Takahashi, T. Nakanishi and N. Mori, *Phys. Rev. Lett.* **81** (1998) 1090.