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# The development of organic conductors: Organic superconductors

Denis Jerome

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## To cite this version:

Denis Jerome. The development of organic conductors: Organic superconductors. Solid State Sciences, 2008, 10, pp.1692 - 1700. 10.1016/j.solidstatesciences.2008.02.001. hal-05010934

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# The development of organic conductors: Organic superconductors

Denis Jérôme

Université Paris-Sud, 91405 Orsay Cedex, France

Received 9 January 2008; accepted 4 February 2008

Available online 13 February 2008

## Abstract

This article provides a brief account of the development of organic conductors since 1970s. This research has been motivated by the quest for high $T_c$ superconductors. Besides superconductivity found in organics in 1980, progresses in this field of research have highlighted the wealth of remarkable properties of low dimensional physics.

© 2008 Elsevier Masson SAS. All rights reserved.

**Keywords:** Organic superconductors; Organic conductors; 1D conductors; Peierls transition; Spin density wave

## 1. Searching for high $T_c$ superconductors

Superconductivity (a new state of matter) has been one of the major discovery of the 20th century in physics. From an experimental point of view the finding by Kamerlingh Onnes [1] in 1911 of a current travelling without resistance through a metal cooled at very low temperature paved the way to a very large number of industrial applications. And this, long before the proposal of a satisfactory theoretical framework by Bardeen, Cooper and Schrieffer (BCS) [2] in 1957 explaining both the zero resistance state and the magnetic flux expulsion (the Meissner effect). Furthermore, the BCS theory has also been the paradigm of modern physical theories based on the importance of the quantum nature at low temperature since it relies on the establishment of a new long range ordered state. BCS emphasized the existence of a two body attractive interaction between charge carriers (either electrons or holes) being a prerequisite for a Bose condensation of electron pairs into the superconducting state. This net attractive coupling in spite of the Coulomb repulsion between carriers of the same sign relies closely on an attraction between electrons mediated by their interaction with excitations of the lattice namely, the phonons.

Consequently, a major achievement of the BCS theory has been the understanding of the ionic mass dependent of the

critical temperature, i.e. ($T_c \propto M^{-1/2}$). Although Fröhlich had proposed earlier in 1954 a model for superconductivity based on the involvement of the lattice [3] this theory was unable to account for the superconductivity of metals but turned out to be quite relevant later for the interpretation of the transport properties in some 1D organic compounds, *vide infra* (Fig. 1). At the beginning of 1960s searching for new materials exhibiting the highest possible values of superconducting $T_c$ was already a strong motivation in materials sciences and the term *high temperature superconductor* was already commonly used referring to the intermetallic compounds of the A15 structure, namely, (Nb$_3$Sn or V$_3$Si) [4]. The hidden 1D nature of the A15 structure provides an enhancement of the density of states at the Fermi level lying close to the van-Hove singularity of the density of states of the 1D d-band. Within the BCS formalism large $T_c$ could be expected. They were actually observed (17–23 K) but an upper limit was found to the increase of $T_c$ since the large value of $N(E_F)$ makes also the structure unstable against a cubic to tetragonal band Jahn–Teller distortion [5,6]. The theory showed that the actual compounds Nb$_3$Sn and V$_3$Si possess their optimum $T_c$ [7]. In this context some papers at the beginning of 1980s, based on metallurgical considerations, regarded 25–30 K as the highest possible value for $T_c$ [8]. However, these attempts to increase $T_c$ were still based on the BCS theory and its strong coupling extension [9,10]. Expending the successful idea of the isotope effect in the BCS theory other

<footnote>E-mail address: jerome@lps.u-psud.fr</footnote>
<figure><img src="image_4.png" /><figcaption>Fig. 1. Evolution of the superconducting critical temperature in metals and intermetallic compounds, cuprates and organic conductors (1 and 2D conductors).</figcaption></figure>

models were proposed in which excitations of the lattice
responsible for the electron pairing had been replaced by
higher energy excitations namely, electronic excitations, with
the hope of new materials with T_c higher than those explained
by the BCS theory. Consequently, the small electronic mass m_e
of the polarizable medium would lead to an enhancement of T_c
of the order of (M/m_e)¹/² times the value which is observed in
a conventional superconductor, admittedly a huge factor. V. L.
Ginzburg [11,12] considered in 1964 the possibility for the
paring of electrons in metal layers sandwiched between polar-
izable dielectrics through virtual excitations at high energy. W.
A. Little made an important suggestion [13,14]: a new mech-
anism for superconductivity expected to lead to a drastic
enhancement of the superconducting T_c to be observed in
especially designed macromolecules. The idea of Little was
indeed strongly rooted in the extension of the isotope effect
proposed by BCS. However, a prerequisite to the model of Lit-
tle was the achievement of conduction in molecular crystals
namely, new types of conductors in which the conduction
would proceed through the transport of charge between molec-
ular orbitals rather than atomic orbitals. Actually, the concept
of a synthetic metal had already been launched by McCoy and
Moore [15] when they proposed to prepare composite metallic
substances from non-metallic constituent elements. As to the
possibility of superconductivity in materials other than metals,
Fritz London in 1937 [16] has been the first to suggest that the
compounds with aromatic rings such as anthracene, naphtha-
lene, ... might exhibit a current running freely around the
rings under a magnetic field. The first successful attempt to
promote metal-like conduction between open shell molecular
species came out in 1954 with the synthesis of the molecular
salt of perylene oxidized with bromine [17] although this salt
was rather unstable. Coming back to 1D materials, the model
of Little was based on the use of a long conjugated polymer
such as a polyacetylene molecule grafted by polarizable side
groups [18] (see Fig. 2). Admittedly, this formidable task in

synthetic chemistry has not reached its initial goal but the
idea to link organic metallicity and one dimensionality was
launched and turned out to be a very strong stimulant for the
development of organic superconductors. Needless to say
that a lot of basic physical problems had been overlooked in
the seminal paper of Little [13] which was still based on the
popular Fermi liquid BCS theory. The first point is that 1D
conductors had been regarded previously as textbook exam-
ples by Peierls [19], who stated a theorem according to which
a gap should spontaneously open at the Fermi level in the con-
duction and lead to the stabilisation of a dielectric insulating
state at low temperature long before it has been established
in the platino-cyanate chains [20]. Bychkov et al. [21] criti-
cized the model of Little regarding its potentiality to lead to
high temperature superconductivity. Following Bychkov
et al., the 1D character of the model system proposed by Little
makes it a unique problem in which there exists a built-in cou-
pling between superconducting and dielectric instabilities. It
follows that each of these instabilities cannot be considered
separately in the mechanism proposed by Little in one dimen-
sion and last but not least, fluctuations should be very efficient
in a 1D conductor to suppress any long range ordering to very
low temperature. A major step has been accomplished in 1970
towards the discovery of new materials for superconductivity
with the synthesis by Wudl et al. [22] of the new molecule tet-
rathiafulvalene (TTF). This molecule containing four sulfur
heteroatoms in the fulvalene skeleton can easily donate elec-
trons when it is combined to electron accepting species. This
discovery has deeply influenced the subsequent evolution of
the chemistry of organic conductors.

## 2. First organic conductors: TTF–TCNQ

The discovery of the molecule TTF triggered the synthesis
of the first stable organic metal, a charge transfer complex,
TTF-TCNQ. The system is made up of two kinds of flat mol-
ecules each forming segregated parallel conducting stacks.

<figure><img src="image_5.png" /><figcaption>Fig. 2. Schematic picture of Little’s suggestion. Charge carriers *a* and *b* moving along a conducting spine are bound *via* a virtual electronic excitation of polarizable side groups.</figcaption></figure>
This compound can be recognized as an organic conductor as the orbitals involved in the conduction (π-HOMO and π-LUMO for TTF and TCNQ, respectively) are associated with the molecule as a whole rather than with a particular atom with carriers in each stacks provided by an interstack charge transfer *at variance* with other organic conductors such as the doped conjugated polymers which came out a little bit later. The overlap of the molecular orbitals being largest along the stack direction and much weaker between them makes the electron dispersion one-dimensional (1D). In a first approximation the energy of this 1D conductor depends only on the electron wave vector along the *b*\* direction in the reciprocal space and due to the inverted band structure coming from TTF and TCNQ, respectively, the 1D bands have to intersect the Fermi energy at Fermi wave-vectors $\pm k_F$. In turn, the band filling is not determined by stoichiometric considerations but rather by a subtle chemical balance. Hence, the band filling does not need to be commensurate with the underlying lattice (Fig. 3).

The announcement of a large and metal-like conduction in TTF-TCNQ was made in 1973, simultaneously by the Baltimore [24] and Pennsylvania [25] groups. The Pennsylvania group made a stimulating claim announcing a giant conductivity peak of the order of $10^5$ (Ω cm)$^{-1}$ at 60 K arising just above a very sharp transition towards an insulating ground state at low temperature. This conductivity peak was attributed by their authors to precursor signs of an incipient superconductor. The claims for superconductivity were made on the basis of 1D superconducting fluctuations enhanced by the softening of the $2k_F$ phonon mode coupled to the electrons. This

was still traditional thinking since it was believed from the study of the A15 superconductors that soft phonons modes were responsible for the high values of their $T_c$ [8]. Unfortunately, a conductivity peak with such a giant amplitude could never be reproduced by other groups who anyway all agreed on the metallic character of this novel molecular material [26]. Quite important has been the finding by the Orsay group from X-ray scattering studies of Bragg satellites allowing to link the metal-insulator transition to the instability predicted by Peierls [28] in a conducting chain. The Peierls transition leading to a charge density wave structure has been recently reexamined in the direct space by STM techniques [27] (see Fig. 4).

High pressure transport measurements have played for the first time a decisive role in the study of organics as shown by the study of the temperature-pressure phase diagram of TTF-TCNQ. As displayed on Fig. 5, the sharp peak of the Peierls transition superimposed to a slow increase under pressure has been attributed to a third order commensurability pinning, the wave length of the Peierls distortion become commensurate with the underlying lattice in the vicinity of 19 kbar ($\lambda_{||} = 3b$) also confirmed by direct neutron scattering data [29]. Taking advantage of a commensurability obtainable under pressure the possibility to perform systematic studies of the conductivity in the metallic temperature domain by sweeping the helium gas pressure at fixed temperatures did show a drop of longitudinal conductivity closely related to the commensurability conditions [31]. This behaviour brought the experimental confirmation of collective Fröhlich fluctuations in a 1D regime above a Peierls transition and, to a large extent, contributed to the understanding of the conduction peak. Furthermore, the role of 1D electron-electron repulsive interactions has been recognized for the first time in organics by the $4k_F$ signature in X-ray diffuse scattering experiments [32].

<figure><img src="image_6.png" /><figcaption>Fig. 3. An artist view of the TTF-TCNQ stacked structure according to Ref. [23].</figcaption></figure>

<figure><img src="image_7.png" /><figcaption>Fig. 4. STM data from Ref. [27] showing the charge density wave at 36 K on TCNQ stacks with longitudinal $\lambda_b = 3.39b$ and commensurate transverse ordering, $\lambda_a = 4a$.</figcaption></figure>
<figure><img src="image_8.png" /><figcaption>Fig. 5. Temperature-pressure phase diagram of TTF-TCNQ showing the commensurability domain around 14–19 kbar (left) [30] and longitudinal conduction measured vs pressure under constant temperature displaying a drop of the conduction in the commensurability (×3) domain due to pinning of the fluctuating Fröhlich contribution.</figcaption></figure>

While high pressure appeared to be a parameter far more influential for organic conductors than for regular metals, it has failed until recently [33] to make TTF-TCNQ sufficiently 3D to prevent the onset of a Peierls instability and enable the stabilisation of superconductivity [34].

### 3. Superconductivity in the Bechgaard salts

While the study of TTF-TCNQ was going on it was recognized that electron polarizability is an important factor to reduce the screened on-site e–e repulsion. Therefore, the redox potential $(\Delta E)_{1/2}$ should be minimized, [35,36] in order to fulfill this goal and obtain potentially conducting materials. Hence, new charge transfer compounds with the acceptor TCNQ were synthesized using other heteroatoms beside sulfur for the donor molecule, i.e., substituting sulfur for selenium in the TTF skeleton thus leading to the TSeF molecule. Much attention was put on the tetramethylated derivative of the TSeF molecule which, when combined to the dimethylated TCNQ gave rise to TMTSF-DMTCNQ. The outcome of this study has been truly decisive for the quest of organic superconductivity [37] since for the first time a metallic state could be stabilized under pressure in an organic compound as a ground state instead of the usual Peierls ground state observed in all the other compounds belonging to the TTF-TCNQ series [38]. In 1979, the Montpellier group synthesized a series of isomorphous radical cationic conductors based on TMTTF (the sulfur analog of the TMTSF molecule) with an inorganic anion [39]. These materials were all insulating at ambient pressure but some of them did show superconductivity under

pressure 20 years later and became quite important for the physics of 1D conductors (Fig. 6). At the same time, the Copenhagen group led by Klaus Bechgaard succeeded in the synthesis of a new series of conducting salts all based on the TMTSF molecule namely, $(TMTSF)_2X$ where X is an inorganic mono-anion with various possible symmetry, spherical $(PF_6, AsF_6, SbF_6, TaF_6)$, tetrahedral $(BF_4, ClO_4, ReO_4)$ or triangular $(NO_3)$ [40]. All these compounds but the one with $X = ClO_4$ did reveal an insulating ground state under ambient pressure. What is special with $(TMTSF)_2PF_6$, the prototype of the so-called Bechgaard salts, unlike previously investigated TTF-TCNQ, is the magnetic origin of the ambient pressure insulating state related to the onset of itinerant antiferromagnetism opening a gap at Fermi level over the planar (1D) Fermi surface (Fig. 7).

### 4. Beyond superconductivity: the generic $TM_2X$ diagram

In the mid-1980s, the isostructural family comprising the sulfur molecule with the same series of monoanions was investigated under pressure and it was realized that $(TMTTF)_2X$ and $(TMTSF)_2X$ salts both belong to a unique class of materials forming the generic $(TM)_2X$ phase diagram [42], Fig. 8. In particular, $(TMTTF)_2PF_6$, although the most insulating compound of the phase diagram can be made superconducting at low temperatures under a pressure of 45 kbar [43,44]. Because of their particular crystal structures, such materials can be considered at first glance as prototypes for 1D physics [45]. What 1D physics means in a nutshell, is the replacement of Landau-Fermi quasi particles low lying
<figure><img src="image_9.png" /><figcaption>Fig. 6. Side view of (TM)₂X conductors along the *b* axis.</figcaption></figure>

excitations by decoupled spin and charge collective modes [46,47]. The model for 1D conductors starting from a linearized energy spectrum for excitations close to the Fermi level and adding the relevant Coulomb repulsions, which are responsible for electron scattering with momentum transfer $0$ and $2k_F$ is known as the popular Tomonaga–Luttinger (TL) model. In this model the spatial variation of all correlation functions (spin susceptibility at $2k_F$ or $4k_F$, CDW, superconductivity) 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) [48]. However, an

important peculiarity of (TM)₂X materials make them different from the usual picture of TL conductors. Unlike two-stacks TTF–TCNQ materials, (TM)₂X conductors exhibit a band filling which is commensurate with the underlying 1D lattice due to the 2:1 stoichiometry imposing half a carrier (hole) per TM molecule. Consequently, given a uniform spacing of the molecules along the stacking axis the unit cell contains 1/2 carrier, i.e., the conduction band is quarter-filled. However, non-uniformity of the intermolecular spacing had been noticed from the early structural studies of (TMTTF)₂X crystals [49] due to the periodicity of the anion packing twice the periodicity of the molecular packing. This non-uniformity amounts to a dismerization of the intrastack overlap between molecules which is important in the sulfur series (prominent 1/2 Umklapp scattering) but still present although less developed in some members of the (TMTSF)₂X series.

A dramatic consequence of this commensurate situation for (TM)₂X materials is the existence of localization channels due to electron–electron Umklapp scatterings with momentum transfer $4k_F$ (two particles scattering) or $8k_F$ (four particles scattering) corresponding to half- or quarter-filled bands, respectively [50,51]. This localization is a typical outcome of 1D physics in the presence of repulsive interactions and leads to a charge gap $Δ_ρ$ in the quasiparticle spectrum although no ordering is expected at any finite temperature for a purely 1D system. The spin sector remains gap-less on account of the separation between spin and charge degrees of freedom in 1D conductors. Fortunately, most features of the 1D localization and the physical properties of this electron gas can be studied by transport and optical properties of the materials. The most salient effect is the existence of a minimum of resistance at $T_ρ$ below which the resistance becomes activated. According to the theory of the commensurate 1D electron gas worked out by a number of authors, using the extended Hubbard, DMRG and bosonisation models, (see Ref. [52] for further reading) it as been shown that both Umklapp mechanisms compete for the establishment of the charge gap, 1/4 and 1/2 Umklapp scatterings leading to charge ordered and Mott insulators, respectively [53]. The experiment is unable to discriminate which of the two localizing channels is the dominant one at low temperature. In spite of the charge gap a metal-like behaviour of the longitudinal resistance can still be observed at $T$ larger than $Δ_ρ$ leading to a resistance

<figure><img src="image_10.png" /><figcaption>Fig. 7. The first observation of organic superconductivity in (TMTSF)₂PF₆ under pressure [41]. The resistance of two samples is displayed. This is a fac-simile of the 1979 lab book, before the common use of computerized data acquisition.</figcaption></figure>
<figure><img src="image_11.png" /><figcaption>Fig. 8. Generic phase diagram for (TM)₂X compounds [42] based on the sulfur compound (TMTTF)₂PF₆ for the ambient pressure value. Phases with long range order are shaded. T<sub>p</sub> and T* mark the onset of 1D charge localization and 1D/2D deconfinement, respectively.</figcaption></figure>

displaying a power law $\rho(T) \approx T^\theta (\theta > 0)$. Experimental studies have revealed such a metallic behaviour for the resistance either in (TMTTF)₂X under pressure [54,55] or in (TMTSF)₂X even at ambient pressure [30]. What has been found is a sublinear exponent, namely $\alpha = 0.93$ for the constant volume temperature dependence of the (TMTSF)₂PF₆ resistance [54,55]. Similarly, the optical conductivity reveals a power law frequency dependence at high frequency above a charge gap, $\sigma(\omega) \approx \omega^{-1.3}$ [56]. Transport and optical data experiments can both be explained within the theoretical framework of commensurate conductors with commensurability one or two. However, in the case of half-filling (commensurability one) the Luttinger parameter $K_\rho$ should be very close to unity (very weak repulsive interactions) since $\theta = 4K_\rho - 3$. A situation, which implies weak coupling $K_\rho \approx 1$ would be hard to reconcile with the exchange enhancement of the spin susceptibility [57]. On the other hand, when quarter-filled Umklapp scatterings prevail at high temperature $\theta = 16K_\rho - 3$ and consequently $K_\rho = 0.23$ according to the data of (TMTSF)₂PF₆.

Furthermore, the temperature dependence of the transverse conduction along the least conducting direction provides additional arguments in favour of a predominance of the quarter-filled Umklapp above $T_\rho$. The behaviour of the (TMTSF)₂PF₆ resistance along the c-direction displays an insulating behaviour below room temperature going through a maximum around $T \approx 80–100$ K and becomes metallic at lower temperatures, although remaining several orders of magnitude above

the Mott-Ioffe critical value which is considered as the limit between metal and insulating-like transport [58]. Transverse transport requires the tunneling of Fermions (at variance with the longitudinal transport which is related to 1D collective modes) between neighbouring chains and therefore probes the amount of quasi particles (QP) existing close to Fermi level. The insulating character of the transverse transport at high temperature can thus be interpreted as the signature of a non Fermi–Landau behaviour [59]. When transport along the c-direction is incoherent, transverse conductivity probes the physics of the *a*–*b* planes namely, the physics of the weakly interacting Luttinger chains in the *a*–*b* planes. The resistivity along the least conducting direction depends on the one-electron spectral function of a single chain and reads $\rho_c(T) \approx T^{1-2\alpha}$ [60] where $\alpha$ is related to $K_\rho$, $\alpha = 1/4(K_\rho + 1/K_\rho - 2)$.

The temperature $T^*$ at which the c-axis transport switches from an insulating to a metallic temperature dependence corresponds to a cross-over between two regimes; a high temperature regime with no QP weight at Fermi energy (possibly a TL liquid in the 1D case) and another regime in which the QP weight increases with decreasing temperature. This picture does not necessarily imply that the transport along the c-direction must also become coherent below the cross-over since the c-axis transport may remain incoherent with a progressive establishment of a Fermi liquid in *a*–*b* planes at temperatures below $T^*$. Consequently, the temperature dependence of transport properties along *a* and *c*-axes in the 1D regime above $T^*$ lead to a determination of $K_\rho$ of order 0.23 in (TMTSF)₂PF₆ and (TMTTF)₂PF₆ under a pressure of 12–15 kbar, respectively. The Luttinger parameter is much smaller in (TMTTF)₂PF₆ at ambient pressure [54], ($K_\rho = 0.18$) due to the enhancement of correlations in the left region of the phase diagram, Fig. 8. Optical data measured throughout the entire generic diagram reach fairly similar conclusions [61].

The decrease of the charge gap under pressure, Fig. 8 is due to a weakening of intrachain correlations moving from left to right in the diagram. Carriers are confined on the 1D chains and the transverse coupling renormalized by intrachain interactions is not pertinent [62]. However, when this gap becomes of the order of the bare kinetic transverse coupling a close interplay between this gap and the transverse coupling occurs giving rise to the sharp suppression of the localization observed around 15 kbar in (TMTTF)₂PF₆. At higher pressures the transverse coupling becomes pertinent and approaches the bare coupling increasing smoothly under pressure. The deconfinement of the carriers is observed around $T^*$. Below this deconfinement temperature charge excitations lose their 1D character and resemble more and more what is expected in Fermi liquids (quasiparticles), leading in turn to a quadratic temperature dependence for the longitudinal resistivity. However, electron excitations of this “Fermi liquid” retain a low energy gap in the far infra-red spectrum in which most of the oscillator strength is carried by states above the gap coexisting with a very narrow and intense zero frequency peak in the conductivity [63,56]. A phase transition towards a long range ordered insulating phase has been observed recently in the
charge localized temperature domain. The phase at low temper-
ature has been ascribed according to NMR data to the onset of
a charge disproportionation between molecules on the molecu-
lar chains [64]. Since the charge of this low temperature phase
(see, the CO state on the left side of the generic (TMTTF)₂PF₆
diagram, Fig. 8) is no longer uniform some ferroelectricity can
be expected as shown by a signature in dielectric measurements
[65]. The stability of the CO state (often called a Wigner state)
is a direct consequence of the long range nature of the Coulomb
repulsion which, in terms of the extended Hubbard model,
amounts to finite on-site U and second-neighbours
V repulsions.

**5. Nature of organic superconductivity**

From the very beginning of organic superconductivity, the
finding of a common border between an insulating SDW phase
and the SC state [30] and even of a coexistence pressure do-
main for both instabilities [66] raised a great and controversial
interest in the nature of the SC pairing (Fig. 9). While NMR
studies in the early days concluded in favour of nodes in the
SC gap [68], thermal conductivity data supported the model
of a s-type BCS gap covering the entire Fermi surface [69].
Regarding the spin part of the SC wave function, a triplet pair-
ing was claimed from a divergence of the critical field H<sub>c2</sub>(T)
exceeding the Pauli limiting value reported at low temperature
in (TMTSF)<sub>2</sub>PF<sub>6</sub> under pressure when H is applied along the
b' or a axes [70] and from the absence of a change in the
<sup>77</sup>Se Knight shift at T<sub>c</sub> [71]. However, the recent finding of
a T<sub>c</sub> determined by the value of the electron lifetime in the
presence of non magnetic lattice defects has conclusively ruled
out the s-wave hypothesis and suggested the existence of no-
des with zeros of the SC gap on the Fermi surface [67].
This latter experiment performed in (TMTSF)<sub>2</sub>ClO<sub>4</sub> could still
not discriminate between the options singlet-d or triplet-p (f)
[73]. However, the clear change of the <sup>77</sup>Se Knight shift ob-
served at T<sub>c</sub> in the compound (TMTSF)<sub>2</sub>ClO<sub>4</sub> [72] at fields

<figure><img src="image_12.png" /><figcaption>Fig. 9. Phase diagram of (TMTSF)₂(ClO₄)ₐ⁻(ReO₄)ₓ governed by non magnetic disorder according to Ref. [67]. All circles (open and close) refer to very slowly cooled samples in the R-state with different ReO₄⁻ contents.</figcaption></figure>

<figure><img src="image_13.png" /><figcaption>Fig. 10. <sup>77</sup>Se T<sub>1</sub><sup>-1</sup> vs T for (TMTSF)<sub>2</sub>ClO<sub>4</sub>, for H||b' and a, according to Ref. [72].</figcaption></figure>

lower than those used in the previous (TMTSF)₂PF₆ studies
[71] provided a conclusive evidence in favour of singlet pair-
ing (Fig. 10). In addition, a sudden increase of the spin lattice
relaxation rate for both field orientations ||a and b' provided
some evidence for a sharp cross-over or even a phase transition
occurring at low temperature under magnetic field between the
low field d-wave singlet phase and a high field regime being
either a triplet-paired state [74,75] or an inhomogenous Ful-
de-Ferrell-Larkin-Ovchinnikov state [76,77]. A very
recent study of the anisotropic superconductivity of
(TMTSF)₂ClO₄ with a magnetic field lying in the b'-a plane
has shown the survival of superconductivity far above Hₚ and

<figure><img src="image_14.png" /><figcaption>Fig. 11. Magnetic fields vs T_c of (TMTSF)₂ClO₄ for H||a (filled squares) and H||b' (filled circles) according to Ref. [78]. The arrow indicates the estimated value of H_p = 26.7 kOe.</figcaption></figure>
<figure><img src="image_15.png" /><figcaption>Fig. 12. Diagrammatic representation of the interchain attractive pairing according to Ref. [80]. Carriers forming singlet pairs belong to neighbouring chains and exchange an antiferromagnetic spin fluctuation with momentum $2k_F$, the double-dotted line in the scattering process.</figcaption></figure>

suggested the existence of modulated superconducting phases when high fields are nearly aligned along this crystallographic plane [78,79] with possibly different kinds of inhomogenous phases for the *a* and *b'* directions. As far as the origin of the coupling is concerned the incipient antiferromagnetism of these (TM)₂X superconductors supports the existence of strong intrachain repulsive interactions making in turn the usual phonon mediated pairing unlikely to be active. In order to explain the stability of superconductivity once the SDW state is suppressed by a large enough departure from good nesting of the Fermi surface, both instabilities, Peierls (electron-hole) and pairing (electron-electron) have to be treated on equal footings and in particular the ladder diagrammatic summation inherent to the BCS treatment can no longer be used. The most salient outcome of such a treatment is an attraction between carriers on neighbouring chains mediated by the interchain exchange of AF intrachain spin fluctuations [80] provided by the existence of a finite transverse coupling $t_⊥$, see Figs. 11 and 12.

## 6. Conclusion

This short presentation as shown how research on organic superconductors has developed and has been boosted by two stimulating proposals: first, the theoretical model suggested by Little for high $T_c$ organic superconductivity and second, the claims made by the Penn group for precursor signs of superconductivity in the first organic conductors TTF-TCNQ. Whereas both statements were probably overemphasized, the discovery of organic superconductivity in 1980 has shown how interesting this class of materials can be in terms of the low dimensional physics. In addition, the determining role of high pressure governing their physical properties, 1D Mott or Wigner localization and the profusion of various instabilities at low temperature have shown how organic conductors compare fairly well in terms of scientific interest with the celebrated class of high $T_c$ cuprates. A short article like this one has not been able to survey other important aspects of the physics of 1D conductors such as the remarkable role played by the magnetic field either on the angular magnetoresistance in the normal phase, (see Ref. [81] for a recent survey) or in the stabilisation of insulating spin density wave phases with a quantization of the Hall effect [82–86]. While the actual pairing mechanism behind organic

superconductivity is still an enigma this whole field remains particularly challenging, even 30 years after its discovery.

The search, discovery and study of organic superconductors have been a major research domain for the Solid State Physics Laboratory at Orsay over the last 30 years. This period has been characterized by an intense and fruitful cooperation between theoreticians and experimentalists. I wish to address my sincere and deepest thanks to all our co-workers.

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