# An Historical Approach to Organic Superconductivity

Denis Jerome

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# An Historical Approach to Organic Superconductivity

**Denis Jérome**

Laboratoire de Physique des Solides, UMR 8502,
Université Paris-Sud, 91405 Orsay, France

January 8, 2010

We give a brief account of how the search for superconductivity in organic matter, motivated by the quest for high $T_c$ superconductors has developed since the 60's. Even if the maximum $T_c$ of organic conductors in 2005 does not exceed 13K these are prototype systems which have contributed remarkably to the progresses in the physics of low dimensional conductors.

## 1 One dimensional conductors

Superconductivity, i.e; the possibility for electrons to flow through a conductor without any energy loss has been the fuel for a very large number of industrial applications of physics in the 20th century. Just to mention some of them which are important in current life there is the power-loss energy transfer, the production of very strong magnetic fields for medical imaging or for plasma confinement (the fusion problem), voltage standards, particle accelerators and magnetic flux sensors.

This new state of matter has been discovered somehow unexpectedly in 1911 in mercury with a critical temperature of $T_c$ = 4.15K by Kamerlingh Onnes just after he had mastered the liquefaction of helium[1]. However, the phenomenon of superconductivity, in particular the value of its $T_c$, is strongly dependent on the material in which it is to be observed. Therefore, over the years a lot of new metals and intermetallic compounds have been
discovered with increasing values of $T_c$ reaching a plateau at about 24 K in the 60s[2], see Figure (1).

<figure><img src="image_2.png" /><figcaption>Figure 1: Evolution of the superconducting critical temperature in metals and intermetallic compounds, cuprates and organic conductors (1 and 2D conductors).</figcaption></figure>

The absence of any satisfactory theory until 1957 did not prevent applications of superconductivity which developed after World War II but they were all bound to the use of expensive liquid helium as the cooling agent necessary for the stabilization of the superconducting state.

In the 60s, when superconductivity had already turned into applications, people were worrying about the possibility to discover new materials to release the constraint of using liquid helium to reach the superconducting state. Then came the idea to look for materials out of the traditional series of metals or alloys compounds namely new types of conductors in which the conduction would proceed through the transport of charge between molecular orbitals rather than atomic orbitals. Actually, the concept of a synthetic metal had already been launched by McCoy and Moore[3] simultaneously with the discovery of superconductivity in mercury 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[4] has been the first to suggest that compounds with aromatic rings such as anthracene, naphtalene,... 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 [5] although this salt was rather unstable. In the early 60s, related to the quest for higher $T_c$ W.A.Little

<figure><img src="image_3.png" /><figcaption>Figure 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>

made an important suggestion[6, 7]: a new mechanism 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 isotope effect, one of the great successes of the theory proposed in 1957 by Bardeen Cooper and Schrieffer (BCS)[8] for the interpretation of superconductivity in materials known up to that date and based on the phonon-mediated electron pairing. In the BCS theory the attractive interaction between electrons (or holes) which is a prerequisite for the Bose condensation of electron pairs into the superconducting state relies on the mass $M$ of the ions which undergo a small displacement when the electrons are passing close to them namely, ($T_c \propto M^{-1/2}$). It can be noticed that Fröhlich proposed in 1954 a mechanism for superconductivity based on the one dimensional case[9]. Although the theory of Fröhlich has not been able to account for the superconductivity of metals it turned out to
be quite successful later for the interpretation of the transport properties
in some one dimensional organic compounds, *vide infra*. In the excitonic
mechanism of Little the charge carriers are moving along a conducting spine
and it is an electronically polarizable medium which is used instead of the
usual polarizable ionic lattice. 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 superconduc-
tor, admittedly a huge factor. This has been the beginning of the concept
of superconductivity at room temperature, still the Graal of scientists in the
beginning of the 21st century. Simultaneously, V.L.Ginzburg[10, 11] consid-
ered the possibility for the paring of electrons in metal layers sandwiched
between polarizable dielectrics through virtual excitations at high energy, see
Figure 3.

<figure><img src="image_4.png" /><figcaption>Figure 3: Sketch of the 2D superconductor suggested by V.L. Ginzburg.</figcaption></figure>

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[12] (see Figure 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[6] which was based on the popular Fermi liquid BCS theory. The first point is that one dimensional conductors had been regarded previously as textbook examples by Peierls[13], who stated a theorem according to which a gap should spontaneously open at the Fermi level in the conduction and lead to the stabilization of a dielectric insulating state at low temperature long before it has been established in the platino-cyanate chains[14]. Bychkov et-al[15] criticized the model of Little regarding its potentiality to lead to high temperature superconductivity. Following Bychkov et-al, the one-dimensional character of the model system proposed by Little makes it a unique problem in which there exists a built-in coupling between superconducting and dielectric instabilities. It follows that each of these instabilities cannot be considered separately in the mechanism proposed by Little in one dimension and last but not least fluctuations should very efficient in a 1D conductor to suppress any long range ordering to very low temperature.

At the beginning of the 60s the term *high temperature superconductor* was already commonly used refering to the intermetallic compounds of the A15 structure, namely, (Nb₃Sn or V₃Si)[16]. 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*<sub>c</sub> could be expected. They were actually observed (17-23K) but an upper limit was found to the increase of *T*<sub>c</sub> since the large value of N(E<sub>F</sub>) makes also the structure unstable against a cubic to tetragonal band Jahn-Teller distortion[17, 18]. The theory showed that the actual compounds Nb₃Sn and V₃Si possess their optimum *T*<sub>c</sub> [19]. In this context some papers at the beginning of the 80s, based on metallurgical considerations, regarded 25-30K as the highest possible value for *T*<sub>c</sub> [2]. Last but not least, the possibility for superconductivity in organic structures was peremptorily rejected by the champion of the A15 materials[20].

Over the subsequent sixteen years before the discovery of superconductiv-
ity in organic matter an intense activity developed in physics and chemistry
communities. A lot of fascinating new compounds have been synthesized by
chemists and studied by physicists. A first step following the ideas of Little
has been the discovery of an inorganic polymer poly(sulfurnitride), which
consists of alternating sulfur and nitrogen atoms covalently bonded in long
chains, (SN)ₓ. The interest in this compound becoming a superconductor
at 0.26K[21] did not last very long first because its T_c could not be signi-
cantly raised and second because the band structure did not reveal marked
one dimensional effects. Nearly at the same time a major step has been accomplished in chemistry with the synthesis of the new molecule tetrathiafulvalene, (TTF), by F.Wudl[22], which has deeply influenced the subsequent evolution of the chemistry of organic conductors. This molecule containing four sulfur heteroatoms in the fulvalene skeleton can easily donate electrons when it is combined to electron accepting molecules allowing the synthesis of the first stable organic metal, the charge transfer complex, TTF-TCNQ. The system is made up of two kinds of flat molecules each forming segregated parallel conducting stacks. 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.

The announcement of a large and metal-like conduction in TTF-TCNQ was made in 1973, simultaneously by the Baltimore[23] and Pennsylvania[24] groups. The Pennsylvania group made a provocating claim announcing a giant conductivity peak of the order of $10^5 (\Omega cm)^{-1}$ 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. Unfortunately, the 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[25]. Besides, the Orsay group showed from X-ray scattering studies that the metal insulator transition was the consequence of the instability of a conducting chain predicted by Peierls[26] and also from the study of transport properties under pressure that the behaviour of the conductivity in the metallic regime could be attributed to collective Fröhlich fluctuations in a 1D regime[27]. Furthermore, the role of 1D electron-electron repulsive interactions has been recognized by the $4k_F$ signature in X-ray diffuse scattering experiments[28]. While high pressure appeared to be a parameter far more influential for organic conductors than for regular metals, it has failed to make TTF-TCNQ sufficiently three dimensional to prevent the onset of a Peierls instability and enable the stabilisation of superconductivity[29].

Since decreasing the size of the Coulombic repulsion is expected to boost the conductivity of metals, other synthetic routes have then been followed. In the 1970s, the leading ideas governing the search for new materials likely to exhibit good metallicity and possibly superconductivity were driven by
the possibility to minimize the role of electron-electron repulsions and at
the same time to increase the electron-phonon interaction while keeping
the overlap between stacks as large as possible. This led to the synthesis
of new series of charge transfer compounds which went beyond the known
TTF – TCNQ system, for example changing the molecular properties while
retaining the same crystal structure. It was recognized that the electron
polarizability is important to reduce the screened on-site e-e repulsion and
that the redox potential (Δ*E*)₁/₂ should be minimized, [30, 31] in order to
fulfill this goal. Hence, new charge transfer compounds with the acceptor
TCNQ were synthesized using other heteroatoms 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 [32] 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 [33]. 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 [34]. 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.

At the same time, the Copenhagen group led by Klaus Bechgaard suc-
ceeded 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 symetry, spherical (PF₆,AsF₆, SbF₆,TaF₆), tetrahedral
(BF₄,ClO₄,ReO₄) or triangular (NO₃)[35]. All these compounds but the one
with X= ClO₄ did reveal an insulating ground state under ambient pressure
, Figure (4).

The compound with X= PF₆ caught much attention since the conductiv-
ity reaches the value of 10⁵Ω⁻¹cm⁻¹ at 12K with still a strong temperature
dependence at this temperature before the onset of an insulating ground
state. This behaviour for the transport properties together with the absence
of any lattice modulation,[36, 37] as precursors to the metal-insulator tran-
sition were new features in a field still dominated by the Peierls philosophy
and stimulated further investigations under pressure suppressing the insu-
lating state at liquid helium temperature under a pressure of about 9kbar.
<figure><img src="image_5.png" /><figcaption>Figure 4: Side view of (TM)₂X conductors.</figcaption></figure>

The finding of a very small and still non-saturating resistivity at 1.3 K was a strong enough motivation to trigger further studies under pressure at even lower temperatures in a dilution refrigerator, which rapidly led to the discovery of a zero resistance state below 1 K. As this zero resistance state was

<figure><img src="image_6.png" /><figcaption>Figure 5: First observation of organic superconductivity in (TMTSF)₂PF₆ under pressure[38]. The resistance of two samples is displayed.</figcaption></figure>

suppressed by a magnetic field, superconductivity was claimed[38].

The discovery of organic superconductivity in (TMTSF)₂PF₆ came out 15 years after the publication of Little and 10 years after the holding of an international symposium organized by W.A. Little at Hawai'i on the Physical
and Chemical Problems of Possible Organic Superconductors [12]. Shortly
after the discovery of superconductivity in (TMTSF)₂PF₆ [38], several other
isostructural compounds with a variety of anions have been found supercon-
ducting in the vicinity of 1 K in the 10 kbar pressure domain [39]. Among
them, (TMTSF)₂ClO₄ is the only exception becoming superconducting at
low temperature without the need of a high pressure. [40].

In addition, it has been realized that the highly insulating isostructural
compound (TMTTF)₂PF₆, the sulfur counterpart of (TMTSF)₂PF₆ can be
made to superconduct at low temperatures provided a pressure of 45 kbar is
applied[41]. As a result, a unified (TM)₂X phase diagram has been proposed
experimentally[42]. This diagram suggests that apparently so different sys-
tems such as (TMTSF)₂ClO₄, superconducting under ambient pressure, and
the strongly insulating (TMTTF)₂PF₆ belong to the same class of materials
i.e. the physical properties of the latter can be made equivalent to those of
the former provided a large enough pressure is applied.

Moreover, according to their crystal structure these 1D conductors could
potentially be protoptypes of Luttinger conductors since the kinetic cou-
pling between molecular stacks is very small compared to the longitudinal
dispersion[43]. However, the commensurate filling of the conduction band, a
consequence of the stoechiometry, makes the conduction band half or quarter-
filled. Consequently, even a moderate value of the electron-electron repulsion
of the order of the bandwidth opens a correlation gap at the Fermi level due
to the half or quarter-filled electron-electron Umklapp scattering[44, 45] and
all members of the (TM)₂X family are actually Mott insulators with a gap
decreasing from sulfur to selenium compounds. In addition, disproportiona-
tion reactions have been observed in several members of the (TM)₂X series
leading to a charge ordering with the onset of an insulating state clearly
evidenced by NMR[46] and dielectric measurements[47].

When the Mott gap becomes small enough (under pressure) to compete
with the transverse kinetic coupling, the charge carriers lose their 1D con-
finement and their behaviour in temperature approaches what is expected
in Fermi liquids namely, a quadratic temperature dependence for the re-
sistivity. However, the electron excitations of this "Fermi liquid" retain a
low energy gap in the far infra red spectrum carrying most of the oscillator
strength together with a very narrow and intense zero frequency peak in the
conductivity[48, 49].

Among the questions which are still of great current interest for 1D or-
ganic superconductors is the nature of the superconducting coupling[50, 51].
Is it a spin singlet as for regular superconductors and high $T_c$ compounds or a triplet coupling as this has been shown to be the case for the superfluidity of $^3$He[52, 53] or some heavy fermion superconductors[54, 55], and ruthenates[56]?. A recent investigation of the Knight shift and of the spin lattice relaxation has shown that the superconducting phase of $(TMTSF)_2ClO_4$ is a spin-singlet state[57]. Together with the existence of nodes for the superconducting gap[58] these recent data strongly suggest that the symmetry of the superconducting wave function is singlet-paired with a d-wave orbital symmetry.

It is also clear that a precursor superconductivity regime exists over an extended temperature domain above $T_c$, although the controversy about its origin (low dimensional fluctuations, pseudogap,...) raised 25 years ago is still going on.

## 2 Two dimensional conductors

At the beginning of the 80s, a new derivative of the fulvalene molecule has been discovered and opened a new series of organic superconductors. This is BEDT – TTF, an elongated version of TTF also called ET which has given a salt with $ReO_4$ becoming superconducting under pressure[59]. The crystal structure of (BEDT – TTF)$_2ReO_4$ bears much similarity with the molecular packing of the $(TMTSF)_2X$ series and shows a pronounced 1D character.

One year later, the same ET molecule contributed to the elaboration of half-filled band organic conductors with quite novel structures namely, materials displaying a two dimensional (2D) conducting (or layer) structure. The first representatives of this 2D class of organic materials were the $(ET)_2X$ compounds where X is a linear triatomic anion such as $I_3$, $IBr_2$, $AuI_2$ etc.,. The salt with $X= I_3$ is particularly interesting as the $\beta_H$-phase, which can be stabilized at low temperature after a special pressure-temperature cycling, has provided a large increase of the superconducting $T_c$ of the organics from 1-2K in the $(TMTSF)_2X$ series up to 8K[60, 61, 62]. When $(ET)_2I_3$ is cooled rapidly without any pressure treatment, random disorder of the ethylene groups of the ET molecules remains in the $\beta_L$-phase and strongly reduces the stability of the superconducting state with a $T_c$ dropping down to 1.4K[63] in agreement with the role of non magnetic defects on the superconductivity of 1D organics[58].

In addition, $\beta_H$(ET)$_2I_3$ has provided a textbook example for Shubnikov-de
Haas oscillations in a 2D conductor with extraordinarily large oscillations of the magnetoresistance [64]. Such a study has also enabled the determination of the overlap integral between conducting planes of 0.5 meV, a clear-cut illustration for the pronounced 2D character and the existence of angular magnetoresistance oscillations due to the c-axis warping of the Fermi surface in these ET conductors [65, 66].

A further increase in the $T_c$ above 10K has been accomplished in the 90s with the discovery of the $\kappa$-phase salts namely, $\kappa - (ET)_2X$ with polymeric anions, $X= \text{Cu(NCS)}_2$ [67], $\text{Cu(N(CN)}_2\text{)Br}$ [68], and even up to 12.5K in $\text{Cu(N(CN)}_2\text{)Cl}$ under a modest pressure of 0.3 kbar [69].

Most $\kappa$-phase salts show an isostructural face to face molecular dimers packing forming a 2D checker-board pattern. Very much like the compounds belonging to the $(\text{TM})_2X$ family most $\kappa$-phase systems can be gathered together in a generic Temperature-Pressure (Chemistry) phase diagram [70]. The compound $X=\text{Cu(N(CN)}_2\text{)Br}$ is thus of particular interest since superconductivity at $T_c = 11.6$K at ambient pressure makes NMR studies in the superconducting state accessible over a decade in temperature. $^{13}$C Knight shift and relaxation studies in a $\kappa$-Cu$(\text{N(CN)}_2\text{)Br}$ single crystal have shown conclusively that the pairing in 2D organic superconductors must be spin singlet with the existence of nodes in the gap [71, 72, 73]. Among all compounds $\kappa$-Cl is also particularly interesting as it is the prototype in the series showing the complete sequence of states, namely, paramagnetic Mott insulating, antiferromagnetic, metallic and superconducting states within a few hundred bars. Magnetic [74], acoustic [75] and transport experiments [76, 77] have shown the existence of a first order transition line in the T-P plane with a phase coexistence regime ending at a Mott critical point.

The study of this material has also enabled to illustrate the concept of bad metal, namely, a strongly correlated metal in the neighbourhood of a Mott transition [78]. A recent reinvestigation of this compound suggests that the critical regime around the critical end point does not appear to obey the 2D Ising model universality class [79] at variance with a similar study of the 3D Mott transition performed in the vanadium sesquioxide [80].

An other aspect of the 2D organics is the experimental realisation of the Jaccarino-Peter mechanism for superconductivity in systems comprising magnetic ions [81]. This mechanism is probably the one which allows the stabilization of superconductivity in $(\text{BETS})_2\text{FeCl}_4$ (where BETS is the selenide parent molecule of ET) under intense magnetic field [82, 83] compensating the exchange field of aligned $\text{Fe}^{3+}$ ions.
## 3 Conclusion

This short presentation for the search for superconductivity in organic matter has been based on an historical approach because it can be instructive to see how the whole story has developed from quite provocative theoretical statements and led finally to the discovery of the first organic superconductor thanks to a very productive work between Chemistry and Physics communities over the past 35 years. It is worth citing V.L.Ginzburg in this conclusion[84]: *I believe that it was undoubtedly the discussion of the possible exciton mechanism of superconductivity that stimulated the search for such superconductors and studies of them* (organic superconductors and intercalated layered superconductors). In addition, the other statement of Ginzburg: *the organic superconductors are clearly interesting by themselves or, to be more precise, irrespective of the high $T_c$ problem* cannot be more appropriate. While the actual mechanism behind organic superconductivity remains an enigma one can be confident saying that the model proposed by Little in 1964 is probably not the one applying to those materials.

Thanks to the determining role of high pressure governing their physical properties, organic conductors (and superconductors) have shown their superiority compared to high $T_c$ cuprates in terms of purity and in the variety of phenomena which can be studied in a single system keeping both structure and chemical purity constant. Recently, it was even believed that the entire physics of organics including superconductivity could be reproduced without the burden of high pressure techniques tuning the carrier concentration with a field effect technique. Unfortunately, the numerous articles published on this subject cannot be cited anymore. They do not represent the most brilliant days of condensed matter physics.

A perfect illustration for the scientific interest of organic conductors is provided by the amount of knowledge and new physical phenomena which came out of the studies of organic superconductors. To summarize, the optimized location for the superconductivity $T_c$ close to the Mott localization resulting from a compromise between the interchain coupling and the strength of the correlations and the likelihood of an unconventional pairing mechanism which remains a challenging topic and awaits new experimental evidences. It is also important to mention the magnetic field confinement discovered in the 1Ds leading to the phenomenon of field induced spin density wave phases[85, 86, 87] and the quantization of the Hall effect in these phases[88, 89, 90]. Furthermore, the angular dependent magnetoresistance
specific to these anisotropic conductors[91, 92], the so-called Lebed oscillations in (TMTSF)₂X provide a nice illustration for the new features of the quasi-1D conductors[93].

The 2D compounds have provided not only higher values of $T_c$ and a textbook example for the two dimensional fermiology but also a playground for the study of the 2D Mott transition as well as the metallic phase in its vicinity.

At the end of this historical perspective of organic superconductors I wish to recall in particular some of my close colleagues who have played a pioneering role in this research, J.Friedel who, 40 years ago, pointed out the relevance of low dimensionality (1 and 2D) in superconductivity and has continuously supported the activity in one dimensional conductors at Orsay, M.Weger who introduced TTF – TCNQ to the physical investigations under pressure at Orsay in 1974, K.Bechgaard with whom the active cooperation established in the 70’s is still very much alive, M.Ribault who understood the importance of linking high pressure and very low temperature measurements. S.Brown has contributed to solve the controversy about organic superconductivity. H.J.Schulz, T.Giamarchi and C.Bourbonnais are all close theoreticians colleagues who made significant contributions to the one dimensional theory and who have been deeply involved in a tight cooperation with experimentalists.

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