# Essay

# European Action on Organic Superconductors

By Denis Jérôme,* and Klaus Bechgaard

## 1. The Interest in Low Dimensional Conductors

Low dimensionality arises when a given structure can be described as a loose assembly of “units” of reduced dimensionality (chains or planes for 1-D and 2-D materials, respectively) in which molecules are more tightly packed and strongly bound than between units.

Such structures have been studied for a fairly long time in compounds from mineral chemistry in the context of magnetism, conductivity and superconductivity. Indeed, until recently, superconductors exhibiting the highest transition temperatures $T_c$ were intermetallic compounds with the β-tungsten structure, Nb$_3$Sn, Nb$_3$Ge, etc.) which are represented by a three-dimensional organization of loosely coupled metal chains. The high-$T_c$ oxide superconductors are also an example of quasi-two dimensionality.

Organic conductors belong to a certain class of molecular solids, namely those in which electrons can flow freely through the material very much like the situation in metals or semiconductors. Most organic conductors of interest for superconductivity are two-component materials containing charged species (anion and cation). Chemistry has so far provided planar molecules exhibiting $π$ orbitals built from the $p_z$ atomic orbitals of carbon and various heteroatoms (Se, S, O). The most remarkable characteristic of these materials is the piling up of segregated columns of donor molecules and acceptor anions which in turn leads to a structure with pronounced 1-D character.

An important aspect in the application of molecular solids is the synthesis of the materials. The building block (the molecule) exhibits enormous flexibility provided by organic chemistry. Small molecular changes can be made by chemical synthesis and thus a particular property (electrical polarizability, crystal symmetry, strength of coulomb repulsions, intermolecular overlap, etc.) can be tailored according to the specific properties which are required for the crystal, for example, the value of the conductivity, the superconducting $T_c$, the sensitivity to irradiation, magnetic field or pressure.

It was actually the prospect of superconductivity at high temperature following the suggestion made by Little in 1964

that superconductivity could be stabilized up to room temperature, provided one could synthesize conducting polymers with highly polarizable side groups,[1a] which has boosted research into one-dimensional organic conductors.[1b]

The Orsay-Copenhagen cooperation, which began in 1976, was focused on TTF-like materials and led, in 1979, to the discovery of the first organic conductor ($TMTSF)_2PF_6$, which is a superconductor below $T_c$ = 1 K under a pressure of 9 kbar (Fig. 1).[2] In the years that followed, superconduc-

<figure><img src="image_1.png" /><figcaption>Fig. 1. The superconducting transition in two organic conductors. ($TMTSF)_2PF_6$ is under a pressure of 9 kbar. The ($ET)_2X$ data have been obtained in the MOLCOM action (at atmospheric pressure).</figcaption></figure>

tivity as well as a wealth of fascinating new physical properties were found in the $TM_2X$ quasi-1-D series:[3] superconductivity at ambient pressure in ($TMTSF)_2ClO_4$,[4] an insulating spin-density wave ground state in ($TMTSF)_2PF_6$ and spin density wave phases stabilized in very high magnetic fields,[1b] the very strong sensitivity of organic superconductivity to ionizing radiation and enhancement of superconductivity in a new series of two-dimensional organic superconductors, ($ET)_2X$ (Fig. 1), where ET is bis(ethylenedithio)tetrathiofulvalene.

<footnote>[*] Dr. D. Jérôme
Laboratoire de Physique des Solides, Université Paris-Sud
F-91405 Orsay (France)
Prof. K. Bechgaard
CISMI
Universitetsparken 5, DK-2100 Copenhagen (Denmark)</footnote>
It is clear that all these new properties are related to two essential features: the one-dimensional architecture and the molecular nature of the building blocks. While a tremendous amount had been learned about each of the materials belonging to the 1-D and also to the 2-D (ET)<sub>2</sub>X families discovered in the mid-1980s, in 1988 we still had a very poor understanding of what the important parameters that are responsible for such large differences in the physical properties were. Before the use of these new materials can be extended to future microelectronic devices it is of great importance to obtain a clear knowledge of the fundamental electronic processes governing the properties of these materials.

## 2. European Cooperation (MOLCOM)

Recognizing the importance of organic conductors, the European Commission Directorate General XIII set up a consortium in 1989 involving Denmark, Portugal and France known as MOLCOM (MOLecular COmponents for Microelectronics), which aimed at studying the various factors governing the flow of electrons in these crystalline organic conductors, in particular the influence of controlled disorder on the conductive and superconductive properties of these low-dimensional conductors. Optimizing the conditions for superconductivity in 2-D organic conductors and understanding the origin of the superconducting pairing was also a task for the consortium.

All partners of this consortium were complementary. More specifically, ADIST, at the University of Lisbon, and LNET, at Sacavem, are groups with expertise in synthesis and magnetic measurements. The Copenhagen group specializes in the design and synthesis of new molecules and their transformation into conducting solids which are investigated at Orsay using powerful and rather unique experimental techniques. These include measurements of transport properties and NMR under extreme conditions of temperature, magnetic field and pressure, diffuse X-ray investigation, and an investigation of the connection between experiment and theory. The synthesis of new molecules and selected inorganic anions as well as X-ray structure determinations are also carried out at Orsay.

## 3. Achievements

An important task of this action is the evaluation of the possibilities of organic conducting compounds as far as superconductivity is concerned. This part of the work has required the preparation of a large number of new compounds. It has become clear that organic superconductors can be classified into different classes of materials depending on whether the superconducting ground state represents the only instability of the electron gas or whether different ground states are accessible. Such a classification is justified by the shape of the Fermi surface. A first class of compounds involves those exhibiting quasi-one-dimensional conduction. This situation is encountered for all conductors built from the tetrathiafulvalene or tetraselenafulvalene molecules. The Fermi surface of these materials comprises two quasi-planar sheets intersecting the $k_{\parallel}$ axis at $\pm k_{\perp}$.

The very special geometry of the Fermi surface is the essential point which makes this series of conductors so different from other compounds where the dimensionality of the conduction is higher than unity. When the Fermi surface is planar or quasi planar, the electron gas may become unstable at low temperature with respect to a superconducting phase but also to an insulating state exhibiting a new modulation with a periodicity which is fixed by the density of electrons and not by the underlying crystalline lattice. The interplay between superconducting and insulating instabilities depends on the chemical species of either the cation radical (TMTTF or TMTSF) or the anion size (simple or complex ion) and also on the amount of coupling existing between conducting chains.[15]

Figure 2 summarizes the main features of the so-called (TM)<sub>2</sub>X series of quasi-1-D conductors. The figure shows that dielectric instabilities are the most stable ones for sulfur based materials, namely (TMTTF)<sub>2</sub>X compounds. Changing S for Se and (or) increasing the interchain coupling using high pressure favors the superconducting ground state which becomes stable below 1 or 2 K. A special effort has been made in the MOLCOM action on the study of new hybrid material which lies between (TMTTF)<sub>2</sub>PF<sub>6</sub> and (TMTSF)<sub>2</sub>-PF<sub>6</sub> as far as physical properties are concerned. This work has been achieved by making extensive use of the mixed sulfur-selenium molecule TMDTDSF which was synthe-

<figure><img src="image_2.png" /></figure>

Denis Jérôme is a Director of Research in the CNRS and heads the research group in Organic Conductors and Superconductors at the Laboratoire de Physique des Solides of the University Paris-Sud, Orsay. He joined the laboratory of Professor A. Abragam at Saclay in 1962 and received his doctorate in 1965 at the University of Paris before working as a postdoc at the University of California, La Jolla. He moved back to Orsay in 1967 to head the high-pressure group in the Solid State Physics Laboratory. He is a laureate of the Holweck Prize of the French Physical Society and Institute of Physics (1985) and became a correspondent of the French Academy of Sciences in 1990. He shared the Europhysics-Hewlett-Packard Prize in 1991 with Professor K. Bechgaard. He is currently a co-editor of Synthetic Metals, a member of the Editorial Board of Journal of Physics Condensed Matter and the Editor in Chief of Journal de Physique.
sized for the purpose of this action. (TMTDTDSF)₂X compounds are very interesting as they show most features which are expected to arise in a 1-D electron gas: one-dimensional Mott-Hubbard localization accompanied by a spin-charge separation, power law divergence of the antiferromagnetic response at low temperature observed by NMR relaxation, and spin-Peierls (SP) and spin density wave (SDW) ground states. The lesson learned from the study of these one-dimensional conductors is that the competition between superconductivity and antiferromagnetic (insulating) properties establishes an upper limit for the onset of the superconducting ground state. Following the data in Figure 2, one would be tempted to think that decreasing the pressure (and therefore the size of the interchain coupling) should lead to a significant increase of T_c. However, an upper limit to this increase is set between 1 and 2 K by the existence of a critical transverse coupling below which the dielectric instability is always dominating the superconductivity.

<figure><img src="image_3.png" /><figcaption>Fig. 2. Generic phase diagram for TM₂X. Notations SP, SDW and SC refer to spin-Peierls, spin density wave and superconducting ground states which are accessible at low temperature. Below the dotted line carriers are localized by Coulombic repulsions.</figcaption></figure>

Consequently, if one is searching for high-T_c superconductors among organic conductors, materials not exhibiting one-dimensional conducting properties should be examined.

Much effort has been devoted in the MOLCOM action to establish the nature of the mechanism responsible for superconducting pairing. The proximity of superconductivity and magnetism makes the attractive electron-electron pairing mediated by the exchange of antiferromagnetic spin fluctuations a likely hypothesis for quasi-1-D superconductors.[6] Because no such proximity with magnetism exists in superconductors belonging to the two-dimensional (ET)₂X series,

a more traditional interaction mediated by phonons can be considered.

A study using isotopically enriched ET molecules has contributed to the clarification of the latter point. Samples of the β-phase of (ET)₂I₃ have been prepared in which the carbon atoms in the central double bond of the molecule were enriched with the ¹³C isotope. Raman shift and superconductivity measurements show that T_c is linked to the frequency of the intramolecular modes involving the vibration of the central double bond.[7] Therefore the nature of the attractive interaction leading to the pair formation may very well be different in 1-D and 2-D organic superconductors.

## 4. Conclusions and Future

It is fair to say that the European cooperation has been remarkably successful in many respects. Even if the magic recipe for the synthesis of high-T_c compounds among organic conductors is not yet at hand, very serious suggestions based on what has been achieved by the MOLCOM action can be proposed for future research.

Through work carried out within the MOLCOM action it has been clearly demonstrated that the one-dimensional electron gas has very interesting properties. Textbook examples of one-dimensional physics have been encountered, such as the competition between various ground states in the TM₂X series. Other remarkable achievements concerning the study of phenomena such as magnetic-field-induced antiferromagnetic phases and the quantification of the Hall effect have not been described in this short essay. Furthermore, quasi 1-D conductors are not the most appropriate materials for achieving the onset of superconductivity at elevated temperature. High-T_c molecular superconductivity should probably be searched for in solids built of conjugated molecules exhibiting strong coupling between symmetrical vibrations and the electron gas. In addition, the intermolecular overlap should be weak in order to get a large density of states at the Fermi level and at least two-dimensional to prevent the formation of density wave states at low temperature. The recently discovered fullerene compounds,[8] with a T_c up to 30 K, are a step in this direction.

<footnote>[1] a) W. A. Little, Phys. Rev. A **1964**, *134*, 1416. b) D. Jérôme, *Adv. Mater.* **1990**, *2*, 321.</footnote>

<footnote>[2] D. Jérôme, A. Mazaud, M. Ribault, K. Bechgaard, *J. Phys. Lett. (Paris)* **1980**, *41*, L-95.</footnote>

<footnote>[3] K. Bechgaard, C. S. Jacobsen, K. Mortensen, H. J. Pedersen, N. Thorup, *Solid State Commun.* **1980**, *33*, 1119.</footnote>

<footnote>[4] K. Bechgaard, K. Carneiro, K. Olsen, F. B. Rasmussen, *Phys. Rev. Lett.* **1981**, *46*, 852.</footnote>

<footnote>[5] D. Jérôme, *Science* **1991**, *252*, 1509.</footnote>

<footnote>[6] C. Bourbonnais, L. G. Caron, *Int. J. Mod. Phys. A* **1991**, *B5*, 1033.</footnote>

<footnote>[7] V. Merzhanov, P. Auban-Senzier, C. Bourbonnais, D. Jérôme, C. Levoir, P. Batail, J. P. Buisson, S. Lefrant, *C. R. Acad. Sci. Paris* **1992**, *314*, 563.</footnote>

<footnote>[8] A. F. Hébard, M. J. Rosseinsky, R. C. Haddon, D. W. Murphy, S. H. Glanum, T. T. M. Palstra, A. P. Ramirez, A. R. Kortan, *Nature* **1991**, *350*, 600.</footnote>