Organic Superconductors

Denis Jerome

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Denis Jerome. Organic Superconductors. Scientific American Monthly, 1982. hal-04967489

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Organic Superconductors

*The ability to carry an electric current without resistance had been known only in metals. Now superconductivity is seen in organic crystals made up of flat molecules in zigzag stacks*

by Klaus Bechgaard and Denis Jéréome

The diversity of synthetic organic compounds in everyday life is a testament to the architectural resources of the carbon atom. By assembling chains or rings of carbon atoms chemists have fabricated more than two million organic substances, which serve as fibers, coatings, adhesives, pharmaceuticals, synthetic rubbers, structural plastics and so on. The collective experience with carbon-based systems offers the molecular designer a vast set of raw materials and a battery of proved experimental techniques with which to create new materials. It seems an organic molecule can be constructed to have almost any physical property.

Recently we and our colleagues have created a series of organic materials with a rare and extraordinary property: superconductivity, or the ability to conduct an electric current without resistance. Superconductivity had been observed previously only in metals and metallic alloys. Even in those materials, most of which are good ordinary conductors, it is a remarkable phenomenon. The resistance of a superconductor is not merely small but exactly zero; a current set up in a superconductor is not dissipated as heat (as it is in an ordinary conductor) but instead continues to flow forever. Establishing superconductivity in an organic solid seems still more remarkable because the great majority of synthetic organic materials are electrical insulators.

The conditions under which we first observed the superconducting state in an organic compound were extreme. The temperature was .9 degree Kelvin (or about -272 degrees Celsius) and the pressure was 12,000 atmospheres (or 12,000 times the atmospheric pressure at the surface of the earth). In less than a year, however, five other synthetic organic compounds were found to be superconducting; one of them is superconducting at normal atmospheric pressure, although a low temperature is still required for all the materials. There is good reason to believe that superconductivity at higher temperatures, perhaps as high as 20 degrees K., can be

achieved. Indeed, now that superconductivity has been added to the list of properties available in organic substances, the full resources of synthetic chemistry can be brought to bear on relaxing the conditions under which superconductivity can be maintained.

Ever since the discovery of superconductivity in 1911 by Heike Kamerlingh Onnes of the University of Leiden, it has been considered a low-temperature phenomenon. Kamerlingh Onnes' discovery was the outcome of a remarkable technological achievement: the liquefaction of helium. At atmospheric pressure helium condenses at 4.2 degrees K. Kamerlingh Onnes employed the new liquid as a refrigerant, and he proceeded to study the electrical resistance of various metals at low temperature. Typically in a metal the resistance decreases with the temperature, and it was predicted even by pre-quantum-mechanical theories that if absolute zero could be achieved in a metal whose interatomic structure was perfectly regular, the electrical resistance would be zero. Kamerlingh Onnes found, however, that the resistance of a mercury wire suddenly fell to zero at 4.2 degrees K., a low temperature but still infinitely far above absolute zero. Soon after the initial discovery investigators at Leiden observed superconductivity in other metals: in lead below 7.2 degrees K. and in tin below 3.7 degrees.

The temperature at which the transition to superconductivity takes place is called the critical temperature. As new superconductors were discovered the greatest measured value of the critical temperature increased slowly at an average rate of .3 degree per year. Relatively high critical temperatures have been found for metallic alloys such as vanadium-silicon (17 degrees), niobium-tin (18 degrees) and niobium-aluminum (18.7 degrees). Nevertheless, the steady rise of the maximum critical temperature since 1911 now shows signs of leveling off. The most recent increase was recorded in 1973 for an alloy of niobium and germanium (Nb₃Ge); the

critical temperature of the material is 23.2 degrees.

It has been apparent since its discovery that if superconductivity could be developed in materials at less severe temperatures, its technological potential would be almost unbounded. Lossless power transmission, more efficient electric motors and enormously powerful electromagnets could become generally available. Supercomputers have been proposed that exploit the small heat output of superconducting circuit elements. The speed of a computer is fundamentally limited by its size, which determines the average distance a signal must travel within the computer. The size of a computer is limited in turn by the ability of its circuit elements to dissipate heat. If the density of circuit elements is too great, the heat they generate will melt the computer. Superconducting elements could largely circumvent the problem. Moreover, such elements can respond to a signal much faster than semiconducting devices.

Superconductivity is also characterized by a magnetic effect, now called the Meissner effect, first observed in 1933 by the German investigators K. W. Meissner and R. Ochsenfeld. A superconductor placed in a magnetic field generates its own internal field that tends to expel the external one. The lines of force of the external magnetic field bend around the surface of the superconductor instead of passing through it. (In a certain class of superconductors the magnetic lines of force may penetrate the surface of the material to a shallow depth.) The Meissner effect also suggests a technological application: a roadway made of a superconducting material would repel a magnetic field generated by a vehicle. Such a vehicle would float above the roadway on a magnetic cushion and move virtually without friction.

The search for high-temperature superconductivity has therefore been one of the driving forces in solid-state physics. Yet in spite of the technological and economic incentive, a fundamental understanding of superconductivity in
metals was not achieved until almost 50 years after the phenomenon was first observed. The understanding, when it came, required a profound application of the ideas of quantum mechanics to the motions of atoms and electrons in a solid. Thus the search for high-temperature superconductivity has also been stimulated by the desire to understand the properties of matter in general, and its investigation has contributed a great deal to the understanding.

The theory of superconductivity in metals was formulated in 1957 by John Bardeen, Leon N. Cooper and J. Robert Schrieffer, who were then at the University of Illinois; they were awarded a Nobel prize for their theory in 1972. According to the Bardeen-Cooper-Schrieffer theory, superconductivity is a state of matter that arises when electrons in a conductor form loosely bound pairs called Cooper pairs. Both electrons in a Cooper pair move with the same speed but in opposite directions. Although the Cooper pair is made up of electrons, its properties are sufficiently different from those of the electron for it to be treated as an entirely new kind of conducting particle. Superconduction is the result of the highly coordinated motion of the Cooper pairs.

It may appear contrary to reason that there can be an attraction between two particles having the same electric charge. When atoms are arranged in a solid, however, the orbitals that define

CARBON

SELENIUM

MOLECULAR CORRIDOR is the path favored by conduction electrons in an organic crystal that has as one of its components the molecule tetramethyltetraselenafulvalene, or TMTSF. Each TMTSF molecule is a flat structure that tends to give up its valence electrons (the outermost electrons that are distributed throughout the molecule). If TMTSF is grown as a crystal with another molecule or an ion that accepts valence electrons, the TMTSF molecules stack in a zigzag pattern. A top view of the pattern is represented in the diagram by the alternating blue and white of the interatomic bonds within each molecule. Because the TMTSF molecules tend to donate electrons to electron-receptor molecules (not shown), the TMTSF stack carries a net positive charge. The orbitals, or electronic shells, of each TMTSF molecule extend above and below the molecular plane, so that the orbitals of adjacent molecules overlap. Hence conduction electrons can readily move only along the corridor formed by the stack, and the TMTSF crystal is highly conductive in only one dimension. In the superconducting state the conduction electrons form bound systems, called Cooper pairs, which move through the material without resistance. The pairs are probably formed first in TMTSF along the molecular corridors.

<figure><img src="image_2.png" /></figure>
the spatial distribution of their outer electrons overlap. Such electrons are called valence electrons; they are the least tightly bound electrons in an atom, and they tend to wander throughout the overlapping orbitals in the solid. The atoms that remain fixed in the lattice are in effect positively charged ions, since they have been stripped of at least one valence electron. The attraction between valence electrons that make up a pair develops through the mediation of the positive ions in the lattice.

As an electron moves through the lattice the electrostatic attraction between the electron and the positive ions generates a ripple or distortion in the lattice. The passage of the negatively charged electron pulls the positive ions together in its wake; interactions among the ions give rise to restorative forces that cause the lattice to vibrate. The vibratory motions of the lattice are much slower than the motions of the electron. Hence for a relatively long period after the passage of the electron the positive ions re-

main close together. They create a region of net positive charge in the lattice to which another electron can be attracted, and a Cooper pair is formed.

As the ions move apart under the influence of the restorative forces a region is created in which the density of positive charge is less than it is in the surrounding parts of the lattice. The two electrons tend to avoid this region. At high temperature the effect is overwhelmed by the random, thermal vibrations of the lattice, but it can emerge below the critical temperature and cause the electrons that make up each Cooper pair to act as if they were connected by a spring. The electrons that form the pair vibrate back and forth in response to the variations in charge density set up by their own motions in the atomic lattice.

Electrical conduction in a superconductor is caused by the net motion of the centers of mass of the Cooper pairs, whereas electrical conduction in an ordinary metal is caused by the net motion

of individual valence electrons. In order to explain the motion of the Cooper pairs in the lattice, however, one must draw on the principles of quantum mechanics. When the electrons enter the bound state, the total energy of the system is minimized if all the Cooper pairs have the same momentum. If no current is flowing through the material, the momentum of each Cooper pair is zero: the electrons in each pair are thereby constrained to move in opposite directions with respect to the lattice, and the velocity of the center of mass of each pair must be exactly zero. When there is a current in the material, the electrons in each pair must move in such a way that the centers of mass of all the Cooper pairs have the same constant momentum. Like the hands of a tightrope walker on his balance bar, every motion of one electron that threatens to break up the Cooper pair must be immediately compensated for by its partner so that equilibrium is always maintained.

Why does a material that resists the flow of individual electrons not also resist the flow of Cooper pairs? Ordinary electrical resistance is a result of the scattering of moving electrons by imperfections in the atomic lattice and by the thermal vibrations of the lattice at any temperature above absolute zero. The current or net motion of the electrons through the lattice is impeded by the scattering, and the energy of the electronic system is correspondingly reduced by an extremely small amount. The scattering of each electron and the resulting decrease in the electric current are accomplished by a small change in the vibrational energy stored in the lattice at a finite temperature. The resistivity of a perfectly regular metallic lattice would theoretically be zero at absolute zero because at that temperature there is no vibrational energy in the lattice available for scattering the electrons.

In a superconductor, on the other hand, the electrical resistivity can be zero at a finite temperature because once the Cooper pairs are put into motion by an electric field they acquire a net momentum. The net momentum cannot decay through scattering: unlike the scattering of an ordinary electron, which costs little energy, the scattering of a Cooper pair requires much more energy than the binding energy that holds the electrons together. Furthermore, in the highly organized superconducting state a change in the momentum of one pair requires a change in the momenta of all the other pairs as well. Hence the energy needed to redistribute the momenta of the Cooper pairs and so to create electrical resistance is much larger than the vibrational energy available in the lattice at low temperature.

The binding of the electrons does not

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

<figure><img src="image_4.png" /><figcaption>TETRATHIAFULVALENE
(TTF)</figcaption></figure>

<figure><img src="image_5.png" /><figcaption>7,7,8,8-TETRACYANO-P-
QUINODIMETHANE
(TCNQ)</figcaption></figure>

<figure><img src="image_6.png" /><figcaption>HEXAFLUOROPHOSPHATE
(PF₆⁻)</figcaption></figure>

<figure><img src="image_7.png" /><figcaption>TETRASELENAFULVALENE
(TSF)</figcaption></figure>

<figure><img src="image_8.png" /><figcaption>2,5-DIMETHYL-7,7,8,8-
TETRACYANO-P-
QUINODIMETHANE
(DMTCNQ)</figcaption></figure>

<figure><img src="image_9.png" /><figcaption>PERCHLORATE
(ClO₄⁻)</figcaption></figure>

<figure><img src="image_10.png" /><figcaption>TETRAMETHYL TETRASELENAFULVALENE
(TMTSF)</figcaption></figure>

CARBON

CHLORINE

FLUORINE

NITROGEN

OXYGEN

PHOSPHORUS

SELENIUM

SULFUR

STRUCTURAL BUILDING BLOCKS of the organic materials in which superconductivity has been detected fit together in long, quasi-one-dimensional structures. Typically two kinds of molecule are combined in one crystal. Because the two molecules differ in their affinity to the valence electrons in the crystal, one kind of molecule tends to accept valence electrons from the other. The resulting structure is a network of stacks of molecules, in which the charge alternates from stack to stack. Perylene bromide was the first highly conductive (but not superconductive) organic substance to be discovered. TTF, TSF and TMTSF are molecules closely related to one another that donate electrons; TCNQ and DMTCNQ accept electrons. PF₆⁻ and ClO₄⁻ have been combined with TMTSF to create the first organic superconductors.
take place above the critical temperature because at such higher temperatures the thermal agitation of the electrons tends to break up the pairs. In other words, if there were enough energy available in the lattice to redistribute the momenta of the pairs, the energy would be sufficient to break them up as well. Below the critical temperature the pairs can persist in their lockstep motion through the lattice for years without detectable change. Above the critical temperature the electrons are scattered as they are in an ordinary conductor; the resistance of the material jumps from zero to some finite value.

Since the condensation of electrons into Cooper pairs depends on deformations of the atomic lattice, the critical temperature for superconductivity is related to the stiffness of the lattice: its resistance to deformation by a passing electron. If the atoms of the lattice have small masses and the interatomic bonds are relatively "soft," the deformations are large compared with the deformations caused by thermal vibrations and the critical temperature is high. More precisely, the critical temperature varies inversely with the square root of the mass of the ions in the lattice.

William L. McMillan of Bell Laboratories has argued, however, that the softening of the lattice may establish an upper limit to the critical temperature for superconductivity. When the lattice becomes softer, its vibration frequency decreases. The lower frequency tends to inhibit the formation of Cooper pairs, an effect McMillan predicts will become dominant between 25 and 30 degrees K. Thus McMillan's theory may account for the failure to find any substance with a critical temperature higher than that of niobium-germanium.

In 1964 W. A. Little of Stanford University proposed an alternative mechanism for the formation of Cooper pairs, which does not depend on lattice distortions [see "Superconductivity at Room Temperature," by W. A. Little; SCIENTIFIC AMERICAN, February, 1965]. According to Little, a superconductor could be fabricated from a chain of organic molecules. A spine of carbon atoms would act as a conducting path for Cooper pairs, and organic molecules attached to the spine would carry out the mediating role of the deforming lattice. Little proposed that each organic molecule be a hydrocarbon (specifically, a dye) with loosely bound valence electrons. Such a molecule can easily be polarized, so that one part acquires a positive charge and another part a negative charge. An electron traveling along the spine of carbon atoms would repel the outer electrons of the hydrocarbon molecules, thereby creating a region of net positive charge on the regions of the hy-

<figure><img src="image_11.png" /></figure>

ELECTRICAL CONDUCTIVITY is one of the most widely varying quantities in nature. Materials can differ in conductivity by a factor of more than 10²⁰. (If superconductivity is considered, the factor is infinite.) Metallic conductors, such as copper, become more conductive as the temperature decreases. In organic conductors, however, structural changes or the onset of magnetic properties can cause the material to become a semiconductor or an insulator at low temperatures. The dependence of conductivity on temperature is plotted for various materials.
<figure><img src="image_12.png" /></figure>

SUPERCONDUCTIVITY in a metal lattice arises through the interaction of conduction electrons with the lattice. A negatively charged electron moving through the metal attracts the fixed, positively charged ions of the lattice, which are therefore drawn together in the wake of the electron. The region of net positive charge formed in this way tends to attract a second electron, and so the motion of the second electron can become correlated with the motion of the first one. The two electrons form a Cooper pair, acting somewhat as if they were connected by a spring. They move at the same speed but in opposite directions. In regions of excess positive charge the electrons move toward each other (upper illustration). Restorative forces on the lattice also act like springs. The positive ions overshoot their equilibrium position and begin to oscillate, briefly creating a region where the density of positive charge is less than it is in the surrounding lattice. The two bound electrons avoid this region and move apart (lower illustration).

drocarbon molecules nearest the carbon spine. A second electron moving along the spine would be attracted by the relatively dense positive charge and so would be attracted indirectly to the first electron.

Little argued that in such a model one could envision superconductivity at room temperature; indeed, he suggested that in principle superconductivity could be established at temperatures as high as 2,000 degrees K. Such a high critical temperature could come about, he maintained, because the region of enhanced positive charge in the wake of the first electron is caused by the displacement of an electron in the hydrocarbon molecule instead of by the displacement of an atom in a lattice. Because the electron is about 100,000 times less massive than a typical positive ion in an ordinary metal the tendency to form Cooper pairs would be increased by a factor of the square root of 100,000, or roughly 300. Although Little's idea continues to be highly controversial, it has generated strong interest in the development of organic superconductors.

No one has yet succeeded in building a molecule even approximating the properties Little envisioned. The first organic superconductors have instead been an outgrowth of attempts to build ordinary electrical conductors from organic materials. The effort is historically linked to the development of electronic components made of silicon and other semiconductors. Organic chemists and solid-state physicists sought to match the range of electrical properties exhibited by the chemical elements and so create from organic substances what Little called a "shadow periodic table." Because most synthetic organic materials such as vinyl or polyethylene are electrically insulating the search for organic conductors and superconductors has been a difficult one.

Electrical conductivity is one of the most widely varying properties in nature: the conductivity of a good conductor such as copper is greater than $10^{20}$ times the conductivity of a good insulator such as glass. As we have mentioned, ordinary conduction depends on a net flow, or drift, of the valence electrons through a material. Under the influence of an electromagnetic field the essentially chaotic motions of the valence electrons are altered slightly to favor a single direction. In an isolated atom each valence electron occupies a discrete orbital, which represents a state with a particular energy. In a molecule or in a solid, however, the principles of quantum mechanics do not allow more than two electrons to have precisely the same energy and momentum. As a result each discrete energy level of the atomic valence electrons is partitioned into many slightly different levels that are spread
<figure><img src="image_13.png" /></figure>

COOPER PAIRS in an organic superconductor may also form by a different mechanism, one based on the polarization of side molecules attached to a spine of carbon atoms. As conduction electrons move along the spine the valence electrons of the side molecules are repelled and tend to concentrate their charge away from the spine. An excess of positive charge is thereby created on the region of each side mole-

cule nearest the conducting spine, so that another electron moving along the spine can become bound to the first one. Once two electrons become bound they oscillate as if they were connected by a spring, just as they do in a metal. The arrows indicate the motion of the electrons during the formation of the bond, before they begin to oscillate. This mechanism was proposed by W. A. Little of Stanford University.

throughout a nearly continuous band.

The drift of electrons through a material corresponds to an increase in the total electronic energy of the material; it can readily take place only if the allowed energy states in the valence energy band are not entirely occupied. The

valence electrons in such a material can drift through the regions of the lattice in which the partially filled atomic orbitals overlap. If the valence band is fully occupied, on the other hand, the total electronic energy can be increased only by raising the energy of some of the valence

electrons across an energy gap: a range of energy levels that is forbidden by the laws of quantum mechanics. Any net motion of the valence electrons in such a material is inhibited because there is no place for them to go within the overlapping atomic orbitals of the material.

<figure><img src="image_14.png" /></figure>

<figure><img src="image_15.png" /></figure>

<figure><img src="image_16.png" /></figure>

<figure><img src="image_17.png" /></figure>

ONE-DIMENSIONAL CONDUCTION can be understood by considering a simple system in which it might be observed: a linear array of sodium atoms whose valence-electron orbitals overlap. Each sodium atom has one valence electron, and when the orbitals overlap, the energy states of the valence electrons form a nearly continuous band of energy levels. The band is only half filled with electrons; if an external electric field is applied, some of the electrons can easily move to unoccupied energy states within the band. The shift in the energy distribution of the electrons corresponds to a net motion of electrons along the overlapping atomic orbitals (upper illustration). Because

the system is one-dimensional, however, its energy can be reduced if the elements of the linear array are not uniformly distributed. The charge density of the valence electrons in such an array may also vary with a periodicity equal to twice the interatomic spacing. The distortions in the structure and in the charge density split the energy band in the middle, leaving a full band and an empty band separated by a gap (lower illustration). Such a configuration cannot readily conduct a current because the required shift in the energy levels is not easy to make. Enough energy to promote valence electrons across the energy gap must be supplied before the array can become conductive.
<figure><img src="image_18.png" /></figure>

PHASE DIAGRAM of the organic compound $TMTSF_2PF_6$ illustrates the typical response of an organic superconductor to variations in temperature and pressure. At low pressure and temperature (gray region) the material is magnetic and becomes an insulator. The effect of magnetic ordering is similar to the effect of the onset of a charge-density wave: an energy gap is created between the filled and the empty parts of the valence-electron energy band. At much higher pressure the material becomes superconductive (solid colored region). The authors have suggested that outside these two regions and up to about 30 degrees Kelvin superconductivity is incipient: Cooper pairs form for short periods over short distances along the stacks of molecules. The incipient superconductivity is manifested by a low but nonzero electrical resistance.

Thus a material readily conducts a current only if some of the energy states in the valence-electron energy band are not occupied by electrons.

In many organic materials partially filled valence orbitals make it energetically favorable for the material to combine chemically with other substances. Hence the requirement that the valence orbitals be partially filled in a conductor can lead to the preparation of a substance that is chemically unstable. The first highly conducting organic compound, for example, was reported by Hideo Akamatu, Hiroo Inokuchi and Yoshia Matsunaga of the University of Tokyo in 1954. The compound was made by treating the organic molecule perylene with bromine, but it broke down almost immediately into a nonconducting substance through continued reaction with the bromine. Because

of such chemical instability relatively few molecules suitable for constructing organic conductors have been found.

The structure of any successful organic conductor is probably dictated by two requirements. First, its molecular building blocks must fit closely together, so that the conduction electrons can move easily from one molecule to another. Second, the energy cost of partially filling or partially opening a valence energy band must be small. All the molecules investigated so far have a large planar skeleton made up of carbon and hydrogen atoms and diverse atoms such as nitrogen, sulfur and selenium. The valence electrons of such a molecule are typically found above and below the planar framework. Hence the molecules can stack close together like pancakes and the valence electrons can propa-

gate a current primarily from plane to plane along the stack.

One of the first molecules of this kind to be synthesized is 7,7,8,8-tetracyano-p-quinodimethane, which is usually abbreviated TCNQ. It was prepared in 1960 by workers at E. I. du Pont de Nemours & Company. Little energy is needed to introduce an extra electron into the TCNQ molecule and so to begin filling a new valence energy band in a stack of such molecules. The negatively charged structure that results is chemically stable. TCNQ by itself, however, is not able to conduct an electric current: all the TCNQ molecules have the same affinity for electrons, and so in an electrically neutral system there is no tendency for electrons to move from one TCNQ molecule to another. If the TCNQ molecules can obtain electrons from other atoms or molecules in their environment that readily give up electrons, the material can become conductive. In the cesium salt of TCNQ ($Cs_2TCNQ_3$), for example, each cesium atom gives up an average of two-thirds of an electron to each TCNQ molecule. Thus two out of every three TCNQ molecules become negatively charged, and if an electric field is imposed, the extra electrons can move from the charged TCNQ molecules to the neutral ones.

In the early 1970's Fred Wudl of the State University of New York at Buffalo prepared an organic molecule called tetrathiafulvalene, or TTF, which incorporates four sulfur atoms. TTF is a counterpart to TCNQ in that it readily gives up an electron to form a stable, positively charged structure. When Wudl combined TTF with chlorine atoms, the chlorine accepted some of the electrons given up by the TTF and the resulting material was highly conductive.

In 1973, shortly after TTF was first synthesized, Dwaine O. Cowan and John P. Ferraris of Johns Hopkins University and Alan J. Heeger and Anthony F. Garito of the University of Pennsylvania independently combined the two organic molecules to form TTF-TCNQ. In the solid form of this substance the TTF and the TCNQ molecules stack in separate columns, and electrons are donated from the TTF stack to the TCNQ stack. X-ray experiments by Robert Comes and his co-workers at the Université de Paris-Sud at Orsay have shown that .59 electron per molecule is transferred from one stack to the other. Because of the electron transfer there can be a net motion of electrons along both stacks; hence the material is conductive.

The architecture of the TTF-TCNQ crystal gives rise to a striking electrical property: the material is highly conductive in one direction only. The most favorable direction is 500 times as conductive as the least favorable direction.
The reason for the anisotropic conduc-
tivity is that all the constituent mole-
cules of the crystal are stacked in paral-
lel planes. The orbitals of the molecules
interact mainly along the stacks.

In view of Little's suggestions about organic superconductivity, the discovery of such a one-dimensional organic conductor is highly suggestive. Could TTF-TCNQ, perhaps under extreme conditions of temperature and pressure or perhaps with slight modifications to its molecular structure, be made superconducting according to the mechanism Little proposed? It is known from theoretical calculations that it is impossible to sustain the formation of Cooper pairs in a strictly one-dimensional conductor at any temperature above absolute zero. Thus the one-dimensional superconductor, as proposed by Little, cannot exist. (The formation of Cooper pairs through the mediation of polarizing electrons, however, which is the heart of Little's theory, is not thereby disproved.) Moreover, the structure of the stacks of TTF-TCNQ is actually quite different from the linear conducting chain Little described.

In spite of such differences the anal-
ogy with Little's hypothetical super-
conductor is not without merit. In any
real molecular system the conductors
are only approximately one-dimension-
al; there is always some interaction be-
tween stacks of molecules that would
allow a Cooper pair to jump from one
stack to another. Hence the theoretical
obstacle to superconductivity in one-di-
mensional systems is not a practical one.
Moreover, investigations of the proper-
ties of TTF-TCNQ have led to the develop-
ment of related compounds that are
quasi-one-dimensional and supercon-
ductive. It is not yet known whether the
mechanism for Cooper pairing in such
systems is the one Little envisioned.

In order to develop superconductiv-
ity in organic materials investigators
had to study and overcome problems
unique to one-dimensional systems. One
of the most important properties of such
systems was noted by Rudolf Peierls of
the University of Birmingham as ear-
ly as 1954. Peierls pointed out that the
uniform separation between adjacent
planes of molecules, characteristic of
substances such as TTF-TCNQ at room
temperature, is not energetically fa-
vored at all temperatures. As the tem-
perature is lowered the total energy of
the solid can be reduced if the interpla-
nar spacing and the distribution of the
charge density of the valence electrons
are modified.

The charge density of the electron un-
dergoes a periodic concentration and
rarefaction that is called a charge-densi-
ty wave. When the charge-density wave
forms and the lattice is distorted, the
partially filled valence-electron energy

<figure><img src="image_19.png" /></figure>

CRYSTALS OF TMTSF₂PF₆ are formed by passing an electric current through a solution in which molecules of TMTSF and negative PF₆⁻ ions are dissolved. Valence electrons from the TMTSF molecules are drawn out through the positive electrode, and crystals of high purity collect there. The needlelike crystals in the photograph are from five to 10 millimeters long.
band splits into two bands, the lower one completely filled and the upper one completely empty. After such a split conduction can occur only if electrons are promoted across the energy gap by an external source of thermal or electromagnetic energy.

The onset of the charge-density wave can thus cause a material to change abruptly from a conductor into a semiconductor or an insulator. Typically the transition takes place between 50 and 100 degrees K.; the mechanism causes TTF-TCNQ to become a semiconductor below 53 degrees.

To modify the electron-donating properties of TTF and similar molecules investigators substituted heavier selenium atoms for the sulfur atoms in TTF. Edward M. Engler of the International Business Machines Corporation

prepared tetraselenafulvalene-TCNQ, or TSF-TCNQ, and one of us (Bechgaard) prepared tetramethyl-TSF, or TMTSF, while working with a group at Johns Hopkins. It is the latter molecule that was finally employed to create superconducting organic molecular crystals.

At first TMTSF looked like a real loser. When it was reacted with TCNQ, red, insulating crystals were obtained. TMTSF-TCNQ is quite stable in this state; with luck and elaborate experimental techniques black, conducting crystals representing a different phase, or structural configuration, can be prepared. Below 60 degrees K. a charge-density wave splits the valence energy band and the material becomes an insulator again.

In collaboration with Jan R. Andersen of the Risø National Laboratory in Roskilde, Denmark, we reacted TMTSF

with a substance closely related to TCNQ called 2,5-dimethyl-TCNQ, or DMTCNQ. The resulting crystals of TMTSF-DMTCNQ conduct current at room temperature along the parallel stacks of molecules in a way similar to that observed in TTF-TCNQ, but the material becomes an insulator below 41 degrees K. At Orsay we had already begun to study the effects of high pressure on the electronic properties of TTF-TCNQ and similar systems. In 1973 Meir Weger of the Hebrew University of Jerusalem had suggested that the insulating phase of such materials, caused by charge-density distortions, could be inhibited by squeezing the chains of molecules together under high pressure. We had therefore hoped that high pressure would induce superconductivity in TTF-TCNQ, but we found instead that high pressure helps to stabi-

<figure><img src="image_20.png" /></figure>

ORGANIC CRYSTALLINE LATTICE of TMTSF₂PF₆ is shown here in a schematic view perpendicular to the one shown on page 53. Regions of negative charge lie along the vertical columns of PF₆⁻ ions (gray shading), whereas regions of positive charge lie along the central stacks of TMTSF molecules (red shading). The orbitals of the valence electrons of the TMTSF molecules overlap along the stack. The conductivity of the crystal in the direction parallel to the axis of the stacks is therefore many times greater than the conductivity along other directions.
lize the insulating phase of the material.

If pressure is applied to TMTSF-DMTCNQ at room temperature, however, a new phase of the crystal is generated. If the material is then cooled under pressure, it does not become an insulator but instead remains highly conducting in the temperature range of liquid helium, that is, below about four degrees K.

Although the phase transition is not yet fully understood, the stabilization of a highly conducting state of TMTSF-DMTCNQ was a milestone in the quest for an organic superconductor. At low temperature and at a pressure of 10,000 atmospheres the conductivity of the material exceeds 200,000 mhos per centimeter, which exceeds the maximum conductivity of TTF-TCNQ by a factor of 10. (The mho, a unit of conductance, is the reciprocal of the ohm, the basic unit of resistance.)

Although the conductivity of the best conducting metals such as copper and silver can be as great as 100 times the conductivity of TMTSF-DMTCNQ, the mobility of the conducting electrons is much greater along the stacks of an organic molecule than it is in a metal. An electron in the stack of an organic molecule is scattered once every 1,000 molecules; an electron in a metal such as copper is scattered once every 50 to 100 lattice spacings, unless the metal is exceptionally pure. (The greater conductivity of a metallic conductor is a result of its far more numerous conduction electrons.) The highly conducting state of TMTSF-DMTCNQ can be suppressed by applying a magnetic field perpendicular to the stacks or by exposing the material to small amounts of radiation.

Superconductivity has not been observed in TMTSF-DMTCNQ, perhaps because of small amounts of impurities in the samples. Its intriguing properties, however, have led us to propose that its high conductivity is a signature of an incipient, transient superconductivity that arises along the one-dimensional stacks of the crystal below about 30 degrees K. Although superconductivity cannot be sustained in a one-dimensional conductor, transient and fluctuating Cooper pairing can take place along a one-dimensional chain of molecules. The transient Cooper pairs can contribute significantly to electrical conduction well above the critical temperature of about 1.2 degrees K. for bulk, or three-dimensional, superconductivity.

The critical temperature for the formation of Cooper pairs in one dimension depends only on the strength of the pairing interactions within the chain. The critical temperature for three-dimensional superconductivity also depends on the strength of interactions between chains. If the strength of the former interactions is much greater than

the strength of the latter, incipient, one-dimensional superconductivity can set in at temperatures well above the three-dimensional critical temperature. The formation of the one-dimensional Cooper pairs is expected to take place gradually as the temperature is decreased.

Our detailed investigations of TMTSF-DMTCNQ indicated that the properties of the material are determined primarily by the stack of TMTSF molecules. We therefore decided to prepare a group of materials in which the properties of the TMTSF stacks are enhanced. To make crystals of high purity we dissolved neutral TMTSF molecules in an organic solvent and added a salt that would release negatively charged ions. We then placed two platinum electrodes in the solution and passed an electric current through it. At the positive electrode electrons were removed from the TMTSF molecules to form positively charged molecules. As soon as a positively charged molecule formed it combined spontaneously with a neutral TMTSF molecule and with one of the negatively charged ions and separated as an insoluble salt at the positive electrode. In this way many so-called TMTSF₂X salts have been prepared, where X represents a negatively charged ion such as hexafluorophosphate (PF₆⁻) or perchlorate (ClO₄⁻).

The TMTSF₂X salts have quite unusual properties. Unlike other stacks of organic molecules such as TTF-TCNQ, most of the TMTSF₂X salts remain highly conducting at temperatures well below 20 degrees K. The transfer of electrons from TMTSF molecules to the other molecules in the solid and the periodic arrangement of the negatively charged ions in the crystal are such that the material is not susceptible to the distortions introduced by the charge-density wave. It turns out that if an organic conductor is not susceptible to such distortions, the material becomes a superconductor instead of an insulator at low temperature. Ordinary conductivity does not persist as the material is cooled.

In December, 1979, we subjected the salt TMTSF₂PF₆ to conditions similar to the ones that resulted in highly conducting TMTSF-DMTCNQ. In an elaborate experiment carried out by Michel Ribault at Orsay, crystals of TMTSF₂PF₆ were first pressurized to 12,000 atmospheres and then cooled slowly at constant pressure over a 12-hour period to below one degree K. As the material cooled we measured its conductivity and its susceptibility to a magnetic field. As the temperature approached one degree K, the electrical resistance began to decrease, and at .9 degree it dropped abruptly to zero. The magnetic measurements also showed that at .9 degree a weak magnetic field was expelled from the sample, which is the sign of the Meissner effect in metallic supercon-

ductors. The material had become superconductive.

Since then we have found superconductivity in crystals of TMTSF₂X made from hexafluoroarsenate (AsF₆⁻), hexafluoroantimonate (SbF₆⁻), hexafluorotantallate (TaF₆⁻), perrhenate (ReO₄⁻) and perchlorate. The properties of the crystals depend somewhat on the properties of the negatively charged ions, although the ions do not take part directly in conduction. At atmospheric pressure the PF₆⁻ salt becomes magnetic and insulating at 12 degrees K., and the ReO₄⁻ salt gives way to charge-density-wave distortions and becomes insulating at 180 degrees. High pressures are needed to make the two kinds of crystal superconductive. The crystals of TMTSF₂ClO₄, on the other hand, become superconductive at 1.2 degree K. at normal atmospheric pressure.

There is now considerable experimental evidence to support our contention that superconductivity is incipient along the one-dimensional molecular stacks, at temperatures well above the onset of bulk superconductivity. Radiation at far-infrared frequencies is absorbed by the materials above a threshold frequency that is related to the bonding energy for Cooper pairs within a single chain. Experiments demonstrating the movement of electrons from the superconductor to a thin layer of semiconductor applied on the surface of the superconductor also suggest that Cooper pairs form at relatively high temperatures within each chain. Finally, the high conductivity of the materials at temperatures well above the critical temperature can be sharply reduced by placing them in a magnetic field.

These results and others all point to a fairly consistent value for the temperature below which electrons in an isolated chain form Cooper pairs, namely about 30 degrees K. Thus the comparatively low critical temperatures found so far in organic superconductors are not upper limits; on the contrary, the experiments done so far strongly indicate a transient, one-dimensional superconductivity at temperatures higher than those found for any other superconductor.

There is much still to be learned about superconductivity in organic materials. It is not known, for example, whether the Cooper pairs form because of interactions mediated by the lattice, as they do in an ordinary superconducting metal, or whether they form through some mechanism of electronic mediation, as Little proposed. As one-dimensional systems come to be better understood, synthetic chemistry may open the way to the development of superconducting materials that have practical technological applications.