An organic superconductor is a synthetic

organic compound In chemistry, organic compounds are generally any chemical compounds that contain carbon-hydrogen or carbon-carbon bonds. Due to carbon's ability to catenate (form chains with other carbon atoms), millions of organic compounds are known. T ...

that exhibits superconductivity at low temperatures. As of 2007 the highest achieved critical temperature for an organic superconductor at standard pressure is , observed in the alkali-doped fullerene RbCs₂C₆₀. In 1979

Klaus Bechgaard Klaus Bechgaard (5 March 1945 – 7 March 2017) was a Danish scientist and chemist, noted for being one of the first scientists in the world to synthesize a number of organic charge transfer complexes and demonstrate their superconductivity, ther ...

synthesized the first organic superconductor (TMTSF)₂PF₆ (the corresponding material class was named after him later) with a transition temperature of T_C = 0.9 K, at an external pressure of 11 kbar. Many materials may be characterized as organic superconductors. These include the

Bechgaard salt In organic chemistry, a Bechgaard salt is any one of a number of organic charge-transfer complexes that exhibit superconductivity at low temperatures. They are named for chemist Klaus Bechgaard, who was one of the first scientists to synthesi ...

s and Fabre salts which are both quasi-one-dimensional, and quasi-two-dimensional materials such as '' k-BEDT-TTF₂X'' charge-transfer complex, '' λ-BETS₂X'' compounds,

graphite intercalation compound Graphite intercalation compounds are complex materials having a formula where the ion or is inserted ( intercalated) between the oppositely charged carbon layers. Typically ''m'' is much less than 1. These materials are deeply colored solids t ...

s and

three-dimensional Three-dimensional space (also: 3D space, 3-space or, rarely, tri-dimensional space) is a geometric setting in which three values (called ''parameters'') are required to determine the position of an element (i.e., point). This is the informa ...

materials such as the alkali- doped

fullerene A fullerene is an allotrope of carbon whose molecule consists of carbon atoms connected by single and double bonds so as to form a closed or partially closed mesh, with fused rings of five to seven atoms. The molecule may be a hollow sphere, ...

s. Organic superconductors are of special interest not only for scientists, looking for room-temperature superconductivity and for model systems explaining the origin of superconductivity but also for daily life issues as

organic compounds In chemistry, organic compounds are generally any chemical compounds that contain carbon-hydrogen or carbon-carbon bonds. Due to carbon's ability to catenate (form chains with other carbon atoms), millions of organic compounds are known. The s ...

are mainly built of

carbon Carbon () is a chemical element with the symbol C and atomic number 6. It is nonmetallic and tetravalent—its atom making four electrons available to form covalent chemical bonds. It belongs to group 14 of the periodic table. Carbon mak ...

and

hydrogen Hydrogen is the chemical element with the symbol H and atomic number 1. Hydrogen is the lightest element. At standard conditions hydrogen is a gas of diatomic molecules having the formula . It is colorless, odorless, tasteless, non-toxic ...

which belong to the most common elements on earth in contrast to

copper Copper is a chemical element with the symbol Cu (from la, cuprum) and atomic number 29. It is a soft, malleable, and ductile metal with very high thermal and electrical conductivity. A freshly exposed surface of pure copper has a pinkis ...

or osmium.

One-dimensional Fabre and Bechgaard salts

Fabre-salts are composed of tetramethyltetrathiafulvalene (TMTTF) and Bechgaard salts of tetramethyltetraselenafulvalene (TMTSF). These two organic molecules are similar except for the sulfur-atoms of TMTTF being replaced by

selenium Selenium is a chemical element with the symbol Se and atomic number 34. It is a nonmetal (more rarely considered a metalloid) with properties that are intermediate between the elements above and below in the periodic table, sulfur and tellurium, ...

-atoms in TMTSF. The molecules are stacked in columns (with a tendency to dimerization) which are separated by

anion An ion () is an atom or molecule with a net electrical charge. The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by conve ...

s. Typical anions are, for example, octahedral PF₆, AsF₆ or tetrahedral ClO₄ or ReO₄. Both material classes are quasi-one-dimensional at room-temperature, only conducting along the molecule stacks, and share a very rich phase diagram containing

antiferromagnetic ordering In materials that exhibit antiferromagnetism, the magnetic moments of atoms or molecules, usually related to the spins of electrons, align in a regular pattern with neighboring spins (on different sublattices) pointing in opposite directions. ...

, charge order, spin-density wave state, dimensional crossover and superconductivity. Only one Bechgaard salt was found to be superconducting at ambient pressure which is (TMTTF)₂ClO₄ with a transition temperature of T_C = 1.4 K. Several other salts become superconducting only under external pressure. The external pressure required to drive most Fabre-salts to superconductivity is so high, that under lab conditions superconductivity was observed only in one compound. A selection of the transition temperature and corresponding external pressure of several one-dimensional organic superconductors is shown in the table below.

Two-dimensional (BEDT-TTF)₂X

BEDT-TTF is the short form of bisethylenedithio-tetrathiafulvalene commonly abbreviated with ET. These molecules form planes which are separated by anions. The pattern of the molecules in the planes is not unique but there are several different phases growing, depending on the anion and the growth conditions. Important phases concerning superconductivity are the α- and θ- phase with the molecules ordering in a fishbone structure and the β- and especially κ-phase which order in a checkerboard structure with molecules being dimerized in the κ-phase. This dimerization makes the κ-phases special as they are not quarter- but half-filled systems, driving them into superconductivity at higher temperatures compared to the other phases. The amount of possible anions separating two sheets of ET-molecules is nearly infinite. There are simple anions such as

triiodide In chemistry, triiodide usually refers to the triiodide ion, . This anion, one of the polyhalogen ions, is composed of three iodine atoms. It is formed by combining aqueous solutions of iodide salts and iodine. Some salts of the anion have bee ...

(), polymeric ones such as the very famous Cu (CN)₂r and anions containing solvents for example Ag(CF₃)₄·112DCBE. The electronic properties of the ET-based crystals are determined by its growing phase, its anion and by the external pressure applied. The external pressure needed to drive an ET-salt with insulating ground state to a superconducting one is much less than those needed for

s. For example, κ-(ET)₂Cu (CN)₂l needs only a pressure of about 300 bar to become superconducting, which can be achieved by placing a crystal in grease frozen below and then providing sufficient

stress Stress may refer to: Science and medicine * Stress (biology), an organism's response to a stressor such as an environmental condition * Stress (linguistics), relative emphasis or prominence given to a syllable in a word, or to a word in a phrase ...

to induce the superconducting transition. The crystals are very sensitive, which can be observed impressively in α-(ET)₂I₃ lying several hours in the sun (or more controlled in an oven at ). After this treatment one gets α_Tempered-(ET)₂I₃ which is superconducting. In contrast to the Fabre or Bechgaard salts universal phase diagrams for all the ET-based salts have only been proposed yet. Such a phase diagram would depend not only on temperature and pressure (i.e. bandwidth), but also on

electronic correlation Electronic correlation is the interaction between electrons in the electronic structure of a quantum system. The correlation energy is a measure of how much the movement of one electron is influenced by the presence of all other electrons. Atom ...

s. In addition to the superconducting ground state these materials show charge-order,

antiferromagnetism In materials that exhibit antiferromagnetism, the magnetic moments of atoms or molecules, usually related to the spins of electrons, align in a regular pattern with neighboring spins (on different sublattices) pointing in opposite directions. ...

or remain

metal A metal (from Greek μέταλλον ''métallon'', "mine, quarry, metal") is a material that, when freshly prepared, polished, or fractured, shows a lustrous appearance, and conducts electricity and heat relatively well. Metals are typicall ...

lic down to lowest temperatures. One compound is even predicted to be a

spin liquid In condensed matter physics, a quantum spin liquid is a phase of matter that can be formed by interacting quantum spins in certain magnetic materials. Quantum spin liquids (QSL) are generally characterized by their long-range quantum entanglem ...

. The highest transition temperatures at ambient pressure and with external pressure are both found in κ-phases with very similar anions. κ-(ET)₂Cu (CN)₂r becomes superconducting at T_C = 11.8 K at ambient pressure, and a pressure of 300 bar drives deuterated κ-(ET)₂Cu (CN)₂l from an antiferromagnetic to a superconducting ground state with a transition temperature of T_C = 13.1 K. The following table shows only a few exemplary superconductors of this class. For more superconductors, see Lebed (2008) in the references. Even more superconductors can be found by changing the ET-molecules slightly either by replacing the sulfur atoms by selenium (BEDT-TSF, BETS) or by oxygen (BEDO-TTF, BEDO). Some two-dimensional organic superconductors of the κ-(ET)₂''X'' and λ(BETS)₂''X'' families are candidates for the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) phase when superconductivity is suppressed by an external magnetic field.Shimahara, H. (2008) "Theory of the Fulde-Ferrell-Larkin-Ovchinnikov State and Application to Quasi-Low-Dimensional Organic Superconductors", in ''The Physics of Organic Superconductors and Conductors''. A. G. Lebed (ed.). Springer, Berlin.

Doped fullerenes

Superconducting

fullerenes A fullerene is an allotrope of carbon whose molecule consists of carbon atoms connected by single and double bonds so as to form a closed or partially closed mesh, with fused rings of five to seven atoms. The molecule may be a hollow sphere, ...

based on C₆₀ are fairly different from other organic superconductors. The building molecules are no longer manipulated

hydrocarbons In organic chemistry, a hydrocarbon is an organic compound consisting entirely of hydrogen and carbon. Hydrocarbons are examples of group 14 hydrides. Hydrocarbons are generally colourless and hydrophobic, and their odors are usually weak or e ...

but pure

molecules. In addition these molecules are no longer flat but bulky which gives rise to a three-dimensional, isotropic superconductor. The pure C₆₀ grows in an fcc-lattice and is an insulator. By placing alkali atoms in the interstitials the crystal becomes metallic and eventually superconducting at low temperatures. Unfortunately C₆₀ crystals are not stable at ambient atmosphere. They are grown and investigated in closed capsules, limiting the measurement techniques possible. The highest transition temperature measured so far was T_C = 33 K for Cs₂RbC₆₀.The highest measured transition temperature of an organic superconductor was found in 1995 in Cs₃C₆₀ pressurized with 15 kbar to be T_C = 40 K. Under pressure this compound shows a unique behavior. Usually the highest T_C is achieved with the lowest pressure necessary to drive the transition. Further increase of the pressure usually reduces the transition temperature. However, in Cs₃C₆₀ superconductivity sets in at very low pressures of several 100 bar, and the transition temperature keeps increasing with increasing pressure. This indicates a completely different mechanism then just broadening of the bandwidth.

More organic superconductors

Next to the three major classes of organic superconductors (SCs) there are more organic systems becoming superconducting at low temperatures or under pressure. A few examples follow.

TTP-based SCs

TMTTF as well as BEDT-TTF are based on the molecule TTF (

tetrathiafulvalene Tetrathiafulvalene is an organosulfur compound with the formula (. Studies on this heterocyclic compound contributed to the development of molecular electronics. TTF is related to the hydrocarbon fulvalene, , by replacement of four CH groups w ...

). Using TTP (tetrathiapentalene) as basic molecules one receives a variety of new organic molecules serving as cations in organic crystals. And some of them are superconducting. This class of superconductors was only reported recently and investigations are still under process.

Phenanthrene-type SCs

Instead of using sulfated molecules or the fairly big Buckminster fullerenes recently it became possible to synthesize crystals from the hydrocarbon

picene Picene is a hydrocarbon found in the Pitch (resin), pitchy residue obtained in the distillation of peat tar and of petroleum. This is distilled to dryness and the distillate repeatedly Recrystallization (chemistry), recrystallized from cymene. It ...

and phenanthrene. Doping the crystal picene and phenanthrene with alkali metals such as

potassium Potassium is the chemical element with the symbol K (from Neo-Latin ''kalium'') and atomic number19. Potassium is a silvery-white metal that is soft enough to be cut with a knife with little force. Potassium metal reacts rapidly with atmosph ...

or rubidium and annealing for several days leads to superconductivity with transition temperatures up to . For AxPhenanthrene, the superconductivity is possible unconventional. Both phenanthrene and picene are called phenanthrene-edge-type

polycyclic aromatic hydrocarbon A polycyclic aromatic hydrocarbon (PAH) is a class of organic compounds that is composed of multiple aromatic rings. The simplest representative is naphthalene, having two aromatic rings and the three-ring compounds anthracene and phenanthrene. ...

. The increasing number of benzene rings results in higher T_c.

Graphite intercalation SCs

Putting foreign molecules or atoms between hexagon

graphite Graphite () is a crystalline form of the element carbon. It consists of stacked layers of graphene. Graphite occurs naturally and is the most stable form of carbon under standard conditions. Synthetic and natural graphite are consumed on lar ...

sheets leads to ordered structures and to superconductivity even if neither the foreign molecule or atom nor the graphite layers are metallic. Several

stoichiometries Stoichiometry refers to the relationship between the quantities of reactants and products before, during, and following chemical reactions. Stoichiometry is founded on the law of conservation of mass where the total mass of the reactants equals ...

have been synthesized using mainly alkali atoms as anions.

Several T_Cs for unusual SCs

{, class="wikitable" , - ! Material !! T_C (K) , - , (BDA-TTP)₂AsF₆ , , align="right", 5.8 , - , (DTEDT)₃Au(CN)₂ , , align="right", 4 , - , K_3.3Picene , , align="right", 18 , - , Rb_3.1Picene , , align="right", 6.9 , - , K₃Phenanthrene , , align="right", 4.95 , - , Rb₃Phenanthrene , , align="right", 4.75 , - , CaC₅ , , align="right", 11.5 , - , NaC₂ , , align="right", 5 , - , KC₈ , , align="right", 0.14

References

Superconductors Organic compounds