An organic superconductor is a organic compound that exhibits superconductivity. All examples exhibit superconductivity only at very low temperatures but are of interest for explaining the origin of superconductivity. As of 2007 the highest achieved critical temperature for an organic superconductor at standard pressure is 33 K (−240 °C; −400 °F), observed in the alkali-doped fullerene RbCs2C60. In 1979 Klaus Bechgaard synthesized the first organic superconductor (TMTSF)2PF6 (the corresponding material class was named after him later) with a transition temperature of Tc = 0.9 K (−272.2 °C; −458.0 °F), at an external pressure of 12,000 bar (170,000 psi). Bechgaard salts and Fabre salts are both quasi-one-dimensional, and quasi-two-dimensional materials such as k-BEDT-TTF2X charge-transfer complex, λ-BETS2X compounds, graphite intercalation compounds and three-dimensional materials such as the alkali-doped fullerenes.
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-atoms in TMTSF. The molecules are stacked in columns (with a tendency to dimerization) which are separated by anions. 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, 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 TC = 1.4 K (−271.8 °C; −457.1 °F). 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 (I−3), polymeric ones such as the very famous Cu[N(CN)2]Br 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 Bechgaard salts. For example, κ-(ET)2Cu[N(CN)2]Cl needs only a pressure of about 300 bar (4,400 psi) to become superconducting, which can be achieved by placing a crystal in grease frozen below 0 °C (32 °F) and then providing sufficient stress 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 40 °C, 104 °F). After this treatment one gets αTempered-(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 correlations. In addition to the superconducting ground state these materials show charge-order, antiferromagnetism or remain metallic down to lowest temperatures. One compound is even predicted to be a spin liquid. The highest transition temperatures at ambient pressure and with external pressure are both found in κ-phases with very similar anions. κ-(ET)2Cu[N(CN)2]Br becomes superconducting at TC = 11.8 K (−261.3 °C; −438.4 °F) at ambient pressure, and a pressure of 300 bar drives deuterated κ-(ET)2Cu[N(CN)2]Cl from an antiferromagnetic to a superconducting ground state with a transition temperature of TC = 13.1 K (−260.0 °C; −436.1 °F). 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)2X and λ(BETS)2X families are candidates for the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) phase when superconductivity is suppressed by an external magnetic field.
Doped fullerenes
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