Fabre-salts are composed of tetramethyltetrathiafulvalene and Bechgaard salts of tetramethyltetraselenafulvalene. 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 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 of course superconductivity. Only one Bechgaard salt was found to be superconducting at ambient pressure which is 2ClO4 with a transition temperature of TC = 1.4 K. Several other salts become superconducting only under external pressure. The external pressure one would have to apply 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.
Material
TC
pext
2SbF6
0.36
10.5
2PF6
1.1
6.5
2AsF6
1.1
9.5
2ReO4
1.2
9.5
2TaF6
1.35
11
2Br
0.8
26
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 I3, polymeric ones such as the very famous CuBr and anions containing solvents for example Ag4·112DCBE. 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 smaller than those needed for Bechgaard salts. For example, κ-2CuCl needs only a pressure of about 300 bar to become superconducting, which can be achieved by placing a crystal in grease which is freezing below 0 °C and then providing sufficient stress to induce the superconducting transition. The crystals are very sensitive which can be observed impressively in α-2I3 lying several hours in the sun. After this treatment one gets αTempered-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. For sure such a phase diagram wouldn’t only depend on temperature and pressure 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-2CuBr becomes superconducting at TC = 11.8 K at ambient pressure, and a pressure of 300 bar drives deuterated κ-2CuCl from an antiferromagnetic to a superconducting ground state with a transition temperature of TC = 13.1 K. The following table restricts to only a few exemplary superconductors of this class. For more superconductors see ref 1.
Material
TC
pext
βH-2I3
1.5
0
θ-2I3
3.6
0
k-2I3
3.6
0
α-2KHg4
0.3
0
α-2KHg4
1.2
1.2
β’’-2SF5CH2CF2SO3
5.3
0
κ-2CuCl
12.8
0.3
κ-2CuCl deuterated
13.1
0.3
κ-2CuBr deuterated
11.2
0
κ-2Cu2
10.4
0
κ-4Hg2.89Cl8
1.8
12
κH-2Cu4·TCE
9.2
0
κH-2Ag4·TCE
11.1
0
Even more superconductors can be found by changing the ET-molecules slightly either by replacing the sulfur atoms by selenium or by oxygen. Some two-dimensional organic superconductors of the κ-2X and λ2X families are candidates for the Fulde-Ferrell-Larkin-Ovchinnikov phase when superconductivity is suppressed by an external magnetic field.
Doped fullerenes
Superconducting fullerenes based on C60 are fairly different from other organic superconductors. The building molecules are no longer manipulated hydrocarbons but pure carbon 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 a 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 TC = 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 TC = 40 K. Under pressure this compound shows a unique behavior. Usually the highest TC 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.
Material
TC
pext
K3C60
18
0
Rb3C60
30.7
0
K2CsC60
24
0
K2RbC60
21.5
0
K5C60
8.4
0
Sr6C60
6.8
0
4Na2CsC60
29.6
0
K3C60
28
14.8
More organic superconductors
Next to the three major classes of organic superconductors there are more organic systems becoming superconducting at low temperatures or under pressure. A few examples shall be presented here.
TTP-based SCs
TMTTF as well as BEDT-TTF are based on the molecule TTF. Using TTP 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 and phenanthrene. Doping the crystal Picene and Phenanthrene with some alkali metals such as potassium or rubidium and annealing for several days leads to superconductivity with transition temperatures up to 18 K. For the AxPhenanthrene, the superconductivity is possible unconventional. Both phenanthrene and picene are called phenanthrene-edge-type polycyclic aromatic hydrocarbon. The increasing number of benzene rings results in higher Tc.
Graphite intercalation SCs
Putting foreign molecules or atoms between hexagon graphite sheets leads to ordered structures and to superconductivity even if neither the foreign molecule or atom nor the graphite layers are metallic. Several stoichiometries have been synthesized using mainly alkali atoms as anions.
