Organic superconductor

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.

[1][2] 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, at an external pressure of 12 kbar.

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 are mainly built of carbon and hydrogen which belong to the most common elements on earth in contrast to copper or osmium.

A selection of the transition temperature and corresponding external pressure of several one-dimensional organic superconductors is shown in the table below.

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 electronic properties of the ET-based crystals are determined by its growing phase, its anion and by the external pressure applied.

In addition to the superconducting ground state these materials show charge-order, antiferromagnetism or remain metallic down to lowest temperatures.

κ-(ET)2Cu[N(CN)2]Br becomes superconducting at TC = 11.8 K 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. The following table shows only a few exemplary superconductors of this class.

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.

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 alkali metals such as potassium or rubidium and annealing for several days leads to superconductivity with transition temperatures up to 18 K (−255 °C; −427 °F).

The layered structure of ET 2 X salts illustrated by κ-(ET) 2 Cu 2 (CN) 3 . The yellow, grey, blue and red ellipsoids represent the sulfur, carbon, nitrogen and copper atoms, respectively. The hydrogen atoms are omitted for clarity. Layers of ET donor molecules are separated by polymeric Cu 2 (CN) 3 − anion sheets. κ-(ET) 2 Cu 2 (CN) 3 is a semiconductor , but a very similar κ'-(ET) 2 Cu 2 (CN) 3 polymorph is an ambient-pressure superconductor with T C ~ 5 K. [ 4 ]
Structure of Cs 3 C 60
Crystal structure of KC 8