Organic Superconductor - Two-dimensional (BEDT-TTF)₂X

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 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 groundstate to a superconducting one is much smaller than those needed for Bechgaard-Salts. For example κ-(ET)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 (never user tweezers on them) which can be observed impressively in α-(ET)₂I₃ lying several hours in the sun (or, more controlled in an oven at 40 °C). After this treatment one gets α_Tempered-(ET)₂I₃ which is superconducting.

In contrast to the Fabre-/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 (i.e. bandwidth) but also on electronic correlations. In addition to the superconducting groundstate 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)₂CuBr becomes superconducting at T_C = 11.8 K at ambient pressure, and a pressure of 300 bar drives deuterated κ-(ET)₂CuCl from an antiferromagnetic to a superconducting groundstate with a transition temperature of T_C = 13.1 K. The following table restricts to only a few exemplary superconductors of this class. For more superconductors see ref 1.

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.