A remarkable pi electron superconductor
Another superconductor with both exotic and otherwise very conventional aspects is a Bi-Te-Cl compound synthesized in the group of M. Ruck (research fellow of the MPI-CPfS, TU Dresden). Te4[Bi0.74Cl4] is a one-dimensional metal  with remarkable structural similarities to the organic metals and superconductors of the (TMTSF)2X family. In the incommensurately modulated structure the Te species form a stack of pi electron systems, leading to a one-dimensional metal with high electrical conductivity already at room temperature, σ(300 K) = 0.43 × 106 Ω-1m-1, even in polycrystalline pellets. Most remarkably, the compound undergoes a sharp superconducting transition at 7.15 K (Fig. 3), the highest Tc among all known pi electron systems. Recently, we have collected evidence that the superconducting state of this structurally highly complex material is surprisingly simple. It is a type-I superconductor with a single s-wave energy gap. This indicates that the superconductivity is based on the BCS mechanism, albeit with strong electron-phonon coupling.