Other Materials

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 [17] 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.

Fig. 1: Electrical resistance vs. temperature for a cold-pressed pellet of polycrystalline Te4[Bi0.74Cl4]. The upper inset is a magnification of the data at low temperatures showing the sharp superconducting transition of the sample. The bulk superconducting state is confirmed by the strong diamagnetic shielding and Meißner effect depicted in the lower inset.
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