One of the greatest successes of the quantum mechanics is the classification of crystalline solids in terms of their electronic properties as metals and insulators.

 

Recently, a new class of materials, the so called topological insulators, has been discovered, showing that this classification is not complete.

They are characterized by the presence of a band gap in their bulk (like the insulators) and gapless Dirac fermions at their surfaces (similarly to the metals) [1-3]. 

Initially, the topological behavior was observed in 2D systems, such HgTe/CdTe quantum. Wells and more recently also in tridimensional systems, as Bi doped by Sb and finally in Particularly, tridimensional topological behavior has been observed in Bi2Se3 and Bi2Te3, already well known for their good thermoelectric properties. These have a relatively large band gap, that would make them particularly interesting, but chemical defects enhance the electrical conductivity: as grown they are metallic. Therefore the bulk contribution can not be neglected and hide the surface states. However the chemical potential can be controlled and the Fermi level shifted into the band gap by chemical doping or by gating in electronic devices, making these systems full insulators. 

Particularly interesting is the trend of physical properties of the solid solution Bi2(Se1-xTex)3 as a function of the composition. Single crystalline samples were grown by the floating zone method.

Finally, superconductivity can be induced in materials related to these topological insulators. by opportune intercalation of a host cation in the structure, in a similar way to superconducting transition metal dichalcogenides (e.g. TiSe2). Superconductivity has been reported in CuxBi2Se3 and PdxBi2Te3, below a Tc of about 3.5 K and 5.5 K, respectively [4,5].

The processing conditions are found to be a crucial parameter for the occurrence of the superconductivity. Whereas single phase CuxBi2Se3 forms, and Cu atoms are actually found to intercalate in the van der Waals structure gap, several (non superconducting) secondary phases are found in PdxBi2Te3, and the intercalation of Pd atoms in Bi2Te3 is not proved. The origin of superconductivity is found not to be the same in CuxBi2Se3 and PdxBi2Te3.