Topological insulator

A topological insulator is a material which behaves as an electrical insulator in the interior thereof, while at the same time allows the movement of charges on its surface.

They were predicted in 2005 by Charles L. Kane and independently in 2006 by Shoucheng Zhang. Zhang said, a realization in Tellurium -cadmium quantum wells before, where they were detected by a group led by Laurens W. Molenkamp at the University of Würzburg in 2007.

Inside an insulator similar topological the electronic band structure of an ordinary insulator having the Fermi energy between the conduction and the valence band. On the surface of the insulator, there are special topological states whose energy lies within the band gap, allowing the charge transport. The carrier in these states have a spin which is directed perpendicular to its direction of movement. For a given power, it is only one further condition with opposite spin, so that scattering is greatly suppressed, and the transport to the surface is almost dissipationslos. These states are characterized by an index similar to the gender of a face in the topology, and are an example of a topologically ordered state.

Topologically protected edge states (1D) were predicted in quantum wells ( very thin layers ) of mercury telluride and cadmium telluride between observed experimentally shortly afterwards. Later, they were predicted in three-dimensional systems of binary compounds with bismuth. The first experimentally realized three-dimensional topological insulator has been observed in bismuth - antimony. A short time later topologically protected surface states were also detected in pure antimony, bismuth selenide, bismuth telluride and antimony telluride by different groups using ARPES. From various other material systems is now believed that they behave like a topological insulator. In some of these materials the Fermi energy in the valence or conduction band is due to naturally occurring defects. In this case, it must be pushed by means of doping or the gate voltage to the band gap.

Similar boundary currents also occur in the quantum Hall effect. However, this requires large magnetic fields, low temperatures and two-dimensional systems.

A helical Dirac fermion, which behaves like a massless relativistic particle, was also observed in a topological insulator.