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Unoccupied Electronic States of Topological Insulators

Sketch of the a typical 2ppe experiment of a topological insulator. A low energy pump photon excites electron from the occupied bulk bands (orange) into the unoccupied part of the topological surface state (grey). The excited electron is scatterd to lower energies inside the dirac cone and then photoemitted by a second probe photon. The photoemitted electron is further measured by an hemispherical electron analyzer. Varying the delay between the pump and probe photon gives direct acces to the electron dynamics inside the Dirac Cone

Recently, a class of materials with novel electronic properties at the surface has been predicted. The so called topological insulators show a unique electronic structure at the surface. The surface states of these materials show a linear dispersion and the spin degeneracy of the bands is lifted. In addition to that the number of fermilevel crossings at the Gamma point is always odd. The structure therefore resembles a Dirac-Cone. This band-structure is famous in graphen, but here we have the advantages of a small parallel momentum of the electrons.

The surface states originate from a band inversion due to the strong spin orbit interaction inside the material. This is different compared to other surface states, which originate from the broken translational symmetry at the surface. The link to the spin orbit interaction makes the surface states of the topological insulators also robust against most pertubations. This means for example that simple adsorbtion at the surface or defects ofthe surface structure itself do not destroy the topological surface states.

Lifting the spin degeneracy of the surface bands and taking time-inversion symmetry into account leads to a an effect called spin-momentum-locking. This implies that the orientation of the spin of an electron is fixed for a fixed direction of movement. If you compare to electrons moving in opposite directions, then these two electrons need to have opposite spin. It is therefore not easy to backscatter an electron, because you would always need a spin-flip.

With all these special electronic properties, topological insulators are a class of materials for a diverse field of new applications. They are especially promising for realization of spin currents and spin transport. Our group is investigating topological insulators with two-photon-photoemission, because this method allows us to directly investigate the dynamics and lifetimes of electrons inside the dirac cone. Using 2ppe, we were able to show that for the two materials Sb2Te3 and Sb2Te2Se there is indeed a linear dispersing surface state and that transport from the surface to the bulk seams to play a substantial role for the electron dynmics inside the dirac cone.



contact: Johannes Reimann, PD Dr. Jens Güdde, Prof. Dr. Ulrich Höfer


J. Reimann, J. Güdde, K. Kuroda, E.V. Chulkov, and U. Höfer,
Spectroscopy and dynamics of unoccupied electronic states of the topological insulators Sb2Te3 and Sb2Te2S,
Phys. Rev. B 90, 081106(R) (2014). DOI: 10.1103/PhysRevB.90.081106 Abstract Reprint (PDF) (© APS)


Zuletzt aktualisiert: 26.02.2016 · armbrusn

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