Colloquium

8831 1485782469

HgTe as a Topological Insulator

  • Date: Jun 18, 2015
  • Time: 17:15
  • Speaker: Prof. Laurens W. Molenkamp
  • University of Würzburg
  • Location: Martin-Luther-Universität Halle-Wittenberg, Institut für Physik, Theodor-Lieser-Str. 9, 06120 Halle
  • Room: Gustav-Mie-Hörsaal


HgTe is a zincblende-type semiconductor with an inverted band structure. While the bulk material is a semimetal, low-ering the crystalline symmetry opens up a gap, turning the compound into a topological insulator. The most straightforward way to do so is by growing a quantum well with (Hg,Cd)Te barriers. Such structures ex-hibit the quantum spin Hall effect, where a pair of spin po-larized helical edge channels develops when the bulk of the material is insulating. Our transport data [1-3] provide very direct evidence for the existence of this third quantum Hall effect, which now is seen as the prime manifestation of a 2-dimensional topo-logical insulator. To turn the material into a 3-dimensional topological insula-tor, we utilize growth induced strain in relatively thick (ca. 100 nm) HgTe epitaxial layers. The high electronic quality of such layers allows a direct observation of the quantum Hall effect of the 2-dimensional topological surface states [4, 5]. Due to the screening properties of Dirac fer-mions, these states turn out to be decoupled from the bulk for a very wide range of densities [5]. This allows us to in-duce a supercurrent in the surface states by contacting these structures with Nb electrodes [6]. AC investigations indicate that the induced superconductivity is strongly influ-enced by the helical character of the charge carriers.

[1] M. König et al., Science 318, 766 (2007). [2] A. Roth et al., Science 325, 294 (2009). [3] C. Brüne et al., Nature Physics 8, 486 (2012). [4] C. Brüne et al., Phys. Rev. Lett. 106, 126803 (2011). [5] C. Brüne et al., Phys. Rev. X 4, 041045 (2014). [6] J.B. Oostinga et al., Phys. Rev. X 3, 021007 (2013).


 
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