Ort:
Martin-Luther-Universität Halle-Wittenberg, Institut für Physik, Theodor-Lieser-Str. 9, 06120 Halle
Raum:
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).