Colloquium Ehrenfestii

Wednesday 20 November 2013

L.W. Molenkamp Physics Institute (EP3), Würzburg University

Title: HgTe as a Topological Insulator 

Abstract: HgTe is a zincblende-type semiconductor with an inverted band structure. While the bulk material is a semimetal, lowering 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 exhibit the quantum spin Hall effect, where a pair of spin polarized 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 topological insulator.

To turn the material into a 3-dimensional topological insulator, 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]. These states appear to be decoupled from the bulk. This allows us to induce a supercurrent is induced in the surface states by contacting these structures with Nb electrodes[5].

[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] L. Maier et al, Phys. Rev. Lett. 109, 186806 (2012);

[6] J.B. Oostinga et al., PRX 3, 021007 (2013).

Please Note:  Before the Colloquium, there is a common dinner in the canteen located on the ground floor of the Oort building. This dinner starts at 18:00 hours sharp and is free of charge, under the condition that one attends the colloquium and that one has made a reservation before noon on the Tuesday preceding the colloquium. The registration form is only accessible from within the University (to block spammers); if you would like to register for the dinner from outside the University, please send an email to fran@lorentz.leidenuniv.nl

The revolving doors of the Oort building, alongside the Kamerlingh Onnes Laboratory, are accessible until 9 pm. After that time, one should instead use the sliding doors directly adjacent to the Huygens building: these can be opened with an electronic key.