This talk aims to provide an overview of our work on HgTe-based devices that show various aspect of topological phenomena in condensed matter physics.
Transport in HgTe-based quantum wells is where experiments on topology in condensed matter started. They exhibit the quantum spin Hall effect, a quantized conductance which occurs when the bulk of the material is insulating. Using various experimental layouts one can show that the transport occurs along one-dimensional, spin-polarized channels at the edges of the sample.
These channels remain intact even in the presence of magnetism, leading to the observation a quantum Hall effect at fields as low as 50 mT. Similarly, adding Mn to the wells does not affect the conductance quantization – but it does allow for a Kondo effect at finite temperatures.
Also thicker, three-dimensional, HgTe samples can be turned into topological insulators, but now the surface states are two-dimensional metallic sheets. The metal in these sheets is rather exotic in that the band structure is Dirac-like – the charge is carried by Dirac fermions. This means that experiments on these layers can be used to test certain predictions from particle theory that are difficult to access otherwise.
As an example, I will describe experiments where a supercurrent is induced in the surface states by contacting these structures with Nb electrodes. AC investigations indicate that the induced superconductivity is strongly influenced by the Dirac nature of the surface states. We present strong evidence for the presence of a gapless Andreev mode in our junctions.
Finally, by inducing strain in the layers, we can modify the band structure to turn HgTe into a Dirac semimetal, which exhibits the chiral anomaly known from particle physics when the Fermi level is tuned to the Dirac points.
Wolfgang Lerche / TH-SP