Topological Physics in HgTe based Quantum Devices

21st October 2020

Timing : 1 pm EST

For zoom link to the talks, please email mjgc@mit.edu with your institute email and mention affiliation


For a list of all talks at the NanoBio seminar Series 2020, see here


This talk aims to provide an overview of our work on HgTe-based devices that show various aspect of topological physics. HgTe-based quantum wells is where experiments on topology started. They exhibit the quantum spin Hall effect, a quantized conductance which occurs when the bulk of the material is insulating. Using various tricks 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 similar to that encountered for elementary particles – the charge is carried by so-called 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 playing with the strain in the layers, we can turn HgTe into a Dirac semimetal, which exhibits the ‘axial anomaly’ known from particle physics when the Fermi level is tuned to the Dirac points.



Snow
Laurens W. Molenkamp
Head of Experimental Physics III
Experimental Physics 3
University of Würzburg
Am Hubland
97074 Würzburg, Germany

Laurens W. Molenkamp is an experimental condensed matter physicist specializing in quantum transport studies. He received his Ph.D. from the University of Groningen, subsequently spent several years with Philips Research Laboratories in Eindhoven and as Associate Professor at the RWTH in Aachen. He came to the University of Würzburg in 1999, where he is the Chair of Experimental Physics III and leads the Institute for Topological Insulators. His research interests include transport in nanostructured magnetic and topological materials, grown in-house by MBE techniques. Since 2012, he has been the Editor of the condensed matter physics journal Physical Review B. He has received numerous awards for his observation of the quantum spin Hall effect, opening up the field of topological materials. Amongst these are the Europhysics Prize, the Oliver E. Buckley Condensed Matter Prize and a DFG Leibniz Award. He is a member of the Dutch and Bavarian Academies of Science.