In this thesis the promising combination of two particular materials is studied: an s-wave superconductor and a topological insulator. ‘We succeeded to induce superconductivity in topological insulator surface states,’ Marieke Snelder says. Next step was to better understand the superconducting correlations, and how these can be observed in conductance measurements. ‘The University of Twente is renowned for its solid expertise on superconductivity,’ Marieke says. ‘This expertise is of importance for future quantum computing applications, and the quest for finding the Majorana fermion.’
Being able to work on topics involving ultra new physics and science, was a fascinating feature of her PhD work, Marieke Snelder can tell. Topological insulators – behaving as an insulator in its interior but whose surfaces contain conducting states - are around only since 2007, when experimental evidence was shown for the very first time.
‘Combining this new generation of materials with superconductors, and studying their interactions, no wonder unpredictable things might happen,’ she says. ‘It is not at all clear what properties the electrons, and other particles involved, will show.’
Mesa+ has a reputation for creating, interpreting and characterising data coming form systems in which superconductivity is involved. When, for example, Fraunhofer characteristics are deviant, theoretical and experimental tools are present to give meaningful explanations and bring scientific research further, step by step.
‘Our contributions are well-recognized and widely appreciated in international conferences,’ Marieke noticed. ‘In Twente we are less pronounced on trying to find the Majorana fermion right away, as is the TU Delft is. We contribute to a significant extent, by facilitating the observation conditions to do so, be it here or elsewhere in the world. In this way we are acting on the subfront internationally, one might say.’
Marieke proceeded her master’s research by pursuing to combine a whole new topological insulator with the superconductor. It took quite some time combining the materials in an optimal way. To do so e-beam lithography functionality had to be taken a major step further, to achieve the etching geometries and dimensions needed for fabrication.
Marieke: ‘More time than expected was invested herein. After that, building the actual devices was less demanding. In the conductance measurements we saw a conductance dip at the induced gap, which could be explained by the presence of p-wave correlations, necessary to create the zero-energy mode. However, more research is needed to finally confirm this.’
The new topological insulator Marieke used, may not be the one to be able to perform the final Majorana experiments, as the mean free path is expected to be too small for this. ‘Dutch universities are collaborating on this subject more and more,’ Marieke noticed. ‘We made our contributions in collaboration with the University of Amsterdam. Also groups within the universities of Delft and Leiden are closely collaborating on this. I firmly believe when all four approaches converge even more, some major breakthrough results will be obtained in the near future.’
As a next step in her career Marieke is starting in industry to go and work at the NXP laboratories. Together with other engineers she will be responsible for modeling and characterizing active and passive components in all NXP-supported technologies.
‘I guess my scientific approach will prove useful,’ Marieke says. ‘The demands I expect to be quite different from academics. In industry - no matter what chip is on the market - they all have to perform to near perfection. A lot of effort is aimed towards this goal, in which collaboration between groups is of main importance, and communication with sales managers and end-user groups is crucial. I am looking forward on playing a role in these developments.’