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  Search for the Fractional Josephson Effect in Topological and Nontopological Materials


Christian Schönenberger (University of Basel (CH))


Time: Tuesay, 19th September 2023 at 2:00 pm


Place : Room Rémy Lemaire K223, Institut Néel


Abstract: I will report on an extensive search of the AC Josephson effect of superconducting junctions and weak links obtained from various materials in a low-ohmic environment allowing for DC bias. The materials are two-dimensional graphene, Al proximitized InAs quantum wells, the Dirac semimetal cadmium arsenide, the Weyl semimetal tungsten telluride WTe2, the 3D topological insulator HgTe, InAs nanowires and carbon nanotubes, as well as conventional Al-based reference Josephson junctions. We have studied AC Josephson emission, Shapiro steps, current-phase relations and Fraunhofer patterns to address the current distribution. We can observe missing Shapiro steps, but there is no AC Josepshon signal appearing concurrently at frequency eV/h, as expected for topological junctions. However, we find strong higher order Josephson terms that go with frequency as 2neV/h with n=1,2,3… These terms stem from simultaneous inelastic tunneling of n Cooper-pairs. The relation to the skewness of the CPR will be addressed. I would also like to stress that we observe missing Shapiro steps even in standard conventional Al-Al-oxide-Al Josephson junctions. Hence, missing odd Shapiro steps cannot serve as a signature for topological superconductivity, although this signature has been used multiple of time for the in the concurrent literature.

If time permits, I would also like to discuss current-phase relation measurements (CPR) in WTe2, which has been predicted to be a higher-order topological insulator. We show that one has to be careful in the interpretation of seemingly 4pi-periodic multivalued CPR data, since also this signature can be generated by spurious, but large, inductances that notoriously appear in contact regions due to alloying effects.

This work was done in collaboration with many colleagues over the last six years. I am grateful to them:(alphabetic order of group members participating in these studies) A. Baumgartner, C. Ciaccia, M. Endres, G. Fülöp, R. Haller, D. Indolese, R. Jha, M. Jung, A. Kononov, P. Karnatak, P. Kumbhakar, F. Oppliger, M. Osterwalder,  J. Ridderbos, C. Schönenberger, D. Sufra,  L. Wang,  D. Weiss, H. Zheng.