- Datum:Startar 13 mars 2023, 10:00Slutar 13 mars 2023, 11:00
Huvudhandledare: Docent Thilo Bauch
Examinator och handledare: Prof. Floriana Lombardi
Hybrid material systems with a conventional superconductor in proximity to a strong spin-orbit semiconductor or a Topological Insulator (TI) have recently acquired a vast interest due to their potential to host exotic phenomena. In a multi-mode hybrid TI Josephson junction with two terminal geometry, Majorana physics manifests as peculiar properties of a part of the Andreev bound states carrying the Josephson current. They give rise to an unconventional 4π periodic current phase relation (CPR) coexisting with a 2π periodic CPR resulting from the conventional Andreev bound states.
We used the asymmetric superconducting quantum interference device (SQUID) technique to extract the CPR of a Josephson junction with a 3D-TI Bi2Se3 nanobelt as a barrier. The obtained CPR shows deviations from the standard sinusoidal CPR with a pronounced forward skewness. At temperatures below 200mK, the junction skewness values are above the zero-temperature limit for short diffusive junctions. Fitting of the extracted CPR shows that most of the supercurrent is carried by ballistic topological surface states (TSSs), with a small contribution of diffusive channels primarily due to the bulk. The nearly ballistic nature of our junction is backed by the observation of Fabry- Pérot like conductance resonance with respect to bias voltage and back gate voltage. Using Fourier analysis of the 2D conductance map, we found that the periodicity of the Fabry-Pérot interference patterns corresponds to transport resonances related to the width of the junction (rather than the length). The same periodicity is also observed in the gate-dependent critical current modulation of TI-junction. These findings are instrumental in engineering devices that can fully exploit the properties of the topologically protected surface states of 3D TIs and in analyzing RF measurements on devices based on TI-Josephson junctions.