Dissertation Committee Chair: Prof. James Williams
Committee:
Prof. Alicia Kollàr
Prof. Johnpierre Paglione
Prof. Frederick Wellstood
Prof. Ichiro Takeuchi
Abstract:
A Josephson junction (JJ) couples the supercurrent flowing between two weakly linked superconductors to the phase difference between them via a current-phase relation (CPR). While a sinusoidal CPR is expected for conventional junctions with insulating weak links, devices made from some exotic materials may give rise to unconventional CPRs and unusual Josephson effects. Here, we experimentally investigate three such cases.
In the first part of the thesis, we fabricate JJs with weak links made of the topological crystalline insulator PbSnTe, and additional JJs with weak links made of its topologically trivial cousin, PbTe. In characterizing these junctions, we find that measurements of the AC Josephson effect reveal a stark difference between the two: while the PbTe JJs exhibit Shapiro steps at the expected values of V=nhf/2e, PbSnTe JJs show more complicated subharmonic structure. We present the skewed sinusoidal CPR necessary to reproduce these measurements, and discuss this alteration to the CPR as a consequence of 1D helical channels arising from the topologically nontrivial surface state.
In the second part of the thesis, we investigate the proximity-induced superconductivity in SnTe nanowires by incorporating them as weak links in Josephson junctions. We report on indications of an unexpected breaking of time-reversal symmetry in these devices, detailing the unconventional characteristics that reveal this behavior. These observations include an asymmetric critical current in the DC Josephson effect, a prominent second harmonic in the AC Josephson effect, and a magnetic diffraction pattern with a minimum in critical current at zero magnetic field. We analyze how multiband effects and the experimentally visualized ferroelectric domain walls may give rise to a nonstandard CPR, giving insight into the Josephson effect in materials that possess ferroelectricity and/or multiband superconductivity.
Finally, we fabricate and measure JJs with weak links made of the topological insulator (BiSb)2Te3. Under low frequency RF radiation, we observe suppression of the first and third Shapiro steps, consistent with the fractional AC Josephson effect. This could indicate a signature of 4π periodicity in the junction's CPR, which may in turn suggest the presence of Majorana bound states. However, not all of our measured devices confirm this observation, as some of them show suppression of only the first step, while still others show probabilistic distortions to the AC Josephson effect which might indicate other nonequilibrium effects at play. We discuss the inconsistency of our measurements with topologically trivial sources of step suppression that have been suggested in the literature.