“Superconducting proximity effect: from transport and fluctuations to topological superconductivity” by So Takei (University of Maryland)

Wednesday, February 20, 2013 - 4:00pm
Condensed Matter Seminars


The study of hybrid structures involving superconducting and non-superconducting materials has recently enjoyed a renewed level of activity. On the one hand, focus is on the physics of superconductor-ferromagnet junctions due to the recent emergence of experiments showing intriguing and unexpected transport behavior through such structures. On another front, the most recent excitement originates from the fact that a rather conventional example of these hybrid systems can harbor long-sought exotic emergent Majorana zero modes. In line with these activities, I will present two connected topics. The first part is motivated by a recent experiment [1] that observed a highly unusual transport behavior of ferromagnetic Cobalt nanowires proximity-coupled to superconducting electrodes. I propose an explanation for both the anomalously-strong proximity effect and the mysterious resistance peak that preempts the superconducting transition in this experiment [2]. In the second part, I theoretically address the origin of the mysterious "soft gap" measured in the tunneling conductance of the proximity-induced superconductivity in a semiconductor-superconductor hybrid structure, where the observed zero-bias conductance peak [3] has created considerable excitement for its possible connection to the elusive Majorana fermions. After systematically studying different possible sources for this soft gap, I show that only the fluctuation effect at the superconductor-semiconductor interface can quantitatively and qualitatively explain the experimental observations [4]. A minimal analytical model for the soft gap that supports our numerical simulations is provided.


[1] J. Wang et al., Nature Physics 6, 389 (2010).

[2] S. Takei and V. Galitski, Phys. Rev. B 86, 054521 (2012).

[3] V. Mourik et al., Science 336, 1003 (2012).

[4] S. Takei et al., arXiv:1211.1029.