So Takei, Quantum over Brunch Friday, Nov. 30, 2012, 11am

Science and Technology I, Room 306, 11am (+ lunch after noon)

Superconducting proximity effect: from transport and fluctuations to topological superconductivity”
So Takei, University of Maryland

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 in the field originates from the fact that a rather conventional example of these hybrid systems can harbor long-sought exotic emergent particles known as Majorana fermions. 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.