About recent experimental evidence of Majorana edge states in topological superconductors

Question

I have a couple of question about the recent experimental evidence of Majorana edge states in topological superconductors.

Which are the main differences between the experimental signatures of Majorana edge states found in the work of Kouwenhoven (http://www.sciencemag.org/content/336/6084/1003.abstract), and in that of Yazdani (http://www.sciencemag.org/content/346/6209/602.abstract)?

Does their experimental signatures (zero-bias conductance) be in principle explained without invoking Majoranas?

And finally, are there other experimental methods which have provided, or could reasonably provide in the next time, experimental evidence of the presence of Majorana edge states in topological superconductors, other than those based on measuring the zero-bias conductance peak?

PS: My question is related with this other "Experimental signature of topological superconductor".

Answer 1

There is no fundamental difference between the signatures found in the two works (Kouwenhoven and Yazdani). Both are tunneling spectroscopy, which roughly measures whether there is a zero-energy single-particle state in the spectrum. Yazdani's setup allowed him to do measurement away from the edge, so that the localization of the edge state can be directly measured.

Just the signatures of zero-bias peak alone can be easily explained by other mechanisms, and Kouwenhoven actually discussed many alternatives. The point is that this signature is supplied by several other measurements (such as the correlation between the zero-bias peak and the direction of the magnetic field, oscillation of splitting of the peak with the magnetic field, etc.), which would be hard to explain without invoking Majoranas. However these are somewhat indirect and although the experimental data has got a lot better, you can argue that none of them is definitive.

The real challenge is to detect the "Majorana" character of the edge state, not just the zero energy. However, experiments of this type are much more difficult than transport, because they all involve quantum interference in one way or another. Easiest among those is probably 4pi Josephson effect, but there has been only a single experiment on that so far.