Monday, August 22, 2011

A Place To Look for Majorana Fermions

This is one of those rare endeavor where both the high energy physics community and the condensed matter physics community are looking for the same thing but with different experiments. They are both looking for tell-tale signs of Majorana fermions but using entirely different scales of experiment. The high energy physics community is looking at remnants from particle physics collisions at huge particle accelerators, while the condensed matter physics community is looking at very small experiments using topological insulators. Who will find it first?

My bet is on the condensed matter physics. This report is another reason why these physicists might have a leg up, because one possible area where they might want to look has been narrowed down. A new theoretical paper indicated that the superconducting vortices on a topological insulator might contain these elusive Majorana fermions.

It is then natural to ask, What is a doped topological insulator good for? While one hopes that many of the topological phenomena of the true insulating state might be manifested in some form in a doped system, many questions still remain unanswered. However, Hosur et al. have made a striking prediction that MBS can still be realized in doped topological insulators under certain mild conditions. A true insulating state is important in the Fu-Kane proposal because if the bulk contains low-energy states then the MBS can tunnel away from the surface and delocalize into the bulk, which effectively destroys the MBS. Hosur et al. circumvent this delocalization by requiring that the entire doped topological insulator become superconducting. They show that as long as the doping is not too large, vortices in superconducting topological insulators will bind MBS at the places where the vortex lines intersect the material surfaces. While this might seem like a big leap in complexity, experimental evidence already shows that, indeed, copper-doped Bi2Se3 is a superconductor below 3.8 K. In this context, Hosur et al. make a strong prediction that vortex lines in superconducting CuxBi2Se3 can harbor MBS.
 (MBS=Majorana bound state)

These topological insulators are definitely a playground for a lot of exotic states that most people typically associate with elementary particles in QCD/QED. This is another example where the so-called "applied" physics of condensed matter can be a source of very fundamental physics.


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