Unconventional Superconductivity and Majorana Fermions in Topological Insulators

So far we have been dealingwith the single-particle properties of the

helical states in TIs. However, the spin helicity can play an important

role in many-body phenomena as well. This chapter focuses on the

interplay of the spin helicity and superconducting pairing in 3DTIs.

This topic has attracted a great deal of interest for several reasons.

First, superconductivity in TIs is unconventional. Unlike typical

superconductors, for example Pb, Al, or Nb, where Cooper pairs

have the ordinary s-wave symmetry, TIs can exhibit spin-triplet p-wave pair correlations. Second, superconducting TI structures support unconventional zero-energy bound states protected by

the TRS. These bound states are the condensed matter analogues

of Majorana fermions of relativistic field theory; hypothetical

particles that are identical to their own antiparticles. Finally, the

superconductivity in TIs can be induced through the proximity

effect, by depositing conventional superconducting metals, such

as Al or Nb, on the surface of a TI material. This offers divers

possibilities for engineering topological superconducting states and

devices.