# Synopsis: Majorana fermions from topological insulator and superconductor junctions

Much attention has recently been afforded to topological insulators, which constitute a unique state of matter. One fascinating aspect is that they show insulating behavior in the bulk, but also spin-dependent conduction channels on the surface, suggesting, among other things, a potential for spintronics applications. Moreover, topological insulators in the proximity of a superconductor could show excitations that satisfy non-Abelian (noncommutative) statistics, the so-called Majorana fermions, which may be used for quantum computation. Why? Unlike a quantum state that decoheres with the smallest perturbation to introduce errors in computation, the topological properties of non-Abelian particles are protected by symmetry, making them robust.

In a recent article published in *Physical Review Letters*, Jacob Linder and collaborators in Norway and Japan have theoretically studied the proximity effects of a ferromagnetic-insulator–unconventional-superconductor junction deposited on a topological insulator. When the superconductor has spin-triplet pairing, charge excitations become gapless and the standard Andreev reflection that occurs at the interface between a superconductor and a nonsuperconducting region is suppressed. However, for a spin-singlet pairing superconductor with ${d}_{\text{xy}}$ symmetry, zero-energy surface states, differently from the case of regular high-${T}_{c}$ cuprates, are now Majorana fermions, and the tunneling conductance for a junction is now strongly dependent on the magnetization direction, due to the proximity of the topological insulator. Linder *et al.* also propose concrete experiments in the paper that could test these predictions. – *Sarma Kancharla*