Half-metallic surface states and topological superconductivity in NaCoO2from first principles

Abstract

Based on the first-principles calculations, we predict a novel half-metallic surface state in layered bulk insulator NaCoO 2 , with tunable surface hole concentration. The half-metallic surface has a single fermi surface with a helical spin texture, similar to the surface state of topological insulators, but with the key difference of time reversal symmetry breaking in the present case. We propose realization of topological superconductivity and Majorana fermions when the half-metallic surface states are in proximity contact with a conventional superconductor.

Majorana fermions emerging from magnetic nanoparticles on a superconductor without spin-orbit coupling

“…4 The difference is that here the p-wave pairing is obtained from s-wave pairing due to the coupling of the electron spin to local magnetic moments. This has the effect of coupling spin to orbital degrees of freedom, but in contrast to existing proposals, [6][7][8][9][10][11][12][13] neither spin-orbit coupling in the superconductor is needed nor a Rashba effect in the nanowire.…”

“…4 While the former symmetry can be readily broken by a magnetic field, it has been argued that Rashba spin-orbit coupling is too weak to effectively break the latter symmetry. 5 We are optimistic that some variation on the InAs nanowire proposal 6,7 will be successfully realized, but alternative proposals [8][9][10][11][12][13][14] continue to play an important role.…”

Momentum relaxation in a semiconductor proximity-coupled to a disordereds-wave superconductor: Effect of scattering on topological superconductivity

“…This is the basic idea underlying the superconducting proximity effect which is used for realizing the topological p + ip superconducting state at the interface. In most of the previous studies [5][6][7][8][9][10][11][12] , the s-wave superconductor has been considered in the clean limit, and the effect of the superconductor disorder on the induced state was not addressed. However, it is well-known that impurity scattering in the active system (i.e.…”

“…[13][14][15][16]18, and 19 for more details on the semiconductor disorder affects) and, thus, τ is entirely determined by the disorder scattering in the superconductor. One can ask the following question: is it possible to realize a topological phase in the sandwich structures [5][6][7][8]11,12,15 , given that both the proximity-induced gap and the disorder scattering rate in the semiconductor induced by the superconductor impurities depend on the interface transparency ? This question is particularly significant in view of the topological superconducting phase in the semiconductor being equivalent to a spinless p-wave superconductor.…”

“…There are many proposals that exploit magnetic, superconducting and other properties for spintronics and quantum information purposes [1][2][3][4] . In particular, the prospect of realizing exotic topological superconducting states carrying Majorana fermions at the interface between a semiconductor and a conventional swave superconductor in sandwich structures [5][6][7][8][9][10][11][12] is very intriguing. The basic concept underlying these proposals is that electrons tunneling between different materials inherit their physical properties.…”