Odd-frequency superconductivity induced in topological insulators with and without hexagonal warping

TL;DR

This paper investigates how Fermi surface anisotropy in topological insulators influences the emergence of odd-frequency spin-triplet pairing and Majorana bound states in superconductor/ferromagnetic insulator hybrid structures.

Contribution

It demonstrates that the boundary orientation relative to the anisotropic Fermi surface determines the presence of odd-frequency pairing and Majorana modes.

Findings

Proper boundary alignment induces odd-frequency pairing.

Majorana bound states depend on boundary orientation.

Anisotropy affects the realization of Majorana modes.

Abstract

We study the effect of the Fermi surface anisotropy on the odd-frequency spin-triplet pairing component of the induced pair potential. We consider a superconductor/ ferromagnetic insulator (S/FI) hybrid structure formed on the 3D topological insulator (TI) surface. In this case three ingredients insure the possibility of the odd-frequency pairing: 1) the topological surface states, 2) the induced pair potential, and 3) the magnetic moment of a nearby ferromagnetic insulator. We take into account the strong anisotropy of the Dirac cone in topological insulators when the chemical potential lies well above the Dirac cone and its constant energy contour has a snowflake shape. Within this model, we propose that the S/FI boundary should be properly aligned with respect to the snowflake constant energy contour to have an odd-frequency symmetry of the corresponding pairing component and to insure the Majorana bound state at the S/FI boundary. For arbitrary orientation of the boundary the Majorana bound state is absent. This provides a selection rule to the realization of Majorana modes in S/FI hybrid structures, formed on the topological insulator surface.