Probing vortex Majorana fermions and topology in semiconductor-superconductor heterostructures

TL;DR

This paper explores signatures of Majorana fermions in vortex cores within semiconductor-superconductor heterostructures, identifying unique spectral and topological features that distinguish topologically non-trivial phases.

Contribution

It provides new criteria for identifying Majorana fermions through momentum distribution and real-space density profiles, and links topological transitions to specific vortex pair amplitudes.

Findings

Majorana fermions show symmetric momentum distribution around high symmetry points.

Vortex core states form an x-shape structure only in topologically non-trivial phases.

Certain spin-triplet p-wave pair amplitudes emerge at topological phase transitions.

Abstract

We investigate the local density of states, spectral function, and superconducting pair amplitudes for signatures of Majorana fermions in vortex cores in ferromagnetic and spin-orbit coupled semiconductor-superconductor heterostructures. We show that the Majorana fermion quasiparticle momentum distribution is always symmetrically distributed at a finite radius around a high symmetry point, thereby providing a necessary condition for a low-energy state to be a Majorana fermion. In real space profiles of the local density of states through the vortex core the Majorana fermion, together with other finite-energy vortex states, form a characteristic x-shape structure only present at non-trivial topology. Moreover, we find that the Mexican hat band structure property of the topologically non-trivial phase translates into multiple high-intensity band edges and also vortex core states located above the superconducting gap in the local density of states. Finally, we find no strong correlation between odd-frequency pairing and the appearance of Majorana fermions, but odd-frequency pairing exists as soon as ferromagnetism is present. In fact, we find that the only vortex superconducting pair amplitude directly related to any phase transition, is the appearance of certain spin-triplet $p$-wave pairing components in the vortex core at a pre-topological vortex core widening transition.