Superconductivity-Induced Ferromagnetism and Weyl Superconductivity in Nb-doped Bi$_2$Se$_3$

TL;DR

This paper investigates how Nb doping in Bi2Se3 induces a chiral topological superconducting phase characterized by spontaneous time-reversal symmetry breaking, Weyl nodes, and surface Majorana arcs, contrasting with nematic phases in Cu-doped variants.

Contribution

It derives a microscopic free energy model showing Nb magnetic moments are polarized by chiral Cooper pairs, expanding the understanding of topological phases in doped Bi2Se3.

Findings

Nb doping induces ferromagnetism in Bi2Se3 below Tc

The chiral phase is a Weyl superconductor with bulk nodes

Surface states include Majorana arcs

Abstract

A recent experiment on Nb-doped Bi2Se3 showed that zero field magnetization appears below the superconducting transition temperature. This gives evidence that the superconducting state breaks time-reversal symmetry spontaneously and possibly be in the chiral topological phase. This is in sharp contrast to the Cu-doped case which is possibly in the nematic phase and breaks rotational symmetry spontaneously. By deriving the free energy of the system from a microscopic model, we show that the magnetic moments of the Nb atoms can be polarized by the chiral Cooper pairs and enlarge the phase space of the chiral topological phase compared to the nematic phase. We further show that the chiral topological phase is a Weyl superconducting phase with bulk nodal points which are connected by surface Majorana arcs.