Time reversal symmetry breaking superconductivity in topological materials

TL;DR

This paper reports the discovery of time reversal symmetry breaking superconductivity in Nb-doped Bi2Se3, a topological insulator, revealing a novel phase with magnetic order and zero-field Hall effect while preserving Dirac surface states.

Contribution

It demonstrates that Nb doping induces bulk superconductivity in Bi2Se3 with spontaneous magnetic order, breaking time reversal symmetry and creating a new topological superconducting phase.

Findings

Nb doping turns Bi2Se3 into a bulk superconductor

Magnetic order appears below 3.2 K, indicating symmetry breaking

A zero-field Hall effect is observed in the superconducting state

Abstract

Fascinating phenomena have been known to arise from the Dirac theory of relativistic quantum mechanics, which describes high energy particles having linear dispersion relations. Electrons in solids usually have non-relativistic dispersion relations but their quantum excitations can mimic relativistic effects. In topological insulators, electrons have both a linear dispersion relation, the Dirac behavior, on the surface and a non-relativistic energy dispersion in the bulk. Topological phases of matter have attracted much interest, particularly broken-symmetry phases in topological insulator materials. Here, we report by Nb doping that the topological insulator Bi2Se3 can be turned into a bulk type-II superconductor while the Dirac surface dispersion in the normal state is preserved. A macroscopic magnetic ordering appears below the superconducting critical temperature of 3.2 K indicating a spontaneous spin rotation symmetry breaking of the Nb magnetic moments. Even though such a magnetic order may appear at the edge of the superconductor, it is mediated by superconductivity and presents a novel phase of matter which gives rise to a zero-field Hall effect.