Unconventional superconductivity and quantum criticality in SmN

TL;DR

This paper investigates nitrogen vacancy doped SmN, revealing a transition from insulating to metallic behavior and providing evidence for unconventional triplet superconductivity near a quantum critical point.

Contribution

It combines experimental and computational methods to identify a defect band and suggests the presence of triplet superconductivity in doped SmN.

Findings

Transition from insulating to metallic state with doping

Evidence of a dispersion-less majority spin defect band

Indications of triplet type superconductivity

Abstract

Nitrogen vacancy doped SmN$_{1-\delta}$ is a semiconductor which lies in the intermediary between insulating-ferromagnetic SmN and metallic-anti-ferromagnetic Sm. The dopant electrons resulting from nitrogen vacancies have recently been predicted to lie in a band precipitated by a majority-spin 4$f$ level on the six Sm ions neighbouring each nitrogen vacancy, an in-gap state $\sim$1 eV below the 4$f$ states in stoichiometric SmN. Optical data reported here corroborate the prediction along with an extended computational study. Electrical transport measurements show the transition from an insulating to metallic state with a hopping type conductivity in dilutely doped films. We provide strong evidence that electron transport is mediated by a dispersion-less majority spin defect band implying triplet type superconductivity, the location of which in the SmN-Sm phase diagram suggests the location of a quantum critical point.