Strain-mediated high conductivity in ultrathin antiferromagnetic metallic nitrides

TL;DR

This study demonstrates strain-controlled electronic phase transitions in ultrathin antiferromagnetic CrN films, revealing that removing strain can recover metallicity even at single-unit-cell thickness, with implications for advanced electronic applications.

Contribution

It provides the first systematic investigation of strain effects on the intrinsic properties of high-quality CrN films, showing how strain manipulation induces phase transitions and enhances conductivity.

Findings

Ultrathin CrN films exhibit a transition from insulating to metallic behavior upon strain removal.

Metallicity is observed in CrN layers as thin as a single unit cell.

Strain-induced orbital splitting modifies the bandgap, enabling phase control.

Abstract

Strain engineering provides the ability to control the ground states and associated phase transition in the epitaxial films. However, the systematic study of intrinsic characters and their strain dependency in transition-metal nitrides remains challenging due to the difficulty in fabricating the stoichiometric and high-quality films. Here we report the observation of electronic state transition in highly crystalline antiferromagnetic CrN films with strain and reduced dimensionality. Shrinking the film thickness to a critical value of ~ 30 unit cells, a profound conductivity reduction accompanied by unexpected volume expansion is observed in CrN films. The electrical conductivity is observed surprisingly when the CrN layer as thin as single unit cell thick, which is far below the critical thickness of most metallic films. We found that the metallicity of an ultrathin CrN film recovers from an insulating behavior upon the removal of as-grown strain by fabrication of first-ever freestanding nitride films. Both first-principles calculations and linear dichroism measurements reveal that the strain-mediated orbital splitting effectively customizes the relatively small bandgap at the Fermi level, leading to exotic phase transition in CrN. The ability to achieve highly conductive nitride ultrathin films by harness strain-controlling over competing phases can be used for utilizing their exceptional characteristics.