Sign-reversed anomalous Nernst effect in the ferromagnetic Weyl-semimetal Fe$_{3-x}$GeTe$_2$: the role of Fe vacancies

TL;DR

This study investigates how Fe vacancies in the ferromagnetic Weyl-semimetal Fe$_{3-x}$GeTe$_2$ influence the anomalous Nernst and Hall effects, revealing vacancy-controlled tuning of Berry curvature and topological properties.

Contribution

It demonstrates the role of Fe vacancies in tuning the Berry curvature and topological effects in Fe$_{3-x}$GeTe$_2$, combining experimental observations with first-principles calculations.

Findings

Fe vacancies cause a sign change in the anomalous Nernst signal.

Fe-vacancy concentration tunes the Berry curvature contribution.

The anomalous Hall conductivity evolves with Fe vacancy levels.

Abstract

Anomalous Nernst effect, as a thermal partner of anomalous Hall effect, is particularly sensitive to the Berry curvature anomaly near the Fermi level, and has been used to probe the topological nature of quantum materials. In this work, we report the observation of both effects in the ferromagnetic Weyl-semimetal Fe$_{3-x}$GeTe$_2$ with tunable Fe vacancies. With decreasing Fe vacancies, the anomalous Hall conductivity evolves as a function of the longitudinal conductivity from the hopping region to the region where the intrinsic Berry curvature contribution dominates. Concomitant evolutions in the anomalous Nernst signal and the anomalous off-diagonal thermoelectric coefficient are observed below the Curie temperature, displaying a unique sign change caused by the Fe vacancies. Combining these results with first-principles calculations, we argue that the Fe-vacancy concentration plays a unique role in simultaneously tuning the chemical potential and ferromagnetism, which in turn controls the Berry curvature contribution in this family of ferromagnetic topological semimetals.